Re: The "loop and a half"

[john polo]

On 10/3/2017 5:08 PM, Stefan Ram wrote:

Steve D'Aprano  writes:

On Wed, 4 Oct 2017 04:45 am, Rhodri James wrote:

On 03/10/17 18:29, Stefan Ram wrote:

Is this the best way to write a "loop and a half" in Python?

Define "best".

I'd start with "define loop and a half".

   I encountered this phenomenon decades ago. But only
   some years ago, I heard someone using a special word
   for it! Let me try to find it ...

   Yes, it was Mehran Sahami who used it, possibly in one of
   his lectures of the course "Programming Methodology"
   (CS 106A) at "Stanford Engineering".

   
   ...


   The history of this problem goes way back into the past.
   In the 1950s (and possibly still inthe 1960s), such mundane
   loops were research topics, and Knuth and others wrote
   research articles about how to write such a loop in such a
   way that no test or statement has to be repeated.
   (I sometimes liked to read old magazines from those years.)

   Thanks for all the answers so far!


I'm adding to what Stefan wrote, simply to point out how a newb like me 
came across this issue formally, besides actually dealing with this in 
my practice.


In Python Programming Fundamentals 2nd ed., the author, Kent D. Lee, 
brings up loop and a half in ch. 3, Repetitive Tasks (p. 82). He wrote:


"Whether you are writing code in Python or some other language, this Reading
Records From a File pattern comes up over and over again. It is 
sometimes called
the loop and a half problem. The idea is that you must attempt to read a 
line from the
file before you know whether you are at the end of file or not. This can 
also be done
if a boolean variable is introduced to help with the while loop. This 
boolean variable
is the condition that gets you out of the while loop and the first time 
through it must

be set to get your code to execute the while loop at least one."


best,

John

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Re: TypeError with map with no len()

[john polo]

On 9/25/2017 5:37 PM, Thomas Jollans wrote:

On 25/09/17 18:44, john polo wrote:

Python List,

I am trying to make practice data for plotting purposes. I am using
Python 3.6. The instructions I have are

import matplotlib.pyplot as plt
import math
import numpy as np
t = np.arange(0, 2.5, 0.1)
y1 = map(math.sin, math.pi*t)

If you use np.sin instead of math.sin, you don't have to use map: Most
numpy functions operate elementwise on arrays (for example, you're
multiplying math.pi with an array - something that wouldn't work with a
list).

Here's the numpy way of doing this:

t = np.arange(0, 2.5, 0.1)
y1 = np.sin(np.pi * t)

Without using numpy at all, this might be

t = [i * 0.1 for i in range(25)]
y1 = [math.pi * a for a in t]

The numpy way looks like a great alternative. Thank you, Thomas.

John
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Re: TypeError with map with no len()

[john polo]

On 9/25/2017 12:03 PM, Paul Moore wrote:

You're using Python 3, and I suspect that you're working from
instructions that assume Python 2. In Python 3, the result of map() is
a generator, not a list (which is what Python 2's map returned). In
order to get an actual list (which appears to be what you need for
your plot call) you just need to call the list constructor:

y1 = list(map(math.sin, math.pi*t))

Although given that you're using numpy, it may be that there's a more
idiomatic numpy way of doing this. I'm not a numpy expert though, so I
can't help on that.

Paul

Paul,
Thank you very much for the explanation.

best regards,
John
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TypeError with map with no len()

[john polo]

Python List,

I am trying to make practice data for plotting purposes. I am using 
Python 3.6. The instructions I have are


import matplotlib.pyplot as plt
import math
import numpy as np
t = np.arange(0, 2.5, 0.1)
y1 = map(math.sin, math.pi*t)
plt.plot(t,y1)

However, at this point, I get a TypeError that says

object of type 'map' has no len()

In [6]: t
Out[6]:
array([ 0. ,  0.1,  0.2,  0.3,  0.4,  0.5,  0.6,  0.7, 0.8,  0.9,  1. ,
    1.1,  1.2,  1.3,  1.4,  1.5,  1.6,  1.7,  1.8, 1.9,  2. ,  2.1,
    2.2,  2.3,  2.4])
In [7]: y1
Out[7]: 
In [8]: math.pi*t
Out[8]:
array([ 0.    ,  0.31415927,  0.62831853,  0.9424778 , 1.25663706,
    1.57079633,  1.88495559,  2.19911486,  2.51327412, 2.82743339,
    3.14159265,  3.45575192,  3.76991118,  4.08407045, 4.39822972,
    4.71238898,  5.02654825,  5.34070751,  5.65486678, 5.96902604,
    6.28318531,  6.59734457,  6.91150384,  7.2256631 , 7.53982237])

