[issue31556] asyncio.wait_for can cancel futures faster with timeout==0

[Viktor Kovtun]

Viktor Kovtun added the comment:

asyncio.wait_for is coroutine itself, to start executing code, no matter with 
this PR or not it needs to be awaited/yield from

import asyncio

@asyncio.coroutine
def foo():
print(1)

loop = asyncio.get_event_loop()

fut = asyncio.wait_for(foo(), 0)

print('it is not raised yet')

try:
loop.run_until_complete(fut)
except asyncio.TimeoutError:
print('raised here')


will print 
it is not raised yet
raised here

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[issue19907] gettext - Non ascii chars in header

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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status: open -> pending

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Re: How to share class relationship representations?

[Pavol Lisy]

On 9/19/17, leam hall  wrote:
> I'm working on designing the classes, sub-classes, and relationships in my
> code. What is a good visual way to represent it so it can be stored in git
> and shared on the list without large images or attachments?
>
> Thanks!
>
> Leam

https://stackoverflow.com/questions/29586520/can-one-get-hierarchical-graphs-from-networkx-with-python-3#29597209

there are probably more ideas useful for you. Maybe this one could be
inspiring too:

import networkx as nx

g=nx.DiGraph()
g.add_edges_from([('class 1','class 2'), ('class 1','class 3'),
('class 1','class 4'),
('class 2','class 5'), ('class 2','class 6'), ('class 2','class 7'),
('class 3','class 8'), ('class 3','class 9'), ('class 4','class 10'),
('class 5','class 11'), ('class 5','class 12'), ('class 6','class 13')])
p=nx.drawing.nx_pydot.to_pydot(g)
p.write_png('example.png')

And you could make function which from list of classes create list of edges! :)

In git you don't need to store images just list of classes and script
to make images (documentation).
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[issue31557] tarfile: incorrectly treats regular file as directory

[Joe Tsai]

New submission from Joe Tsai:

The original V7 header only allocates 100B to store the file path. If a path 
exceeds this length, then either the PAX format or GNU formats must be used, 
which can represent arbitrarily long file paths. When doing so, most tar 
writers just store the first 100B of the file path in the V7 header.

When reading, a proper reader should disregard the contents of the V7 field if 
a previous and corresponding PAX or GNU header overrode it.

This currently not the case with the tarfile module, which has the following 
check 
(https://github.com/python/cpython/blob/c7cc14a825ec156c76329f65bed0d0bd6e03d035/Lib/tarfile.py#L1054-L1057):
# Old V7 tar format represents a directory as a regular
# file with a trailing slash.
if obj.type == AREGTYPE and obj.name.endswith("/"):
obj.type = DIRTYPE

This check should be further constrained to only activate when there were no 
prior PAX or GNU records that override that value of obj.name. This check was 
the source of a bug that caused tarfile to report a regular as a directory 
because the file path was extra long, and when the tar write truncated the path 
to the first 100B, it so happened to end on a slash.

--
messages: 302778
nosy: Joe Tsai
priority: normal
severity: normal
status: open
title: tarfile: incorrectly treats regular file as directory

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[issue31555] Windows pyd slower when not loaded via load_dynamic

[S Safihre]

New submission from S Safihre:

I have a Python 2.7 C-extension that was made for performance, however, I 
noticed something strange on Windows:
When I run locally python setup.py install and it installs it the "egg" way, 
the performance of calling the function in the module 500x in a loop is:

 yEnc C New  took   579 ms
 yEnc C New  took   580 ms
 yEnc C New  took   580 ms

But when I install it as a wheel locally or via pip without a single change to 
the C-code, this is the result:

 yEnc C New  took   702 ms
 yEnc C New  took   694 ms
 yEnc C New  took   691 ms

That's a 10-15% difference. I also checked macOS and Ubuntu, here it does not 
seem to be the case.
By investigating (https://github.com/pypa/setuptools/issues/1154) I found that 
the difference is only that the "egg" way the module gets loaded via 
imp.load_dynamic 

I cannot test under Python 3, since the module is (not yet) converted to Python 
3-style.


To reproduce run and look at the yEnc C New score between the 2 ways (on 
Windows only of course):

git clone https://github.com/sabnzbd/sabyenc.git
cd sabyenc
python setup.py install
python .\tests\speed_compare.py; python .\tests\speed_compare.py;python 
.\tests\speed_compare.py;
pip uninstall sabyenc -y

pip install sabyenc
# Or this:
# python setup.py install bdist_wheel
# pip install .\dist\sabyenc-3.3.1-cp27-cp27m-win_amd64.whl
python .\tests\speed_compare.py; python .\tests\speed_compare.py;python 
.\tests\speed_compare.py;

--
components: Extension Modules, Windows
messages: 302768
nosy: S Safihre, paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: Windows pyd slower when not loaded via load_dynamic
type: performance
versions: Python 2.7

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[issue1612262] Class Browser doesn't show internal classes

[Terry J. Reedy]

Changes by Terry J. Reedy :


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[issue31556] asyncio.wait_for can cancel futures faster with timeout==0

[Yury Selivanov]

Yury Selivanov added the comment:

I think this is a backwards incompatible change and thus will be rejected.  
Currently there's a guarantee that "wait_for" can throw a TimeoutError *only* 
when when you await it.

   fut = wait_for(something, 0)

   # some important code

   try:
   await fut
   except TimeoutError:
   # do something

With your PR merged, the above asyncio code would be broken, because asyncio 
users can guard with try..except only the await expression.

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[issue1612262] Class Browser doesn't show internal classes

[Terry J. Reedy]

Terry J. Reedy added the comment:

I settled on 'transform_children' for the new function. #31461 is the master 
issue for improving the browsers.

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue31423] Error while building PDF documentation

[Ned Deily]

Ned Deily added the comment:

Thanks for looking into this.  See also 
https://github.com/python/psf-salt/issues/118 where the question arose whether 
the version of Tex Live on docs is new enough.  I'm no expert at anything Tex 
but the odd thing is that I had been able to build dist docs on my current 
Debian (testing) system without this change.  Let's see what happens on docs 
after the daily full doc build.

--
nosy: +ned.deily
status: pending -> open

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Re: Issues with beginning Django use

[mm0fmf]

On 22/09/2017 19:50, Joey Steward wrote:

Hello,

I've been attempting to begin learning Django but have been having running
Django and Python commands on windows.

For example, when I run django-admin startproject mytestsite, I get the
following error message

django-admin : *The term 'django-admin' is not recognized as the name of a
cmdlet, function, script file, or operable program. Check the spelling of
the name, or if a path was included,*
*verify that the path is correct and try again.*
*At line:1 char:1*
+ django-admin startproject mytestsite
+ 
 + CategoryInfo  : ObjectNotFound: (django-admin:String) [],
CommandNotFoundException
 + FullyQualifiedErrorId : CommandNotFoundException



Just curious if anyone has encountered a similar problem and can give some
advice on getting moving in the right direction.

Thanks a lot!



This is a repeat of your post of 20/9/2017.

I cut and pasted your error message "django-admin : *The term 
'django-admin' is not " into Google and the first hit was the fix to 
the problem.


Pasting error messages into a Google search gives instant results. 
Posting to the list/newsgroup may give better, more personalised 
responses but you may have to wait sometime before anyone replies.




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[issue12301] Use :role:`sys.thing` instead of ``sys.thing`` throughout

[Éric Araujo]

Changes by Éric Araujo :


--
resolution:  -> rejected
stage: patch review -> resolved
status: pending -> closed

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[issue31556] asyncio.wait_for can cancel futures faster with timeout==0

[Viktor Kovtun]

Changes by Viktor Kovtun :


--
keywords: +patch
pull_requests: +3687
stage:  -> patch review

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Re: Assertions

[jladasky]

On Thursday, September 21, 2017 at 9:29:19 AM UTC-7, Tobiah wrote:
> Are these completely equivalent?
> 
> def foo(thing):
> 
> assert(thing > 0), "Thing must be greater than zero"
> 
> 
> def foo(thing):
> 
> if not (thing > 0): raise AssertionError("Thing must be greater than 
> zero")
> 
> 
> Other than the fact that the assertion can be turned off
> with -O?

For that reason, I would prefer to raise ValueError instead of AssertionError.  
The -O flag is nice for turning off test code.  If the code is essential to the 
functionality of the program, you probably don't want to misidentify it as test 
code.
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[issue31556] asyncio.wait_for can cancel futures faster with timeout==0

[Yury Selivanov]

Yury Selivanov added the comment:

Do you have a use case where this optimization is important?

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[issue31556] asyncio.wait_for can cancel futures faster with timeout==0

[Viktor Kovtun]

Viktor Kovtun added the comment:

If coroutine function has some blocking calls before first await/yield from 
statement maybe makes sense do no let them be executed, if timeout equals 0

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[issue1612262] Class Browser doesn't show internal classes

[Terry J. Reedy]

Terry J. Reedy added the comment:


New changeset fa1cae5832cbcfafedc4b1879c2abc85452f4edd by Terry Jan Reedy in 
branch '3.6':
 [3.6] bpo-1612262: IDLE: Class Browser shows nested functions, classes 
(GH-2573)  (#3702)
https://github.com/python/cpython/commit/fa1cae5832cbcfafedc4b1879c2abc85452f4edd


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[issue31556] asyncio.wait_for can cancel futures faster with timeout==0

[Viktor Kovtun]

New submission from Viktor Kovtun:

There is no need to create extra future and use loop.call_later to cancel base 
future from asyncio.wait_for when timeout==0. If loop is heavy loaded it can be 
cancelled simnifically later then 0 seconds later.

--
components: asyncio
messages: 302770
nosy: asvetlov, hellysmile, yselivanov
priority: normal
severity: normal
status: open
title: asyncio.wait_for can cancel futures faster with timeout==0
versions: Python 3.6, Python 3.7

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[issue31556] asyncio.wait_for can cancel futures faster with timeout==0

[Yury Selivanov]

Yury Selivanov added the comment:

You're right!  Let's work on the PR then, I've left a review comment.

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[issue31556] asyncio.wait_for can cancel futures faster with timeout==0

[Viktor Kovtun]

Viktor Kovtun added the comment:

Actually provided example without patch will print 1, which is not expected

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[issue1612262] Class Browser doesn't show internal classes

[Terry J. Reedy]

Terry J. Reedy added the comment:


New changeset 058de11360ea6816a6e978c7be0bcbea99a3f7da by Terry Jan Reedy 
(Cheryl Sabella) in branch 'master':
bpo-1612262: IDLE: Class Browser shows nested functions, classes (#2573)
https://github.com/python/cpython/commit/058de11360ea6816a6e978c7be0bcbea99a3f7da


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[issue31534] python 3.6.2 installation failed 0x80070002 error

[Steve Dower]

Steve Dower added the comment:

If you look in your %TEMP% directory, there should be more log files near the 
one you attached (most of the filename will be the same).

Could you zip them up and attach all of them here? I think there's some 
information in one of the others that I'll need to see.

Also, you shouldn't have to run this installer as admin. If you are not 
planning to install for all users on your machine (and according to the log, 
you did not select that option), then it may work better if you just run it as 
yourself.