At the start of creating y1, it appears there is an array to iterate 
through for the math.sin function used in map(), but y1 doesn't appear 
to be saving any values. I expected y1 to hold a math.sin() output for 
each item in math.pi*t, but it appears to be empty. Am I 
misunderstanding map()? Is there something else I should be doing 
instead to populate y1?



John

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Re: Re: errors with json.loads

[john polo]

On 9/21/2017 10:11 AM, Ned Batchelder wrote:


I can only assume that the actual data being read is different than 
the data they put into the message here.


--Ned.

There was a typo in the file that I had made and saved; an extra comma 
before one of the ":". Apologies to the list for not catching that sooner.



John
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Re: Re: errors with json.loads

[john polo]

On 9/21/2017 4:24 AM, Thomas Jollans wrote:


It looks to me like the root cause of the problem was that they copied
the code from a web page, and the web page contained invalid JSON.


Thank you, Thomas.


John

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Re: Re: errors with json.loads

[john polo]

On 9/20/2017 6:40 PM, Dennis Lee Bieber wrote:

On Wed, 20 Sep 2017 17:13:41 -0500, john polo <jp...@mail.usf.edu>
declaimed the following:



and the example code for reading the file is:


file = open('books.json','r')

What encoding is the file? I did a cut from your post into a
file, and the contents (Python 2.7) failed with

Dennis,
I typed it into notepad++ and saved as .json. On the bottom of the 
notepad++ window it says:

Windows (CR LF) UTF-8

I think that answers your question?

Thanks,
John

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and he'll be silent for a moment.
Feed a man to a red dragon
and he'll be silent for a lifetime."
-Anne Isabella Thackeray Ritchie

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Re: Re: errors with json.loads

[john polo]

On 9/20/2017 5:58 PM, Bill wrote:

Interesting problem, John.

I have probably even less experience with json than you do, so I'm 
taking this as an opportunity to learn with you.


Suggestions:

1. Try your example with Python 2 rather than Python 3.

Bill,
Thanks for the reply. I wasn't sure how to get Python 2 through the cmd 
or IPython, so I went through ArcGIS, but it's mostly the same result:


>>> file = open('books.json','r')
>>> text = file.read()
>>> text = json.loads(text)
Runtime error
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'json' is not defined
>>> import json   #whoops, forget to do that first in this go 'round.
>>> text = json.loads(text)
Runtime error
Traceback (most recent call last):
  File "", line 1, in 
  File "C:\Python27\ArcGIS10.4\Lib\json\__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "C:\Python27\ArcGIS10.4\Lib\json\decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "C:\Python27\ArcGIS10.4\Lib\json\decoder.py", line 382, in 
raw_decode

    obj, end = self.scan_once(s, idx)
ValueError: Expecting : delimiter: line 5 column 50 (char 161)
>>>

2. Take your file and make it into a string literal in your program, 
and try calling json.loads with that as an argument. 

I am not sure I follow what you meant, but this was what I did:
In [26]: file2 = open('books.txt')

In [27]: text2 = file2.read()

In [28]: text2 = json.loads(text2)
---
JSONDecodeError   Traceback (most recent call last)
 in ()
> 1 text2 = json.loads(text2)

c:\users\jpolo\appdata\local\programs\python\python36\lib\json\__init__.py 
in lo
ads(s, encoding, cls, object_hook, parse_float, parse_int, 
parse_constant, objec

t_pairs_hook, **kw)
    352 parse_int is None and parse_float is None and
    353 parse_constant is None and object_pairs_hook is 
None and not

 kw):
--> 354 return _default_decoder.decode(s)
    355 if cls is None:
    356 cls = JSONDecoder

c:\users\jpolo\appdata\local\programs\python\python36\lib\json\decoder.py 
in dec

ode(self, s, _w)
    337
    338 """
--> 339 obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    340 end = _w(s, end).end()
    341 if end != len(s):

c:\users\jpolo\appdata\local\programs\python\python36\lib\json\decoder.py 
in raw