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Re: [Tutor] beginning to code

[Bill]

Marko Rauhamaa wrote:

Bill :


I figure that, internally, an address, a pointer, is being passed by
value to implement pass by reference. Why do you say "they are right"
above? Are you saying it's not pass by reference?


Thank you for your examples.  I studied them carefully (and I'm not 
through with them yet).
I just wanted to mention that my comment was made in the context that 
Python is implemented by an interpreter written in C.   I realize that 
this may not always be the case.  However, I haven't heard anyone 
mention a Python interpreter written in Python yet.

"Pass by reference" could be "pass by reference to object" (Python,
Java, JavaScript, Lisp) or "pass by reference to memory slot" (available
to Pascal and C++).

Memory slots (or lvalues, as they are known in C) are not first class
objects in Python, which makes "pass by reference to memory slot" a bit
tricky in Python. Python *could* add memory slots to its sanctioned
collection of object types, and it *could* add special syntax to express
a memory slot reference and dereference ("&" and "*" in C).

However, Python doesn't need any language changes to implement memory
slots. A memory slot could be defined as any object that implements
"get()" and "set(value)" methods:

C: 

Python:
   class Xref:
   def get(self): return x
   def set(self, value): nonlocal x; x = value
   ref_x = Xref()


C: >y[3]

Python:
   class XY3ref:
   def get(self): return x.y[3]
   def set(self, value): x.y[3] = value
   ref_xy3 = XY3ref()

The examples could be simplified:

   ref_x = slot_ref(locals(), "x")
   ref_xy3 = slot_ref(x.y, 3)

by defining:

def slot_ref(dict_or_array, key_or_index):
class SlotRef:
def get(self): return dict_or_array[key_or_index]
def set(self, value): dict_or_array[key_or_index] = value
return SlotRef()


Marko


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[issue31092] multiprocessing.Manager() race condition

[Lang]

Lang added the comment:

# code reproduce bug
# KeyError in lib\multiprocessing\managers.py in incref 

import multiprocessing as mp
from time import sleep

def func(queue):
pass

if __name__ == '__main__':
manager = mp.Manager()

pool = mp.Pool(1)

queue = manager.Queue()
r = pool.apply_async(func, args = [queue])
#sleep(1)
queue = None

pool.close()
pool.join()

--
nosy: +tlxxzj

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Re: [Tutor] beginning to code

[Steve D'Aprano]

On Fri, 22 Sep 2017 07:53 pm, Bill wrote:

> Marko Rauhamaa wrote:
>> Bill :
>>
>>> I figure that, internally, an address, a pointer, is being passed by
>>> value to implement pass by reference. Why do you say "they are right"
>>> above? Are you saying it's not pass by reference?
> 
> Thank you for your examples.  I studied them carefully (and I'm not
> through with them yet).
> I just wanted to mention that my comment was made in the context that
> Python is implemented by an interpreter written in C.   I realize that
> this may not always be the case.  However, I haven't heard anyone
> mention a Python interpreter written in Python yet.

I don't see what the implementation language has to do with anything (except
perhaps performance). You can implement any language in any other language,
with greater or less difficulty, but its still possible.

CPython is written in C, as is Stackless.

Jython is written in Java.

IronPython is written in C# for .Net.

PyPy is written in a custom version of Python, RPython.

Nuitka is written in C++.

CLPython is written in Lisp.

Berp and Hope are written in Haskell.

Skulpt is written in Javascript.

Vyper was a really old implementation written in OCaml, apparently now lost and
no longer visible on the internet.

Some of these may been no longer supported, but the Big Four python
implementations are CPython, Jython, IronPython and PyPy.



-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.

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Re: Old Man Yells At Cloud

[Paul Rubin]

Steve D'Aprano  writes:
> Having to spend a few hours being paid to migrate code using "print x"
> to "print(x)", or even a few months, is not a life-changing experience.

Didn't someone further up the thread mention some company that had spent
1.5 years porting a py2 codebase to py3?

The issue of breaking the print statement isn't the difficulty of
converting old programs, or that the print statement is superior to the
print function or vice versa.  Reasonable people might believe that one
is slightly better than the other, but it would be hard to argue that
one is overwhelmingly better than the other.  So there's not a
convincing reason to change.

That calls the whole py3 movement into question, since its advocates so
vigorously defend unnecessary changes.  It's fine to fix broken stuff
(Unicode was broken, indexes escaping list comprehensions was broken)
but fixing highly visible stuff that wasn't broken makes the more subtle
changes easier to ignore.

Py3 imo would have been more successful if it introduced even more
breakage, but produced dramatic benefits (for example a 10x speedup) as
a result.  That would have been doable.  Instead we got minor benefits
and useless breakage.  Py4 as a result of learning the wrong lesson
won't break anything, so it won't be able to introduce dramatic benefits
either.  Will the 5th time (Py5) be the charm?  (I don't mean Pycharm).

Python is gaining ground in numerics and data science, which is great.
Micropython for embedded MCUs is also poised for popularity.  I don't
know how Python is doing at general systems stuff which is what I mostly
use it for.  I think Ruby is losing ground, and some of the ground it
has lost has been to Elixir.  An Elixir-like reimplementation of Python
might be an interesting avenue to pursue.  So would a dialect with less
pervasive dynamism than Python, but that could be compiled to fast
machine code with traditional Lisp techniques.  The dynamism would still
be available "opt-in" so you could turn it on when you wanted it, and
only those parts of your program would slow down.
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Re: [Tutor] beginning to code

[Marko Rauhamaa]

Bill :

> I figure that, internally, an address, a pointer, is being passed by
> value to implement pass by reference. Why do you say "they are right"
> above? Are you saying it's not pass by reference?

"Pass by reference" could be "pass by reference to object" (Python,
Java, JavaScript, Lisp) or "pass by reference to memory slot" (available
to Pascal and C++).

Memory slots (or lvalues, as they are known in C) are not first class
objects in Python, which makes "pass by reference to memory slot" a bit
tricky in Python. Python *could* add memory slots to its sanctioned
collection of object types, and it *could* add special syntax to express
a memory slot reference and dereference ("&" and "*" in C).

However, Python doesn't need any language changes to implement memory
slots. A memory slot could be defined as any object that implements
"get()" and "set(value)" methods:

   C: 

   Python:
  class Xref:
  def get(self): return x
  def set(self, value): nonlocal x; x = value
  ref_x = Xref()


   C: >y[3]

   Python:
  class XY3ref:
  def get(self): return x.y[3]
  def set(self, value): x.y[3] = value
  ref_xy3 = XY3ref()

The examples could be simplified:

  ref_x = slot_ref(locals(), "x")
  ref_xy3 = slot_ref(x.y, 3)

by defining:

   def slot_ref(dict_or_array, key_or_index):
   class SlotRef:
   def get(self): return dict_or_array[key_or_index]
   def set(self, value): dict_or_array[key_or_index] = value
   return SlotRef()


Marko
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Re: How to ingore "AttributeError: exception

[Paul Moore]

On 22 September 2017 at 10:05, Thomas Jollans  wrote:
> On 2017-09-22 10:55, Ganesh Pal wrote:
>> I have two possible values for Z_block in the block code i.e
>> disk_object.data.data.di_data[0] or block.data.data.di_data.data[0][0]
>>
>>
>> def get_block():
>> ''' Get Z block ''
>>
>> if block.data.data.di_data.data[0][0] is not None:
>> Z_block = block.data.data.di_data.data[0][0]
>>
>> else:
>>  Z_block = disk_object.data.data.di_data[0]
>>
>> if not Z_block:
>> return False
>>
>> return Z_block
>>
>>
>>
>>
>> I have a problem with if and else  satement i.e  if  IF codition fails  the
>> code would exit with  "AttributeError: 'list' object has no attribute
>> 'data' " error
>>
>> Any suggestion on how this can be handled better , Will ignoring the
>> exceptions in try -except with pass be good or are there easier ways ) ,
>>
>>
>> try:
>> Z_block = block.data.data.di_data.data[0][0]except AttributeError as e:
>>
>> pass
>
>
>
> try:
> return self.some.attribute.or.another
> except AttributeError:
> return DEFAULT_VALUE
>
> is a perfectly good pattern to use.

getattr(obj, 'attr_name', DEFAULT_VALUE) may also be useful, although
if more than one of the attribute lookups in the chain you show could
fail, then catching the exception is probably simpler.

Paul
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[issue31551] test_distutils fails if current directory contains spaces

[Serhiy Storchaka]

New submission from Serhiy Storchaka:

Running tests from the directory containing a space: '/home/serhiy/py/cpy thon'.

$ ./python -m test -vuall test_distutils
== CPython 3.7.0a1+ (heads/master:5e02c7826f, Sep 22 2017, 13:23:33) [GCC 6.3.0 
20170406]
== Linux-4.10.0-35-generic-x86_64-with-debian-stretch-sid little-endian
== cwd: /home/serhiy/py/cpy thon/build/test_python_11135
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 load avg: 0.09 [1/1] test_distutils
test_no_optimize_flag (distutils.tests.test_bdist_rpm.BuildRpmTestCase) ... 
/var/tmp/rpm-tmp.ufRiE4: 28: /var/tmp/rpm-tmp.ufRiE4: /home/serhiy/py/cpy: not 
found
error: Bad exit status from /var/tmp/rpm-tmp.ufRiE4 (%build)
Bad exit status from /var/tmp/rpm-tmp.ufRiE4 (%build)
ERROR
test_quiet (distutils.tests.test_bdist_rpm.BuildRpmTestCase) ... 
/var/tmp/rpm-tmp.uBva85: 28: /var/tmp/rpm-tmp.uBva85: /home/serhiy/py/cpy: not 
found
error: Bad exit status from /var/tmp/rpm-tmp.uBva85 (%build)
Bad exit status from /var/tmp/rpm-tmp.uBva85 (%build)
ERROR
...
==
ERROR: test_no_optimize_flag (distutils.tests.test_bdist_rpm.BuildRpmTestCase)
--
Traceback (most recent call last):
  File "/home/serhiy/py/cpy thon/Lib/distutils/tests/test_bdist_rpm.py", line 
120, in test_no_optimize_flag
cmd.run()
  File "/home/serhiy/py/cpy thon/Lib/distutils/command/bdist_rpm.py", line 366, 
in run
self.spawn(rpm_cmd)
  File "/home/serhiy/py/cpy thon/Lib/distutils/cmd.py", line 365, in spawn
spawn(cmd, search_path, dry_run=self.dry_run)
  File "/home/serhiy/py/cpy thon/Lib/distutils/spawn.py", line 36, in spawn
_spawn_posix(cmd, search_path, dry_run=dry_run)
  File "/home/serhiy/py/cpy thon/Lib/distutils/spawn.py", line 159, in 
_spawn_posix
% (cmd, exit_status))
distutils.errors.DistutilsExecError: command 'rpmbuild' failed with exit status 
1

==
ERROR: test_quiet (distutils.tests.test_bdist_rpm.BuildRpmTestCase)
--
Traceback (most recent call last):
  File "/home/serhiy/py/cpy thon/Lib/distutils/tests/test_bdist_rpm.py", line 
77, in test_quiet
cmd.run()
  File "/home/serhiy/py/cpy thon/Lib/distutils/command/bdist_rpm.py", line 366, 
in run
self.spawn(rpm_cmd)
  File "/home/serhiy/py/cpy thon/Lib/distutils/cmd.py", line 365, in spawn
spawn(cmd, search_path, dry_run=self.dry_run)
  File "/home/serhiy/py/cpy thon/Lib/distutils/spawn.py", line 36, in spawn
_spawn_posix(cmd, search_path, dry_run=dry_run)
  File "/home/serhiy/py/cpy thon/Lib/distutils/spawn.py", line 159, in 
_spawn_posix
% (cmd, exit_status))
distutils.errors.DistutilsExecError: command 'rpmbuild' failed with exit status 
1

--

See also issue31548.

--
components: Distutils, Tests
messages: 302741
nosy: dstufft, merwok, serhiy.storchaka
priority: normal
severity: normal
status: open
title: test_distutils fails if current directory contains spaces
type: behavior
versions: Python 2.7, Python 3.6, Python 3.7

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Re: How to ingore "AttributeError: exception

[Thomas Jollans]

On 2017-09-22 10:55, Ganesh Pal wrote:
> I have two possible values for Z_block in the block code i.e
> disk_object.data.data.di_data[0] or block.data.data.di_data.data[0][0]
> 
> 
> def get_block():
> ''' Get Z block ''
> 
> if block.data.data.di_data.data[0][0] is not None:
> Z_block = block.data.data.di_data.data[0][0]
> 
> else:
>  Z_block = disk_object.data.data.di_data[0]
> 
> if not Z_block:
> return False
> 
> return Z_block
> 
> 
> 
> 
> I have a problem with if and else  satement i.e  if  IF codition fails  the
> code would exit with  "AttributeError: 'list' object has no attribute
> 'data' " error
> 
> Any suggestion on how this can be handled better , Will ignoring the
> exceptions in try -except with pass be good or are there easier ways ) ,
> 
> 
> try:
> Z_block = block.data.data.di_data.data[0][0]except AttributeError as e:
> 
> pass



try:
return self.some.attribute.or.another
except AttributeError:
return DEFAULT_VALUE

is a perfectly good pattern to use.


> 
> 
> I am a Linux user on Python 2.7

Have you considered moving to Python 3?

> 
> 
> Regards,
> Ganesh
> 


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Re: Assertions

[alister via Python-list]

On Fri, 22 Sep 2017 03:44:59 +1000, Steve D'Aprano wrote:

> On Fri, 22 Sep 2017 02:29 am, Tobiah wrote:
> 
>> Are these completely equivalent?
>> 
>> def foo(thing):
>> 
>> assert(thing > 0), "Thing must be greater than zero"
>> 
>> 
>> def foo(thing):
>> 
>> if not (thing > 0): raise AssertionError("Thing must be greater
>> than zero")
>> 
>> 
>> Other than the fact that the assertion can be turned off with -O?
> 
> As I replied to Ned just now, the two may compile to slightly different
> byte-code. But (apart from the -O optimization) they ought to be
> semantically the same, as far as the interpreter goes. I'd be very
> surprised if the byte-code differences were ever more than trivial.
> 
> But I don't think they are the same as far as the human reader goes. If
> you believe the truism that code is written for other people, and only
> incidentally to be run by computers, then I think we should say that the
> two above are very different.
> 
> The first is an actual assertion. It has to be an assertion, because it
> uses the assert keyword!
> 
> The second is testing a condition, and if need be, raising an exception.
> Which looks like an inappropriate exception. Why AssertionError? It
> looks like it ought to be a ValueError.
> 
> Assertions aren't merely a synonym for testing a condition and raising
> an exception if the condition fails. Assertions have semantics to the
> human reader. I wrote a blog post explaining why you should use
> assertions:
> 
> - you want a checked comment;
> - you have code that can't possibly fail, unless it does;
> - you are testing the program logic;
> - you are checking a contract;
> 
> and when you should not. By using `assert`, you are signalling your
> intent to do one of the above.
> 
> http://import-that.dreamwidth.org/676.html
> 
> But by using an ordinary `if ... raise AssertionError`, the intention is
> ambiguous. If you meant for it to be an assertion, why not use assert?
> If it is not an assertion, then AssertionError is probably the wrong
> exception.
> 
> (It is okay in a testing framework, like unittest, but a bad idea in a
> regular library or an application. Your end-users should never see an
> AssertionError.)
> 
> 
> The bottom line is, if I saw
> 
> if not (thing > 0): raise AssertionError(...)
> 
> in a code review, I'd probably insist that either it be changed to use
> `assert`,
> or the exception be changed to ValueError, whichever better expresses
> the intention of the code.

In a code review I would want the condition changed to be less noisy/
confusing to the reader.

if thing <=0: whatever




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How to ingore "AttributeError: exception

[Ganesh Pal]

I have two possible values for Z_block in the block code i.e
disk_object.data.data.di_data[0] or block.data.data.di_data.data[0][0]


def get_block():
''' Get Z block ''

if block.data.data.di_data.data[0][0] is not None:
Z_block = block.data.data.di_data.data[0][0]

else:
 Z_block = disk_object.data.data.di_data[0]

if not Z_block:
return False

return Z_block




I have a problem with if and else  satement i.e  if  IF codition fails  the
code would exit with  "AttributeError: 'list' object has no attribute
'data' " error

Any suggestion on how this can be handled better , Will ignoring the
exceptions in try -except with pass be good or are there easier ways ) ,


try:
Z_block = block.data.data.di_data.data[0][0]except AttributeError as e:

pass


I am a Linux user on Python 2.7


Regards,
Ganesh
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[issue31423] Error while building PDF documentation

[linkid]

Changes by linkid :


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pull_requests: +3680
stage:  -> patch review

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Re: [Tutor] beginning to code

[bartc]

On 22/09/2017 10:23, Marko Rauhamaa wrote:

Bill :


I figure that, internally, an address, a pointer, is being passed by
value to implement pass by reference. Why do you say "they are right"
above? Are you saying it's not pass by reference?


"Pass by reference" could be "pass by reference to object" (Python,
Java, JavaScript, Lisp) or "pass by reference to memory slot" (available
to Pascal and C++).

Memory slots (or lvalues, as they are known in C) are not first class
objects in Python, which makes "pass by reference to memory slot" a bit
tricky in Python. Python *could* add memory slots to its sanctioned
collection of object types, and it *could* add special syntax to express
a memory slot reference and dereference ("&" and "*" in C).

However, Python doesn't need any language changes to implement memory
slots. A memory slot could be defined as any object that implements
"get()" and "set(value)" methods:


I didn't understand your examples.

Can Python be used to write, say, a swap() function that works with any 
argument types (not just classes or lists)? Example:


def swap(,):# made up syntax
a, b = b, a

x=10
y="Z"
swap(x,y)
print (x,y) # "Z" and "10"

If not, then it doesn't have reference passing as it is normally understood.

A simpler example:

def set(,value):
   a = value

set(x,100)

Here the type of x is irrelevant anyway. It doesn't even need to be 
initialised to anything first (that might violate something in Python, I 
don't know what).


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Re: [Tutor] beginning to code

[Steve D'Aprano]

On Fri, 22 Sep 2017 08:48 pm, bartc wrote:

> Can Python be used to write, say, a swap() function that works with any
> argument types (not just classes or lists)? Example:
> 
>  def swap(,):# made up syntax
>  a, b = b, a
> 
>  x=10
>  y="Z"
>  swap(x,y)
>  print (x,y) # "Z" and "10"
> 
> If not, then it doesn't have reference passing as it is normally understood.

No it cannot, and does not. You can emulate it by adding a layer of redirection,
but that's it.



-- 
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“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.

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Re: [Tutor] beginning to code

[Bill]

Bill wrote:

Stefan Ram wrote:

Bill  writes:

Stefan Ram wrote:

bartc  writes:

On 20/09/2017 02:31, Bill wrote:
it's implementation, I would say that C++ has it all over Python 
from
the point of view of "intuitiveness".  It's much easier to tell 
what's

going on, at a glance, in a C++ program.

You're being serious, aren't you?

For one example, this is a part of a C++ program:
template< typename C >C T( void ( C::* )() );
. It defines a template T, that can be used in a
class as follows:
struct example { void f(); typedef decltype( T(  )) S; };
. The type »S« now has a certain property, that can
be useful sometimes. What is this property (asking Bill)?As

has already been pointed out, one can write "obfuscating code" in any
language, with little effort.  I strive to write code which is easily
understandable--and I document it. I don't wish to debate whether I
could make more of a mess in Python, or not.

   From the point of view of a C++ programmer, the above
   is not obfuscated, but it is readable and simple C++.
   It is of course not readable for readers who do not know
   C++. Just as Python's »string[::-1]« appears "obfuscated"
   to readers who don't know Python.

   It was the answer to the question "How can I express the
   class I'm in in, when I can't write that classes name
   literally?


I would try to use virtual cast in place of the *&%, I mean code, you 
wrote.


Sorry, I mean dynamic_cast().

"Clever code" is a losing game--just look at your explanation below.  
Simple==Good.




So, »S« is »example«.

   It works like this: The type of »« is »void ( example::*
   )()«. So, the function-declaration template »T« infers »C«
   to be »example«, and the type of »T(  )« is »example«,
   which then is transferred to the name »S« using typedef.

   This is obvious for C++ programmers, but it takes a lot
   of time to become a C++ programmer, maybe more than it
   takes to become a Python programmer.





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[issue31423] Error while building PDF documentation

[linkid]

Changes by linkid :


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Re: [Tutor] beginning to code

[Bill]

Steve D'Aprano wrote:

On Fri, 22 Sep 2017 02:57 pm, Bill wrote:


I find Python to be more more
like Java, with regard to "passing objects by reference".

Which is not a surprise, since both Python and Java use the same value passing
style: pass by object reference, or pass by sharing if you prefer.

Java people don't call it that. They call it pass by value, and categorically
deny that it is pass by reference. (They're right about the second point.)


I figure that, internally, an address, a pointer, is being passed by 
value to implement pass by reference.  Why do you say "they are right" 
above? Are you saying it's not pass by reference?


What really screws the terminology up is that it's not parameters that 
are being passed, it's arguments! %-)





It also the exact same passing style as used by Swift, except Swift calls it
pass by reference, and denies it is pass by value. (They're right about the
second point.)