_decode(self, s, idx)
    353 """
    354 try:
--> 355 obj, end = self.scan_once(s, idx)
    356 except StopIteration as err:
    357 raise JSONDecodeError("Expecting value", s, 
err.value) from

None

JSONDecodeError: Expecting ':' delimiter: line 5 column 50 (char 161)


best,
John

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Re: Re: errors with json.loads

[john polo]

On 9/20/2017 5:56 PM, John Gordon wrote:

In <mailman.101.1505945630.2730.python-l...@python.org> john polo 
<jp...@mail.usf.edu> writes:


JSONDecodeError: Expecting ':' delimiter: line 5 column 50 (char 161)
?json.loads says that the method is for deserializing "s", with "s"
being a string, bytes, or bytearray.
In [24]: type(text)
Out[24]: str
So "text" seems to be a string. Why does json.loads return an error?

Because, although text is the right type, it does not contain a
valid json string.


Thank you for the reply, John. I am not familiar with how to check 
validity for json. I typed the example into notepad++ and saved as .json 
and didn't take any further steps after that. I'll look into that.



John

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errors with json.loads

[john polo]

Greetings,

I am using IPython 6.1.0 with Python 3.6.2 on a Windows 7 machine. I am 
not a programmer. I am using a book called Python Data Analytics to try 
to learn some of Python. I am at a section for reading and writing JSON 
data. The example JSON file is:



Listing 5-13.  books.json
[{"writer": "Mark Ross",
 "nationality": "USA",
 "books": [
 {"title": "XML Cookbook", "price": 23.56},
 {"title": "Python Fundamentals", "price": 50.70},
 {"title": "The NumPy library", "price": 12.30}
 ]
},
{"writer": "Barbara Bracket",
 "nationality": "UK",
 "books": [
 {"title": "Java Enterprise", "price": 28.60},
 {"title": "HTML5", "price": 31.35},
 {"title": "Python for Dummies", "price": 28.00}
 ]
}]

and the example code for reading the file is:

>>> file = open('books.json','r')
>>> text = file.read()
>>> text = json.loads(text)

When I use those 3 lines, I get the following:

JSONDecodeError   Traceback (most recent call last)
 in ()
> 1 text = json.loads(text)

c:\users\..\python\python36\lib\json\__init__.py in loads(s, encoding, 
cls, object_hook, parse_float, parse_int, parse_constant, 
object_pairs_hook, **kw)

    352 parse_int is None and parse_float is None and
    353 parse_constant is None and object_pairs_hook is 
None and not kw):

--> 354 return _default_decoder.decode(s)
    355 if cls is None:
    356 cls = JSONDecoder

c:\users\..\python\python36\lib\json\decoder.py in decode(self, s, _w)
    337
    338 """
--> 339 obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    340 end = _w(s, end).end()
    341 if end != len(s):

c:\users\..\python\python36\lib\json\decoder.py in raw_decode(self, s, idx)
    353 """
    354 try:
--> 355 obj, end = self.scan_once(s, idx)
    356 except StopIteration as err:
    357 raise JSONDecodeError("Expecting value", s, 
err.value) from None


JSONDecodeError: Expecting ':' delimiter: line 5 column 50 (char 161)

?json.loads says that the method is for deserializing "s", with "s" 
being a string, bytes, or bytearray.


In [24]: type(text)
Out[24]: str

So "text" seems to be a string. Why does json.loads return an error?


John

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Re: script output appears correct but still raises, AssertionError

[john polo]

ChrisA,

Thank you for pointing out my error: using print() when I should have 
used return().


John

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script output appears correct but still raises AssertionError

[john polo]

Python People,

I am learning about assertions. I wrote a small script that takes 2 
inputs, an amino acid sequence and one residue symbol. The script should 
return what percent of the sequence is residue in output. The point of 
this script is to use assert for debugging. My script seems to work 
correctly, but when I use the assert statements that are supposed to 
test the script, the assertions indicate there is a problem with the 
script.