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Re: Convert pandas series to string and datetime object

[Pavol Lisy]

On 9/21/17, Peter Otten <__pete...@web.de> wrote:
> zljubi...@gmail.com wrote:
>
>> I have sliced the pandas dataframe
>>
>> end_date = df[-1:]['end']
>>
>> type(end_date)
>> Out[4]: pandas.core.series.Series
>>
>> end_date
>> Out[3]:
>> 48173   2017-09-20 04:47:59
>> Name: end, dtype: datetime64[ns]
>>
>> 1.   How to get rid of index value 48173 and get only "2017-09-20
> 04:47:59"
>> string? I have to call REST API with "2017-09-20 04:47:59" as a
>> parameter,
>> so I have to get string from pandas datetime64 series.
>> 2.   How to get rid of index value 48173 and get only datetime object
>> [something like datetime.datetime.strptime('2017-09-20 04:47:59',
>> '%Y-%m-%d %H:%M:%S')]. Later I will have to check if '2017-09-20
>> 04:47:59'
>> < datetime.datetime(2017,1,9)
>>
>> How to do these conversions?
>
> After a web search and some trial and error:
>
 d = pd.DataFrame([[1, datetime.datetime.now()], [2,
> datetime.datetime.now()]], columns=["whatever", "end"])
 d
>whateverend
> 0 1 2017-09-21 22:36:52.342757
> 1 2 2017-09-21 22:36:52.349973
>
> [2 rows x 2 columns]
 d["end"].astype(datetime.datetime).values[-1]
> datetime.datetime(2017, 9, 21, 22, 36, 52, 349973)

Or:

>>> d.iloc[-1]['end'].strftime('%Y-%m-%d %H:%M:%S')
'2017-09-22 09:03:40'

PS.
pandas is one of reasons why python is so popular these days. But
"there is only milion way how to do it" (and other unpythonic issues)
I see there every time I am looking at it. :)
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Re: [Tutor] beginning to code

[Bill]

Stefan Ram wrote:

Bill  writes:

Stefan Ram wrote:

bartc  writes:

On 20/09/2017 02:31, Bill wrote:

it's implementation, I would say that C++ has it all over Python from
the point of view of "intuitiveness".  It's much easier to tell what's
going on, at a glance, in a C++ program.

You're being serious, aren't you?

For one example, this is a part of a C++ program:
template< typename C >C T( void ( C::* )() );
. It defines a template T, that can be used in a
class as follows:
struct example { void f(); typedef decltype( T(  )) S; };
. The type »S« now has a certain property, that can
be useful sometimes. What is this property (asking Bill)?As

has already been pointed out, one can write "obfuscating code" in any
language, with little effort.  I strive to write code which is easily
understandable--and I document it. I don't wish to debate whether I
could make more of a mess in Python, or not.

   From the point of view of a C++ programmer, the above
   is not obfuscated, but it is readable and simple C++.
   It is of course not readable for readers who do not know
   C++. Just as Python's »string[::-1]« appears "obfuscated"
   to readers who don't know Python.

   It was the answer to the question "How can I express the
   class I'm in in, when I can't write that classes name
   literally?


I would try to use virtual cast in place of the *&%, I mean code, you 
wrote.  "Clever code" is a losing game--just look at your explanation 
below.  Simple==Good.




So, »S« is »example«.

   It works like this: The type of »« is »void ( example::*
   )()«. So, the function-declaration template »T« infers »C«
   to be »example«, and the type of »T(  )« is »example«,
   which then is transferred to the name »S« using typedef.

   This is obvious for C++ programmers, but it takes a lot
   of time to become a C++ programmer, maybe more than it
   takes to become a Python programmer.



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[issue31478] assertion failure in random.seed() in case the seed argument has a bad __abs__() method

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

The more I think about this the more I like the idea of using int.__abs__() 
directly (PyLong_Type.tp_as_number->nb_absolute() in C). The C code doesn't 
call potentially overridable methods bit_length() and to_bytes(), it uses 
concrete implementations directly. I don't see reasons why it should obey 
overriding the __abs__() method.

This will save us from the problem with the wording of the error message.

I mentioned a drawback, but the current implementation has the same drawback. 
We can avoid copying positive integer subtypes by using more complex code, but 
I think it isn't worth.

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Re: [Tutor] beginning to code

[Steve D'Aprano]

On Fri, 22 Sep 2017 02:57 pm, Bill wrote:

> I find Python to be more more
> like Java, with regard to "passing objects by reference".

Which is not a surprise, since both Python and Java use the same value passing
style: pass by object reference, or pass by sharing if you prefer.

Java people don't call it that. They call it pass by value, and categorically
deny that it is pass by reference. (They're right about the second point.)

It also the exact same passing style as used by Swift, except Swift calls it
pass by reference, and denies it is pass by value. (They're right about the
second point.)



-- 
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enough, things got worse.

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[issue30844] selectors: Add urgent data to read event

[Pim Klanke]

Pim Klanke added the comment:

In the selectors module, the winsock select method is wrapped. The second 
parameter (writefds) is passed to both writefds and exceptfds and the returned 
number of fds for write and except are summed and returned in the second 
element of the tuple.

Can anyone explain to me why the winsock select method is wrapped like this?

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[issue31459] IDLE: Rename Class Browser as Module Browser

[Cheryl Sabella]

Changes by Cheryl Sabella :


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pull_requests: +3689
stage: test needed -> patch review

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Re: How to share class relationship representations?

[Stephan Houben]

Op 2017-09-22, Pavol Lisy schreef :
> On 9/19/17, leam hall  wrote:
>> I'm working on designing the classes, sub-classes, and relationships in my
>> code. What is a good visual way to represent it so it can be stored in git
>> and shared on the list without large images or attachments?
>>
>> Thanks!
>>
>> Leam
>
> https://stackoverflow.com/questions/29586520/can-one-get-hierarchical-graphs-from-networkx-with-python-3#29597209

For direct inclusion in source code, what about plain text with 
Unicode box drawing characters?

┏━━┓
┃object┃
┗━━━┳━━┛
┃
 ┏━━┻━━┓
 ┃float┃
 ┗━┛

Stephan
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Re: Fw: Problems Installing Python36

[Irmen de Jong]

On 09/22/2017 08:34 PM, Stephan Houben wrote:

> I was vaguely tempted to offer the Mingw-w64 (GCC) Python as an
> alternative, since it doesn't rely on any optionally-installed Microsoft
> DLLs and so avoids this issue. But I suppose that is not really the
> newbie-friendly solution the OP was looking for...

Mingw? Perhaps better to choose Anaconda/Miniconda instead if you go for
an alternative implementation...

Irmen
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[issue31558] gc.freeze() - an API to mark objects as uncollectable

[Łukasz Langa]

New submission from Łukasz Langa:

When you're forking many worker processes off of a parent process, the 
resulting children are initially very cheap in memory.  They share memory pages 
with the base process until a write happens [1]_.

Sadly, the garbage collector in Python touches every object's PyGC_Head during 
a collection, even if that object stays alive, undoing all the copy-on-write 
wins.  Instagram disabled the GC completely for this reason [2]_.  This fixed 
the COW issue but made the processes more vulnerable to memory growth due to 
new cycles being silently introduced when the application code is changed by 
developers.  While we could fix the most glaring cases, it was hard to keep the 
memory usage at bay.  We came up with a different solution that fixes both 
issues.  It requires a new API to be added to CPython's garbage collector.


gc.freeze()
---

As soon as possible in the lifecycle of the parent process we disable the 
garbage collector.  Then we call a new API called `gc.freeze()` to move all 
currently tracked objects to a permanent generation.  They won't be considered 
in further collections.  This is okay since we are assuming that (almost?) all 
of the objects created until that point are module-level and thus useful for 
the entire lifecycle of the child process.

After calling `gc.freeze()` we call fork. Then, the child process is free to 
re-enable the garbage collector.

Why do we need to disable the collector on the parent process as soon as 
possible?  When the GC cleans up memory in the mean time, it leaves space in 
pages for new objects.  Those pages become shared after fork and as soon as the 
child process starts creating its own objects, they will likely be written to 
the shared pages, initiating a lot of copy-on-write activity.

In other words, we're wasting a bit of memory in the shared pages to save a lot 
of memory later (that would otherwise be wasted on copying entire pages after 
forking).


Other attempts
--

We also tried moving the GC head to another place in memory.  This creates some 
indirection but cache locality on that segment is great so performance isn't 
really hurt.  However, this change introduces two new pointers (16 bytes) per 
object.  This doesn't sound like a lot but given millions of objects and tens 
of processes per box, this alone can cost hundreds of megabytes per host.  
Memory that we wanted to save in the first place.  So that idea was scrapped.


Attribution
---

The original patch is by Zekun Li, with help from Jiahao Li, Matt Page, David 
Callahan, Carl S. Shapiro, and Chenyang Wu.


.. [1] https://en.wikipedia.org/wiki/Copy-on-write
.. [2] 
https://engineering.instagram.com/dismissing-python-garbage-collection-at-instagram-4dca40b29172

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priority: normal
severity: normal
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title: gc.freeze() - an API to mark objects as uncollectable
type: resource usage
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Re: [Tutor] beginning to code

[Bill]

Mark Lawrence wrote:

On 22/09/2017 08:01, Bill wrote:

Steve D'Aprano wrote:

On Fri, 22 Sep 2017 02:57 pm, Bill wrote:


I find Python to be more more
like Java, with regard to "passing objects by reference".
Which is not a surprise, since both Python and Java use the same 
value passing

style: pass by object reference, or pass by sharing if you prefer.

Java people don't call it that. They call it pass by value, and 
categorically
deny that it is pass by reference. (They're right about the second 
point.)


I figure that, internally, an address, a pointer, is being passed by 
value to implement pass by reference.  Why do you say "they are 
right" above? Are you saying it's not pass by reference?




Please see 
http://jeffknupp.com/blog/2012/11/13/is-python-callbyvalue-or-callbyreference-neither/ 
and http://effbot.org/zone/call-by-object.htm





I would would agree with the description provided for the C++ example 
provided


string some_guy = "fred";
 is replaced by
char* some_guy="fred";

To see that this is correct, note the some_guy may subsequently be 
assigned to a character string much longer then "fred".  An additional 
note: A character string literal, like "cat", never occurs more than 
once in compiled C++ program unit.  This also shows that the provided 
description can't be completely correct. One last thing,


string some_guy = "fred"

is really the same thing as

string some_guy("fred");

and both equivalently call the string constructor.

The data type of "fred" is const char*, not (class) string.
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Re: [Tutor] beginning to code

[Bill]

Bill wrote:

Mark Lawrence wrote:

On 22/09/2017 08:01, Bill wrote:

Steve D'Aprano wrote:

On Fri, 22 Sep 2017 02:57 pm, Bill wrote:


I find Python to be more more
like Java, with regard to "passing objects by reference".
Which is not a surprise, since both Python and Java use the same 
value passing

style: pass by object reference, or pass by sharing if you prefer.

Java people don't call it that. They call it pass by value, and 
categorically
deny that it is pass by reference. (They're right about the second 
point.)


I figure that, internally, an address, a pointer, is being passed by 
value to implement pass by reference.  Why do you say "they are 
right" above? Are you saying it's not pass by reference?




Please see 
http://jeffknupp.com/blog/2012/11/13/is-python-callbyvalue-or-callbyreference-neither/ 
and http://effbot.org/zone/call-by-object.htm





I would would agree with the description provided for the C++ example 
provided


string some_guy = "fred";
 is replaced by
char* some_guy="fred";


On second thought, so that the description is correct (matches the 
semantics), replace it by


char some_guy[10]="fred";

But then you need to use std::strcpy to reassign some_guy
to "george".








To see that this is correct, note the some_guy may subsequently be 
assigned to a character string much longer then "fred".  An additional 
note: A character string literal, like "cat", never occurs more than 
once in compiled C++ program unit.  This also shows that the provided 
description can't be completely correct. One last thing,


string some_guy = "fred"

is really the same thing as

string some_guy("fred");

and both equivalently call the string constructor.

The data type of "fred" is const char*, not (class) string.


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[issue31553] Extend json.tool to handle jsonlines (with a flag)

[Raymond Hettinger]

Raymond Hettinger added the comment:

This seems like a reasonable enhancement.

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Re: Python Boolean Logic

[Steve D'Aprano]

On Sat, 23 Sep 2017 03:01 pm, Bill wrote:

> s='(20 - 10)  > 15'
> b=(20 - 10)  > 15
> print(s, " is ", ("true" if b else "false") );  ## inside parentheses
> may be removed.
> 
> I am new to Python.  Maybe someone here is familiar with an elegant way
> to get the the value of b directly from the string s?  Hmm... It appears
> that eval() would work


Indeed it will, but don't get into the habit of using eval willy-nilly. While it
is absolutely fine to use it with data you provide yourself, it is a HUGE
security risk to eval strings that came from an untrusted user.


eval("__import__('os').system('echo """rm-rf /"""')")


Also, for what its worth, it's about ten times slower to run:

eval('(20 - 10)  > 15')

than to simply run 

(20 - 10)  > 15



-- 
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“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.

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[issue31423] Error while building PDF documentation

[Ned Deily]

Ned Deily added the comment:

Good news! It appears that the changes supplied by @linkid have indeed fixed 
the daily downloadable doc build failures, 3.7, 3.6, and 2.7 docs were built 
successfully and the 404s are gone.  Thanks everyone for helping with this!

--
stage: commit review -> resolved
status: open -> closed

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[issue31461] IDLE: Enhance module browser

[Terry J. Reedy]

Changes by Terry J. Reedy :


--
title: IDLE: Enhance class browser -> IDLE: Enhance module browser

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Re: Python Boolean Logic

[Bill]

Cai Gengyang wrote:

Hey guys, I'm testing this on CodeAcademy, but I cant get the program to output 
a result even after pressing the run button. Just wanted to check if my logic 
is correct. Thanks alot
Your answers appear correct, but you could write Python statements to 
test them (or any you encounter in the future). For instance,


if (20 - 10)  > 15 :
print("true")
else:
print("false");

Or,

s='(20 - 10)  > 15'
b=(20 - 10)  > 15
print(s, " is ", ("true" if b else "false") );  ## inside parentheses 
may be removed.


I am new to Python.  Maybe someone here is familiar with an elegant way 
to get the the value of b directly from the string s?  Hmm... It appears 
that eval() would work (see "Python: Essential Reference", p. 115).  I 
just read about that for the first time last night! I may try that, for 
practice,  after I post this.


Bill



# Assign True or False as appropriate on the lines below!

# (20 - 10) > 15
bool_one = False# We did this one for you!

# (10 + 17) == 3**16
# Remember that ** can be read as 'to the power of'. 3**16 is about 43 million.
bool_two = False

# 1**2 <= -1
bool_three = False

# 40 * 4 >= -4
bool_four = True

# 100 != 10**2
bool_five = False


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Re: Python Boolean Logic

[Rick Johnson]

On Friday, September 22, 2017 at 11:46:59 PM UTC-5, Cai Gengyang wrote:
> Input :
> 
> # Assign True or False as appropriate on the lines below!
> 
> # (20 - 10) > 15
> bool_one = False# We did this one for you!
> 
> # (10 + 17) == 3**16
> # Remember that ** can be read as 'to the power of'. 3**16 is about 43 
> million.
> bool_two = False
> 
> # 1**2 <= -1
> bool_three = False
> 
> # 40 * 4 >= -4
> bool_four = True
> 
> # 100 != 10**2
> bool_five = False
> 
> print ("bool_one = ", bool_one) 
> print ("bool_two = ", bool_two) 
> print ("bool_three = ", bool_three) 
> print ("bool_four = ", bool_four) 
> print ("bool_five = ", bool_five) 
> 
> 
> Output :
> 
> ('bool_one = ', False)
> ('bool_two = ', False)
> ('bool_three = ', False)
> ('bool_four = ', True)
> ('bool_five = ', False)
> 
> 
> Is my logic / input / output correct ? Thanks a lot ...

This looks like homework, and not a very well designed
homework either. It seems the instructor wants you to
provide the value of each "rich comparison expression" by
assigning a boolean to an enumerated variable. As far as
your "logic and IO being correct", i don't see a request for
any of those things, based on what you provided here. If you
want to check the expressions, just copy/paste them into a
Python console.

# Python2.x (with dependancy)
>>> from sanity import *
>>> if isbadidea('typehints'):
... print 'Duh!'
... else:
... print 'Duh!'
Duh

Yep. My sanity module could have saved the BDFL a lot of
headaches. And possibly his cushy job at GooglePlex...
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[issue31558] gc.freeze() - an API to mark objects as uncollectable

[Łukasz Langa]

Changes by Łukasz Langa :


--
components: +Interpreter Core

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Re: Python Boolean Logic

[Cai Gengyang]

Input :

# Assign True or False as appropriate on the lines below!

# (20 - 10) > 15
bool_one = False# We did this one for you!

# (10 + 17) == 3**16
# Remember that ** can be read as 'to the power of'. 3**16 is about 43 million.
bool_two = False

# 1**2 <= -1
bool_three = False

# 40 * 4 >= -4
bool_four = True

# 100 != 10**2
bool_five = False

print ("bool_one = ", bool_one) 
print ("bool_two = ", bool_two) 
print ("bool_three = ", bool_three) 
print ("bool_four = ", bool_four) 
print ("bool_five = ", bool_five) 


Output :

('bool_one = ', False)
('bool_two = ', False)
('bool_three = ', False)
('bool_four = ', True)
('bool_five = ', False)


Is my logic / input / output correct ? Thanks a lot ...








On Saturday, September 23, 2017 at 12:40:10 PM UTC+8, steve.ferg...@gmail.com 
wrote:
> You have a lot of assignment statements, but nothing that produces output.  
> Try adding statements like this at appropriate places...
> 
> print ("bool_one = ", bool_one)

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Re: Python Boolean Logic

[Bill]

Bill wrote:

Cai Gengyang wrote:
Hey guys, I'm testing this on CodeAcademy, but I cant get the program 
to output a result even after pressing the run button. Just wanted to 
check if my logic is correct. Thanks alot
Your answers appear correct, but you could write Python statements to 
test them (or any you encounter in the future). For instance,


if (20 - 10)  > 15 :
print("true")
else:
print("false");

Or,

s='(20 - 10)  > 15'
b=(20 - 10)  > 15
print(s, " is ", ("true" if b else "false") );  ## inside parentheses 
may be removed.


I am new to Python.  Maybe someone here is familiar with an elegant 
way to get the the value of b directly from the string s? Hmm... It 
appears that eval() would work (see "Python: Essential Reference", p. 
115).  I just read about that for the first time last night! I may try 
that, for practice,  after I post this.



Update: Yes,
b=(20 - 10)  > 15
may be replaced by eval(s).

We can write:

print(s, " is ", ("true" if eval(s)  else "false") )







# Assign True or False as appropriate on the lines below!

# (20 - 10) > 15
bool_one = False# We did this one for you!

# (10 + 17) == 3**16
# Remember that ** can be read as 'to the power of'. 3**16 is about 
43 million.

bool_two = False

# 1**2 <= -1
bool_three = False

# 40 * 4 >= -4
bool_four = True

# 100 != 10**2
bool_five = False




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[issue31558] gc.freeze() - an API to mark objects as uncollectable

[Roundup Robot]

Changes by Roundup Robot :


--
keywords: +patch
pull_requests: +3690

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Re: Fw: Problems Installing Python36

[Stephan Houben]

Op 2017-09-22, Irmen de Jong schreef :
> On 09/22/2017 08:34 PM, Stephan Houben wrote:
>
>> I was vaguely tempted to offer the Mingw-w64 (GCC) Python as an
>> alternative, since it doesn't rely on any optionally-installed Microsoft
>> DLLs and so avoids this issue. But I suppose that is not really the
>> newbie-friendly solution the OP was looking for...
>
> Mingw? Perhaps better to choose Anaconda/Miniconda instead if you go for
> an alternative implementation...

Anaconda is still based on MSVC and so still relies on the MSVC C library.

If installing the MSVC C linrary is the problem, then I don't think
Anaconda solves that.

Stephan
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[issue31478] assertion failure in random.seed() in case the seed argument has a bad __abs__() method

[Raymond Hettinger]

Raymond Hettinger added the comment:

>  I don't see reasons why it should obey overriding the __abs__() method.

I concur.

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[issue31553] Extend json.tool to handle jsonlines (with a flag)

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Jsonlines is an external to JSON format. You should split the data on lines and 
pass every line to the JSON parser separately. The same you should do with 
json.tool.

$ echo -e '{"ingredients":["frog", "water", "chocolate", 
"glucose"]}\n{"ingredients":["chocolate","steel bolts"]}' | while read -r line; 
do echo -n "$line" | python -m json.tool; done
{
"ingredients": [
"frog",
"water",
"chocolate",
"glucose"
]
}
{
"ingredients": [
"chocolate",
"steel bolts"
]
}

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[issue31425] Expose AF_QIPCRTR in socket module

[Roundup Robot]

Changes by Roundup Robot :


--
keywords: +patch
pull_requests: +3691
stage:  -> patch review

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Python Boolean Logic

[Cai Gengyang]

Hey guys, I'm testing this on CodeAcademy, but I cant get the program to output 
a result even after pressing the run button. Just wanted to check if my logic 
is correct. Thanks alot

# Assign True or False as appropriate on the lines below!

# (20 - 10) > 15
bool_one = False# We did this one for you!

# (10 + 17) == 3**16
# Remember that ** can be read as 'to the power of'. 3**16 is about 43 million.
bool_two = False

# 1**2 <= -1
bool_three = False

# 40 * 4 >= -4
bool_four = True

# 100 != 10**2
bool_five = False
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Re: Python Boolean Logic

[steve . ferg . bitbucket]

You have a lot of assignment statements, but nothing that produces output.  Try 
adding statements like this at appropriate places...

print ("bool_one = ", bool_one)
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[issue31452] asyncio.gather does not cancel tasks if one fails

[Andrew Lytvyn]

Andrew Lytvyn added the comment:

Guido, look. The point is that if you change run_forever with 
run_until_complete, then behavior changes: success_coro(5) will not be executed.

I think that it's strange that behavior differs depending on entrypoint: 
run_forever or run_untill_complete

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Re: [Tutor] beginning to code

[Mark Lawrence via Python-list]

On 22/09/2017 08:01, Bill wrote:

Steve D'Aprano wrote:

On Fri, 22 Sep 2017 02:57 pm, Bill wrote:


I find Python to be more more
like Java, with regard to "passing objects by reference".
Which is not a surprise, since both Python and Java use the same value 
passing

style: pass by object reference, or pass by sharing if you prefer.

Java people don't call it that. They call it pass by value, and 
categorically
deny that it is pass by reference. (They're right about the second 
point.)


I figure that, internally, an address, a pointer, is being passed by 
value to implement pass by reference.  Why do you say "they are right" 
above? Are you saying it's not pass by reference?