>>> def aminosleft(sequence, res):
... sequp = sequence.upper()
... lens1 = len(sequp)
... rep = res.upper()
... reps2 = sequp.replace(rep,"")
... lens2 = len(reps2)
... resid = 100* (lens1 - lens2) / lens1
... print(int(resid))
...
...
>>> aminosleft("MSRSLLLRFLLFPPLP", "M")
5
>>> assert aminosleft("MSRSLLLRFLLFPPLP", "M") == 5
5
Traceback (most recent call last):
  File "", line 1, in 
AssertionError

>>> aminosleft("MSRSLLLRFLLFPPLP", "r")
10
>>> assert aminosleft("MSRSLLLRFLLFPPLP", "r") == 10
10
Traceback (most recent call last):
  File "", line 1, in 
AssertionError

>>> aminosleft("msrslllrfllfpplp", "L")
50
>>> assert aminosleft("msrslllrfllfpplp", "L") == 50
50
Traceback (most recent call last):
  File "", line 1, in 
AssertionError

>>> aminosleft("MSRSLLLRFLLFPPLP", "Y")
0
>>> assert aminosleft("MSRSLLLRFLLFPPLP", "Y") == 0
0
Traceback (most recent call last):
  File "", line 1, in 
AssertionError

The script returns an integer. I don't know if the assertion uses an 
integer or if that matters. What is causing the AssertionError?



John
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Re: read in a list in a file to list

[john polo]

On 4/8/2017 3:21 PM, breamore...@gmail.com wrote:

On Saturday, April 8, 2017 at 7:32:52 PM UTC+1, john polo wrote:

Hi,

I am using Python 3.6 on Windows 7.

I have a file called apefile.txt. apefile.txt's contents are:

apes =  "Home sapiens", "Pan troglodytes", "Gorilla gorilla"

I have a script:

apefile =  open("apefile.txt")
apelist =  apefile.read()
for ape in apelist:
 print("one of the apes is " + ape)
apefile.close()

The output from the script does not print the ape names, instead it
prints each letter in the file. For example:

one of the apes is a
one of the apes is p
one of the apes is e

What should I do instead to get something like

one of the apes is Home sapiens
one of the apes is Pan troglodytes
one of the apes is Gorilla gorilla
  
John

I'll start you off.

with open("apefile.txt") as apefile:
 for line in apefile:
 doSomething(line)

String methods and/or the csv module might be used here in doSomething(line), 
but I'll leave that to you so that you can learn.  If you get stuck please ask 
again, we don't bite :)

Kindest regards.

Mark Lawrence.

Mark,
Thanks for the reply. I looked back through the methods for strings. I 
also looked at the csv module, but I couldn't tell which one of those 
would help. The small .txt file does have commas, but with the weird 
form of listname = [1] , [2], [3], etc. for a .csv, I don't know how 
that would be read in a like a .csv. But now that I think about it, 
datObj2 in my script prints just the list elements, so maybe the 
'listname=' part wouldn't affect it...


Anyway, after reviewing string methods some more, I came up with this. 
If I understood your hint, the loop was supposed to somehow break up the 
long string that came from the apelist file, but it seemed that trying 
to use the split method in a loop wouldn't work, especially since one of 
my attempts started a list and then the interpreter said there were no 
split methods for list.


The new attempt gives me a list, now I have to figure out how to deal 
with unwanted quotation marks and spaces.


datFil =  open("apelist.txt")
datObj =  datFil.read()


datObj2 = datObj.replace('" ','')  #added this comment while writing 
this email: I guess I could have used 'datObj = datFil.read().replace('" 
','')'. Can you make two replacements in the same statement, for example 
'datObj=datFil.read().replace('" ','').replace('"','')?


datObj2 = datObj2.replace('"','')  #this was here to try to get rid of " 
that didn't have a subsequent space.


datObj2 = datObj2.split(',')

print(datObj2)

for i in datObj2:
print("one of the apes is " + i)

datFil.close()

Output:
['Homo sapiens', ' Pan troglodytes', ' Gorilla gorilla']
one of the apes is Homo sapiens
one of the apes is  Pan troglodytes
one of the apes is  Gorilla gorilla

Per reply of Rick Johnson, I altered variable names to something generic.

Thanks for the prodding from the replies.

John
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read in a list in a file to list

[john polo]

Hi,

I am using Python 3.6 on Windows 7.