Please see 
http://jeffknupp.com/blog/2012/11/13/is-python-callbyvalue-or-callbyreference-neither/ 
and http://effbot.org/zone/call-by-object.htm


--
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence

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Re: xml: TypeError: write() got an unexpected keyword argument 'short_empty_elements'

[Thomas Jollans]

On 2017-09-22 14:29, Nagy László Zsolt wrote:
> 
>> Result:
>>
>> Traceback (most recent call last):
>>   File "C:/not_telling/c14n.py", line 16, in 
>>     short_empty_elements=False
>>   File "lxml.etree.pyx", line 1869, in lxml.etree._ElementTree.write
>> (src\lxml\lxml.etree.c:57004)
>> TypeError: write() got an unexpected keyword argument 'short_empty_elements'
> Well, it looks like etree does not implement the short_empty_elements
> argument in its write method:
> 
> https://github.com/lxml/lxml/blob/master/src/lxml/etree.pyx#L1954
> 
> But it should (see
> https://github.com/lxml/lxml/blob/master/src/lxml/etree.pyx#L5 - The
> ``lxml.etree`` module implements the extended ElementTree API for XML. )
> 
> Can somebody please confirm that this is a bug? Also, how can I send a
> bug report? ( I'm not able to add an issue to lxml, lack of permissions.  )
> 


https://docs.python.org/3/library/xml.etree.elementtree.html#xml.etree.ElementTree.ElementTree.write

The argument was added in Python 3.4. Presumably, lxml implemented the
API before this change.

Maybe this would be considered a bug by lxml. Maybe it won't.

http://lxml.de/ links to a bug tracker and a mailing list.

-- 
Thomas Jollans
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what is happening in panda "where" clause

[Exposito, Pedro (RIS-MDW)]

This code does a "where" clause on a panda data frame...

Code:
import pandas as pd;
col_names = ['Name', 'Age', 'Weight', "Education"];
# create panda dataframe
x = pd.read_csv('test.dat', sep='|', header=None, names = col_names);
# apply "where" condition
z = x[ (x['Age'] == 55) ]
# prints row WHERE age == 55
print (z);

What is happening in this statement:
z = x[ (x['Age'] == 55) ]

Thanks,
Pedro Exposito




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Re: xml: TypeError: write() got an unexpected keyword argument 'short_empty_elements'

[Nagy László Zsolt]

> https://docs.python.org/3/library/xml.etree.elementtree.html#xml.etree.ElementTree.ElementTree.write
>
> The argument was added in Python 3.4. Presumably, lxml implemented the
> API before this change.
>
> Maybe this would be considered a bug by lxml. Maybe it won't.
Maybe it is not a bug, just a feature not implemented. After testing it
a bit, I realized that for my chosen method (c14n) it cannot be
specified anyway, because it must be False and actually it works that way.
>
> http://lxml.de/ links to a bug tracker and a mailing list.
>
All right, I'm subscribing to the list (I have other questions regarding
C14N)
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Re: [Tutor] beginning to code

[Steve D'Aprano]

On Fri, 22 Sep 2017 10:27 pm, Marko Rauhamaa wrote:

> r...@zedat.fu-berlin.de (Stefan Ram):
> 
>> Marko Rauhamaa  writes:
>>>swap(slot_ref(locals(), "x"), slot_ref(locals(), "y"))
>>
>>   You need to be able to write the call as
>>
>> swap( x, y )
> 
> Why?

Because that's the whole point of the exercise.

The exercise isn't to demonstrate how to swap two variables. In Python, that's
easy:

a, b = b, a

The exercise is to demonstrate pass by reference semantics. That requires
demonstrating the same semantics as the Pascal swap procedure:

procedure swap(var a, var b):
  begin
tmp := a;
a := b;
b := tmp;
  end;

(ignoring the type declarations for brevity).

Call by reference semantics enable you to call swap(x, y) to swap x and y in the
caller's scope. You don't call swap('x', 'y', scope_of_x, scope_of_y) or any
other variant. That isn't call by reference semantics.

The whole point of call by reference semantics is that the *compiler*, not the
programmer, tracks the variables and their scopes. The programmer just
says "swap x and y", and the compiler works out how to do it.


-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.

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Re: [Tutor] beginning to code

[Marko Rauhamaa]

bartc :

> On 22/09/2017 10:23, Marko Rauhamaa wrote:
>> However, Python doesn't need any language changes to implement memory
>> slots. A memory slot could be defined as any object that implements
>> "get()" and "set(value)" methods:
>
> I didn't understand your examples.
>
> Can Python be used to write, say, a swap() function that works with any
> argument types (not just classes or lists)? Example:
>
> def swap(,):# made up syntax
> a, b = b, a
>
> x=10
> y="Z"
> swap(x,y)
> print (x,y) # "Z" and "10"

Yes, following my recipe:

   def swap(ref_a, ref_b):
   a, b = ref_a.get(), ref_b.get()
   ref_a.set(b)
   ref_b.set(a)

   x = 10
   y = "Z"
   swap(slot_ref(locals(), "x"), slot_ref(locals(), "y"))
   print(x, y) # "Z" and 10


Marko
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Re: [Tutor] beginning to code

[Steve D'Aprano]

On Fri, 22 Sep 2017 05:01 pm, Bill wrote:

> Steve D'Aprano wrote:
>> On Fri, 22 Sep 2017 02:57 pm, Bill wrote:
>>
>>> I find Python to be more more
>>> like Java, with regard to "passing objects by reference".
>> Which is not a surprise, since both Python and Java use the same value
>> passing style: pass by object reference, or pass by sharing if you prefer.
>>
>> Java people don't call it that. They call it pass by value, and categorically
>> deny that it is pass by reference. (They're right about the second point.)
> 
> I figure that, internally, an address, a pointer, is being passed by
> value to implement pass by reference.  Why do you say "they are right"
> above? Are you saying it's not pass by reference?

Are you suggested that the Java people are wrong to say it isn't?

Last time I suggested that the Java community had made a mistake in this regard,
I got blasted by certain folks here for insulting the intelligence of compiler
experts.

But yes, I agree with the Java community that what they do is NOT pass by
reference. It clearly is not: the canonical test for pass by reference is
whether you can write a "swap" procedure which swaps two variables:

a = 1
b = 2
swap(a, b)
assert a == 2 and b == 1

A general swap procedure cannot be written in Python or Java. (We don't need
one, in Python, because we have alternatives which are arguably better, but
that's not the point.)

So I am in 100% agreement with the Java people when they say what they do (and
what Python does) is not pass by reference.


> What really screws the terminology up is that it's not parameters that
> are being passed, it's arguments! %-)

I don't believe that I mentioned parameters or arguments, so that's a red
herring.

The terminology is fine, if we don't insist on ignoring it. There are many more
kinds of passing semantics than just by value and by reference, and the
confusion only occurs when people insist on taking the round peg of a
language's actual behaviour, and hammering it in by brute-force into the square
hole of "pass by value" or "pass by reference". Quoting Wikipedia:

In some cases, the term "call by value" is problematic, as
the value which is passed is not the value of the variable as 
understood by the ordinary meaning of value, but an 
implementation-specific reference to the value.


Some examples:

- call by sharing
- call by need
- call by name
- call by copy-restore ("copy in, copy out")

etc.


https://en.wikipedia.org/wiki/Evaluation_strategy


-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.

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Re: [Tutor] beginning to code

[bartc]

On 22/09/2017 12:47, Chris Angelico wrote:

On Fri, Sep 22, 2017 at 9:24 PM, Marko Rauhamaa  wrote:



Yes, following my recipe:

def swap(ref_a, ref_b):
a, b = ref_a.get(), ref_b.get()
ref_a.set(b)
ref_b.set(a)

x = 10
y = "Z"
swap(slot_ref(locals(), "x"), slot_ref(locals(), "y"))
print(x, y) # "Z" and 10


Sure, let me just put that into a function. CPython 3.7, although I'm
pretty sure most CPython versions will do the same, as will several of
the other Pythons.

(Side point: Your slot_ref function is rather bizarre. It's a closure
AND a class, just in case one of them isn't sufficient. The following
code is copied and pasted from two of your posts and is unchanged
other than making try_swapping into a function.)


def slot_ref(dict_or_array, key_or_index):

...class SlotRef:
...def get(self): return dict_or_array[key_or_index]
...def set(self, value): dict_or_array[key_or_index] = value
...return SlotRef()
...

def swap(ref_a, ref_b):

...a, b = ref_a.get(), ref_b.get()
...ref_a.set(b)
...ref_b.set(a)
...

def try_swapping():

...x = 10
...y = "Z"
...swap(slot_ref(locals(), "x"), slot_ref(locals(), "y"))
...print("x, y =", x, y)
...

try_swapping()

x, y = 10 Z

Strange... doesn't seem to work. Are you saying that Python is
pass-by-reference outside of a function and pass-by-value inside?
Because that'd be just insane.

Or maybe what you have here isn't a real reference at all.


It also looks too complicated to be really useful. And I'm not sure it 
would work (when it does work), with more complex terms. For simple given:


 A = [10,20,30]
 B = ["X","Y","Z"]

and wanting to swap A[0] and B[2]. Although these being list elements, 
they /could/ be exchanged via a function:


 def swapelems(a,i,b,j):
 a[i],b[j]=b[j],a[i]

 swapelems(A,0,B,2)

Just not using a general-purpose swap function.

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Re: Assertions

[alister via Python-list]

On Fri, 22 Sep 2017 21:15:54 +1000, Steve D'Aprano wrote:

> On Fri, 22 Sep 2017 08:50 pm, alister wrote:
> 
>>> The bottom line is, if I saw
>>> 
>>> if not (thing > 0): raise AssertionError(...)
>>> 
>>> in a code review, I'd probably insist that either it be changed to use
>>> `assert`,
>>> or the exception be changed to ValueError, whichever better expresses
>>> the intention of the code.
>> 
>> In a code review I would want the condition changed to be less noisy/
>> confusing to the reader.
>> 
>> if thing <=0: whatever
> 
> Fair point, assuming they are the same.
> 
> Actually, even for floats they're not the same.
> 
> py> not (NAN > 0)
> True py> NAN <= 0 False

I bet those are conditions the original author did not expect 

actually I am not sure how you got them on my system

python 2.7

>>> not (NAN >0)
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'NAN' is not defined

>>> NAN <=0
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'NAN' is not defined


Python 3

Python 3.6.2 (default, Aug 11 2017, 11:59:59) 
[GCC 7.1.1 20170622 (Red Hat 7.1.1-3)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> not (NAN > 0)
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'NAN' is not defined
>>> NAN <= 0
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'NAN' is not defined


I would also say that if your examples did indeed return different 
results then pythons logic model has a bug.



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Re: [Tutor] beginning to code

[Mark Lawrence via Python-list]

On 22/09/2017 10:53, Bill wrote:
I just wanted to mention that my comment was made in the context that 
Python is implemented by an interpreter written in C.   I realize that 
this may not always be the case.  However, I haven't heard anyone 
mention a Python interpreter written in Python yet.


The reference implementation is written in C but there's also Jython 
(Java) and IronPython (.Net).


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what you can do for our language.

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Re: [Tutor] beginning to code

[Marko Rauhamaa]

r...@zedat.fu-berlin.de (Stefan Ram):

> Marko Rauhamaa  writes:
>>swap(slot_ref(locals(), "x"), slot_ref(locals(), "y"))
>
>   You need to be able to write the call as
>
> swap( x, y )

Why?


Marko
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Re: Assertions

[Thomas Jollans]

On 2017-09-22 14:03, alister via Python-list wrote:
> On Fri, 22 Sep 2017 21:15:54 +1000, Steve D'Aprano wrote:
>> On Fri, 22 Sep 2017 08:50 pm, alister wrote:
>>> [snip]
>>>
>>> In a code review I would want the condition changed to be less noisy/
>>> confusing to the reader.
>>>
>>> if thing <=0: whatever
>>
>> Fair point, assuming they are the same.
>>
>> Actually, even for floats they're not the same.
>>
> [snip]
>
> I would also say that if your examples did indeed return different 
> results then pythons logic model has a bug.


No, it doesn't (in Python 3).
NaN is not a number. It is neither larger nor smaller than zero. This
behavior is correct:

Python 3.6.0 |Continuum Analytics, Inc.| (default, Dec 23 2016, 12:22:00)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from math import nan
>>> nan < 0
False
>>> nan > 0
False
>>> nan == 0
False
>>> not (nan > 0)
True


Python 2.7 is a bit more fickle:

Python 2.7.5 (default, Aug  2 2017, 11:05:32)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-16)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from numpy import nan
>>> nan < 0
False
>>> nan > 0
False
>>> nan == 0
False
>>> not(nan > 0)
True
>>> cmp(nan, 0)
-1
>>> cmp(0, nan)
1


Of course I see why the behavior of NaN is hard to swallow: in a sense
the logical thing to do would be to raise a ValueError (or TypeError)
when comparing to NaN - seeing as the operation doesn't make much sense
in the first place - but it's better to not have an edge case where
comparing two floats can raise, but only under very unusual circumstances.


Cheers,
Thomas

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Re: [Tutor] beginning to code

[Steve D'Aprano]

On Fri, 22 Sep 2017 09:24 pm, Marko Rauhamaa wrote:

> bartc :
> 
>> On 22/09/2017 10:23, Marko Rauhamaa wrote:
>>> However, Python doesn't need any language changes to implement memory
>>> slots. A memory slot could be defined as any object that implements
>>> "get()" and "set(value)" methods:
>>
>> I didn't understand your examples.
>>
>> Can Python be used to write, say, a swap() function that works with any
>> argument types (not just classes or lists)? Example:
>>
>> def swap(,):# made up syntax
>> a, b = b, a
>>
>> x=10
>> y="Z"
>> swap(x,y)
>> print (x,y) # "Z" and "10"
> 
> Yes, following my recipe:
> 
>def swap(ref_a, ref_b):
>a, b = ref_a.get(), ref_b.get()
>ref_a.set(b)
>ref_b.set(a)
> 
>x = 10
>y = "Z"
>swap(slot_ref(locals(), "x"), slot_ref(locals(), "y"))
>print(x, y) # "Z" and 10


No, you failed to follow Bart's instructions and answered a different question.

"Can you pulverise this granite boulder with your bare hands?"

"Sure! I just need to use this sledge hammer, and pulverise this small sandstone
rock instead."


You have to pass x and y, not strings 'x' and 'y'. The swap procedure needs to
accept any variable, given as ordinary bare names swap(x, y), not written as
strings, or by hard-coding x and y as the variables to swap, or by using a
string and passing it to exec, or any other loophole.

Also, you're assuming that locals() is writable, which in the most general case
it is not. Try using locals from inside a function, rather than in the global
scope.

You made a good attempt to emulate pass by reference via an extra layer of
indirection. Pity that it doesn't solve the problem and doesn't work.

Here's an easy way to emulate pass by reference which works in any scope: use a
list.


def swap(a, b):
a[0], b[0] = b[0], a[0]


a = [1]
b = [2]
swap(a, b)
assert a[0] == 2 and b[0] == 1


It's not proper pass by reference, and so will still fail Bart's test, because
you need to manually add an extra layer of redirection (using a list). Its like
using a pointer in C to emulate references, compared to having actual language
support for them like in Pascal or C++.

But if I wanted to write a Pascal interpreter in Python, I could use something
like this to implement Pascal var parameters.



-- 
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“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.

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Re: How to ingore "AttributeError: exception

[Ganesh Pal]

>
>
> is a perfectly good pattern to use.
>

Thanks looks  nice :)


>
>
> >
> >
> > I am a Linux user on Python 2.7
>
> Have you considered moving to Python 3?
>


Not yet , but  Is there something that I need to consider  in the current
context?

Regards,
Ganesh
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[issue31351] ensurepip discards pip's return code which leads to broken venvs

[Mariatta Wijaya]

Mariatta Wijaya added the comment:


New changeset eef49f5dd021d15396551880cf451042a79a1107 by Mariatta (Miss 
Islington (bot)) in branch '3.6':
bpo-31351: Set return code in ensurepip when pip fails (GH-3626) (GH-3683)
https://github.com/python/cpython/commit/eef49f5dd021d15396551880cf451042a79a1107


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Re: Assertions

[Steve D'Aprano]

On Fri, 22 Sep 2017 08:50 pm, alister wrote:

>> The bottom line is, if I saw
>> 
>> if not (thing > 0): raise AssertionError(...)
>> 
>> in a code review, I'd probably insist that either it be changed to use
>> `assert`,
>> or the exception be changed to ValueError, whichever better expresses
>> the intention of the code.
> 
> In a code review I would want the condition changed to be less noisy/
> confusing to the reader.
> 
> if thing <=0: whatever

Fair point, assuming they are the same.

Actually, even for floats they're not the same.

py> not (NAN > 0)
True
py> NAN <= 0
False


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enough, things got worse.

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[issue31149] Add Japanese to the language switcher

[Julien Palard]

Changes by Julien Palard :


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status: open -> closed

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Re: xml: TypeError: write() got an unexpected keyword argument 'short_empty_elements'

[Nagy László Zsolt]

> Result:
>
> Traceback (most recent call last):
>   File "C:/not_telling/c14n.py", line 16, in 
>     short_empty_elements=False
>   File "lxml.etree.pyx", line 1869, in lxml.etree._ElementTree.write
> (src\lxml\lxml.etree.c:57004)
> TypeError: write() got an unexpected keyword argument 'short_empty_elements'
Well, it looks like etree does not implement the short_empty_elements
argument in its write method:

https://github.com/lxml/lxml/blob/master/src/lxml/etree.pyx#L1954

But it should (see
https://github.com/lxml/lxml/blob/master/src/lxml/etree.pyx#L5 - The
``lxml.etree`` module implements the extended ElementTree API for XML. )

Can somebody please confirm that this is a bug? Also, how can I send a
bug report? ( I'm not able to add an issue to lxml, lack of permissions.  )
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Re: [Tutor] beginning to code

[bartc]

On 22/09/2017 13:34, Steve D'Aprano wrote:

On Fri, 22 Sep 2017 09:24 pm, Marko Rauhamaa wrote:



Yes, following my recipe:

def swap(ref_a, ref_b):
a, b = ref_a.get(), ref_b.get()
ref_a.set(b)
ref_b.set(a)

x = 10
y = "Z"
swap(slot_ref(locals(), "x"), slot_ref(locals(), "y"))
print(x, y) # "Z" and 10



No, you failed to follow Bart's instructions and answered a different question.



You have to pass x and y, not strings 'x' and 'y'. The swap procedure needs to
accept any variable, given as ordinary bare names swap(x, y), not written as
strings, or by hard-coding x and y as the variables to swap, or by using a
string and passing it to exec, or any other loophole.


And being able to pass any lvalue expression, not just simple variable 
names. Such as a[i] or x.y, provided the term is mutable.


(To make it work would require a new reference type. And extra versions 
of the byte-codes or whatever is used to evaluate terms such as:


a, a[i], x.y

that evaluate a reference rather than the value. So LOADFASTREF as well 
as LOADFAST)


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Re: [Tutor] beginning to code

[Marko Rauhamaa]

Chris Angelico :

> Sure, let me just put that into a function. CPython 3.7, although I'm
> pretty sure most CPython versions will do the same, as will several of
> the other Pythons.
> [demonstration that it didn't work]

Ok. The reason is this:

   Note: The contents of this dictionary should not be modified; changes
   may not affect the values of local and free variables used by the
   interpreter.

   https://docs.python.org/3/library/functions.html#locals>

So the language specification explicitly ruled it out, unfortunately.

> (Side point: Your slot_ref function is rather bizarre. It's a closure
> AND a class, just in case one of them isn't sufficient.

I don't see anything bizarre in it at all. I use the pattern all the
time. It's called an inner class:

   In Python, it is possible to nest a class within another class,
   method or function.

   https://en.wikipedia.org/wiki/Inner_class#Programming_languages>


Marko
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Re: [Tutor] beginning to code

[Marko Rauhamaa]

r...@zedat.fu-berlin.de (Stefan Ram):

> Marko Rauhamaa  writes:
>>r...@zedat.fu-berlin.de (Stefan Ram):
>>>Marko Rauhamaa  writes:
swap(slot_ref(locals(), "x"), slot_ref(locals(), "y"))
>>>You need to be able to write the call as
>>>swap( x, y )
>>Why?
>
>   You responded to Bart:
>
> | swap(x,y)
> | print (x,y) # "Z" and "10"
> |
> |If not, then it doesn't have reference passing as
> |it is normally understood.
>
>   , and Bart required this form. Moreover, if you allow other
>   forms, such as
>
> swap( ,  )
>
>   , then even C, would have "call by reference",
>   but it has not.

There's two things: syntax and semantics.

Obviously, Bart's syntax couldn't work syntactically unless Python added
the syntactic facilities. But the bigger question is a semantic one: is
it possible regardless of syntactic considerations.

As Chris pointed out, Python has explicitly ruled it out with the
immutability caveat to locals().


Marko
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Re: Assertions

[Steve D'Aprano]

On Fri, 22 Sep 2017 10:03 pm, alister wrote:

> On Fri, 22 Sep 2017 21:15:54 +1000, Steve D'Aprano wrote:
> 
>> On Fri, 22 Sep 2017 08:50 pm, alister wrote:
>> 
 The bottom line is, if I saw
 
 if not (thing > 0): raise AssertionError(...)
 
 in a code review, I'd probably insist that either it be changed to use
 `assert`,
 or the exception be changed to ValueError, whichever better expresses
 the intention of the code.
>>> 
>>> In a code review I would want the condition changed to be less noisy/
>>> confusing to the reader.
>>> 
>>> if thing <=0: whatever
>> 
>> Fair point, assuming they are the same.
>> 
>> Actually, even for floats they're not the same.
>> 
>> py> not (NAN > 0)
>> True py> NAN <= 0 False
> 
> I bet those are conditions the original author did not expect

Yes, everyone forgets about NAN ("Not A Number").
 
> actually I am not sure how you got them on my system

Oops, sorry, NAN is defined in my startup.py file, together with INF.

NAN = float('nan')
INF = float('inf')

Apologies.


> I would also say that if your examples did indeed return different
> results then pythons logic model has a bug.

No, Python does the right thing here.

Not all classes of values have a total ordering. With numbers, we can safely say
that if x > y, and y > z, then x > z too. But that doesn't necessarily hold for
everything.

E.g. if x, y and z are sports teams, and we identify ">" with "beats", then x
can beat y, and y beat z, but x lose to z. This isn't *numeric* order, but it
is a valid form of order.

In the case of floating point NANs, they are unordered with respect to all
numbers. So for any number x, we always have:

NAN == x
NAN < x
NAN > x
NAN <= x
NAN >= x

all return False, and 

NAN != x

return True.




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Re: Assertions

[Thomas Jollans]

On 2017-09-22 14:43, Steve D'Aprano wrote:
> In the case of floating point NANs, they are unordered with respect to all
> numbers. So for any number x, we always have:
> 
> NAN == x
> NAN < x
> NAN > x
> NAN <= x
> NAN >= x
> 
> all return False, and 
> 
> NAN != x
> 
> return True.

Just to make the implication explicit:

>>> from math import nan
>>> nan is nan
True
>>> nan == nan
False
>>> nan != nan
True
>>>


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Re: [Tutor] beginning to code

[Chris Angelico]

On Fri, Sep 22, 2017 at 9:24 PM, Marko Rauhamaa  wrote:
> bartc :
>
>> On 22/09/2017 10:23, Marko Rauhamaa wrote:
>>> However, Python doesn't need any language changes to implement memory
>>> slots. A memory slot could be defined as any object that implements
>>> "get()" and "set(value)" methods:
>>
>> I didn't understand your examples.
>>
>> Can Python be used to write, say, a swap() function that works with any
>> argument types (not just classes or lists)? Example:
>>
>> def swap(,):# made up syntax
>> a, b = b, a
>>
>> x=10
>> y="Z"
>> swap(x,y)
>> print (x,y) # "Z" and "10"
>
> Yes, following my recipe:
>
>def swap(ref_a, ref_b):
>a, b = ref_a.get(), ref_b.get()
>ref_a.set(b)
>ref_b.set(a)
>
>x = 10
>y = "Z"
>swap(slot_ref(locals(), "x"), slot_ref(locals(), "y"))
>print(x, y) # "Z" and 10

Sure, let me just put that into a function. CPython 3.7, although I'm
pretty sure most CPython versions will do the same, as will several of
the other Pythons.

(Side point: Your slot_ref function is rather bizarre. It's a closure
AND a class, just in case one of them isn't sufficient. The following
code is copied and pasted from two of your posts and is unchanged
other than making try_swapping into a function.)

>>> def slot_ref(dict_or_array, key_or_index):
...class SlotRef:
...def get(self): return dict_or_array[key_or_index]
...def set(self, value): dict_or_array[key_or_index] = value
...return SlotRef()
...
>>> def swap(ref_a, ref_b):
...a, b = ref_a.get(), ref_b.get()
...ref_a.set(b)
...ref_b.set(a)
...
>>> def try_swapping():
...x = 10
...y = "Z"
...swap(slot_ref(locals(), "x"), slot_ref(locals(), "y"))
...print("x, y =", x, y)
...
>>> try_swapping()
x, y = 10 Z

Strange... doesn't seem to work. Are you saying that Python is
pass-by-reference outside of a function and pass-by-value inside?
Because that'd be just insane.

Or maybe what you have here isn't a real reference at all.

ChrisA
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xml: TypeError: write() got an unexpected keyword argument 'short_empty_elements'

[Nagy László Zsolt]

Here is an MWE:

import io
from lxml import etree

test_node = etree.fromstring('''
http://schemas.xmlsoap.org/soap/envelope/;
xmlns:ns1="http://docs.oasis-open.org/ws-sx/ws-trust/200512;>
  
 
  
''')
output = io.BytesIO(b'')
test_node.getroottree().write(output,
 encoding="UTF-8",
 xml_declaration=None,
 default_namespace=None,
 method="c14n",
 short_empty_elements=False
 )
output.seek(0)
print(output.read())

Result:

Traceback (most recent call last):
  File "C:/not_telling/c14n.py", line 16, in 
    short_empty_elements=False
  File "lxml.etree.pyx", line 1869, in lxml.etree._ElementTree.write
(src\lxml\lxml.etree.c:57004)
TypeError: write() got an unexpected keyword argument 'short_empty_elements'

I have tracked down this to:

https://github.com/python/cpython/blob/master/Lib/xml/etree/ElementTree.py#L721

This method does have a "short_empty_elements" argument, but if I set it
to True, then it fails with TypeError.

Is this a bug?


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[issue31550] Inconsistent error message for TypeError with subscripting

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I'm not sure that this is an enhancement. It makes an error message 
inconsistent with other error messages.

>>> class I(int): pass
... 
>>> I()[0]
Traceback (most recent call last):
  File "", line 1, in 
TypeError: 'I' object does not support indexing
>>> 0[0] = 0
Traceback (most recent call last):
  File "", line 1, in 
TypeError: 'int' object does not support item assignment
>>> del 0[0]
Traceback (most recent call last):
  File "", line 1, in 
TypeError: 'int' object does not support item deletion
>>> ''['']  
>>> 
>>> 
>>>  
Traceback (most recent call last):  

 
  File "", line 1, in

 
TypeError: string indices must be integers, not str 

 
>>> iter(0)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: 'int' object is not iterable
>>> reversed(0)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: argument to reversed() must be a sequence
>>> next(0)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: int object is not an iterator
>>> {}[[]]
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unhashable type: 'list'

And actually this message is misleading. An attribute '__getitem__' is not 
looked up on objects.

>>> class I(int): pass
... 
>>> x = I()
>>> x.__getitem__ = lambda *args: None
>>> I()[0:]
Traceback (most recent call last):
  File "", line 1, in 
TypeError: 'I' object has no attribute '__getitem__'
>>> x.__getitem__
 at 0x7f11b62dd648>

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Re: what is happening in panda "where" clause

[Peter Otten]

Exposito, Pedro (RIS-MDW) wrote:

> This code does a "where" clause on a panda data frame...
> 
> Code:
> import pandas as pd;
> col_names = ['Name', 'Age', 'Weight', "Education"];
> # create panda dataframe
> x = pd.read_csv('test.dat', sep='|', header=None, names = col_names);
> # apply "where" condition
> z = x[ (x['Age'] == 55) ]
> # prints row WHERE age == 55
> print (z);
> 
> What is happening in this statement:
> z = x[ (x['Age'] == 55) ]
> 
> Thanks,

Let's take it apart into individual steps:

Make up example data:

>>> import pandas as pd
>>> x = pd.DataFrame([["Jim", 44], ["Sue", 55], ["Alice", 66]], 
columns=["Name", "Age"])
>>> x
Name  Age
0Jim   44
1Sue   55
2  Alice   66

Have a look at the inner expression:

>>> x["Age"] == 55
0False
1 True
2False

So this is a basically vector of boolean values. If you want more details: 
in numpy operations involving a a scalar and an array work via 
"broadcasting". In pure Python you would write something similar as

>>> [v == 55 for v in x["Age"]]
[False, True, False]

Use the result as an index:

>>> x[[False, True, True]]
Name  Age
1Sue   55
2  Alice   66

[2 rows x 2 columns]

This is again in line with numpy arrays -- if you pass an array of boolean 
values as an index the values in the True positions are selected. In pure 
Python you could achieve that with

>>> index = [v == 55 for v in x["Age"]]
>>> index
[False, True, False]
>>> [v for b, v in zip(index, x["Age"]) if b]
[55]


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[issue31351] ensurepip discards pip's return code which leads to broken venvs

[Mariatta Wijaya]

Mariatta Wijaya added the comment:

This has been backported to 3.6. Is backport to 2.7 needed?

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[issue31552] IDLE: Convert browswers to use ttk.Treeview

[Terry J. Reedy]

Terry J. Reedy added the comment:

>From #31461: Browsers currently use idlelib.tree.TreeWidget.  Treeview is not 
>a drop-in replacement because TreeWidget has some of the higher-level 
>app-specific functions that users are expected to add to Treeview.  But could 
>Treeview replace the low-level parts of TreeWidget that actually display stuff 
>on the screen?

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search and replace first amount of strings instances with one thing and a second amount of instances with another thing-

[validationmail1]

i have a code in python to search and replace what i need though is to replace 
the first say 10 instances of the number 1 with 2 and the second 10 instances 
with the number 3. anybody knows how to do that?

fin = open(r'F:\1\xxx.txt')
fout = open(r'F:\1\xxx2.txt', "wt")
for line in fin:
fout.write( line.replace('1', '2') )
fin.close()
fout.close()
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Re: [Tutor] beginning to code

[Chris Angelico]

On Fri, Sep 22, 2017 at 10:26 PM, Marko Rauhamaa  wrote:
> Chris Angelico :
>> (Side point: Your slot_ref function is rather bizarre. It's a closure
>> AND a class, just in case one of them isn't sufficient.
>
> I don't see anything bizarre in it at all. I use the pattern all the
> time. It's called an inner class:
>
>In Python, it is possible to nest a class within another class,
>method or function.
>
>https://en.wikipedia.org/wiki/Inner_class#Programming_languages>

Sure you *can* do it. It's perfectly legal Python syntax. But what's
the point? The class alone will do it.

class slot_ref:
def __init__(self, ns, key):
self.ns = ns; self.key = key
def get(self): return self.ns[self.key]
def set(self, value): self.ns[self.key] = value

That should be drop-in compatible with your original function/class
hybrid, complete with following the naming convention for functions
rather than classes.

ChrisA
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[issue31443] Possibly out of date C extension documentation

[Christoph Reiter]

Christoph Reiter added the comment:

Building the following with gcc from msys2 (cygwin) worked fine here: 
https://bpaste.net/show/0cafd5fa8211

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Re: Convert pandas series to string and datetime object

[Peter Otten]

Pavol Lisy wrote:

> pandas is one of reasons why python is so popular these days. But
> "there is only milion way how to do it" (and other unpythonic issues)
> I see there every time I am looking at it. :)

Yeah, such a useful tool with such a byzantine API, completely at odds with 
the zen -- I wonder what prevented that the authors chose to write it in 
Perl ;)

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[issue31461] IDLE: Enhance class browser

[Terry J. Reedy]

Terry J. Reedy added the comment:

#31552 ttk.Treeview

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Re: [Tutor] beginning to code

[Marko Rauhamaa]

Chris Angelico :

> On Fri, Sep 22, 2017 at 10:26 PM, Marko Rauhamaa  wrote:
>> Chris Angelico :
>>> (Side point: Your slot_ref function is rather bizarre. It's a closure
>>> AND a class, just in case one of them isn't sufficient.
>>
>> I don't see anything bizarre in it at all.

> Sure you *can* do it. It's perfectly legal Python syntax. But what's
> the point? The class alone will do it.
>
> class slot_ref:
> def __init__(self, ns, key):
> self.ns = ns; self.key = key
> def get(self): return self.ns[self.key]
> def set(self, value): self.ns[self.key] = value
>
> That should be drop-in compatible with your original function/class
> hybrid, complete with following the naming convention for functions
> rather than classes.

You are right. That one would be the way to go here.


Marko
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