I have a file called apefile.txt. apefile.txt's contents are:

apes =  "Home sapiens", "Pan troglodytes", "Gorilla gorilla"

I have a script:

apefile =  open("apefile.txt")
apelist =  apefile.read()
for ape in apelist:
   print("one of the apes is " + ape)
apefile.close()


The output from the script does not print the ape names, instead it 
prints each letter in the file. For example:


one of the apes is a
one of the apes is p
one of the apes is e


What should I do instead to get something like

one of the apes is Home sapiens
one of the apes is Pan troglodytes
one of the apes is Gorilla gorilla



John
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Re: syntax error in first-time user script

[john polo]

I had a misconception of how the Python interpreter works. If I have a 
script, call it example.py, in order to run it, I shouldn't be in the 
interpreter? In other words, I should be at the Windows command prompt, 
for example


C:/test> python example.py

and not

>>> python example.py

?

So, if I am already on >>> and have a problem with a script and need to 
edit it and want to run the script again, how do I do that? Do I exit 
the interpreter and start it again? Or is there a different way to run 
the script once the interpreter is active and I am at >>> ?


Cheers,
John
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Re: syntax error in first-time user script

[john polo]

On 3/24/2017 9:11 AM, Bob Gailer wrote:


On Mar 24, 2017 4:53 AM, "john polo" <jp...@mail.usf.edu 
<mailto:jp...@mail.usf.edu>> wrote:

>
> Greetings,
> I am attempting to learn Python. I have no programming background. 
I'm on a computer with Windows 7. I checked the PATH in System 
Variables and Python and Python\Scripts are in the path. I have a 
book, Python for Biologists and it supplies example scripts, one is 
called comment.py. The output from comment.py is

>
> Comments are very useful!
>
> I use cd to get to the directory with the example scripts

We can assume you are at a Windows command prompt. It' a good idea too 
tell us that,  so we don't have to assume!


and open a command prompt with "python" and get the prompt.

Assumptions / terminology again! Typing "python" normally invokes the 
python interpreter. Since you did not pass any arguments you get a 
window displaying the python interpreter prompt (>>>).


I type
>
> "python comment.py"
>
> and get
>
> "File "", line 1
> python comment.py
> ^
> SyntaxError: invalid syntax

It is great (as John said) that you provided the traceback. It would 
have been even greater if you had given us the rest e.g,


Microsoft Windows [Version 10.0.14393]
(c) 2016 Microsoft Corporation. All rights reserved.

C:\Users\bgailer>cd /examples

C:\examples>python
Python 3.3.5 (v3.3.5:62cf4e77f785, Mar  9 2014, 10:35:05) [MSC v.1600 
64 bit (AMD64)] on win32

Type "help", "copyright", "credits" or "license" for more information.
>>> python comment.py
  File "", line 1
python comment.py
 ^
SyntaxError: invalid syntax
>>>

Which brings up one other important matter: when writing us also tell 
us which python version you are using. In my example it is 3.3.5.



I'm using 3.6.0.


>
> Sorry, I realize this may not all line up like it does when I'm 
typing this in the email. The caret/error pointer is under the t in 
comment. I am not sure what is causing this error.

>
> If I type
>
> "import comment.py"
>
> the output is
>
> "Comments are very useful!
> Traceback (most recent call last):
> File "", line 1, in 
> ModuleNotFoundError: No module named 'comment.py'; 'comment'
> "
>
> If I use IDLE shell:
>
> "python comment.py"
>
> "SyntaxError: invalid syntax"
>
> I know I can open the file in the editor and use F5, but shouldn't I 
be able to use a command from shell? How do I run comment.py from a 
command line without error? Or is that not possible?

>
> cheers,
> John
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<https://mail.python.org/mailman/listinfo/python-list>





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over the truth, but most of them
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syntax error in first-time user script

[john polo]

Greetings,
I am attempting to learn Python. I have no programming background. I'm 
on a computer with Windows 7. I checked the PATH in System Variables and 
Python and Python\Scripts are in the path. I have a book, Python for 
Biologists and it supplies example scripts, one is called comment.py. 
The output from comment.py is


Comments are very useful!

I use cd to get to the directory with the example scripts and open a 
command prompt with "python" and get the prompt. I type


"python comment.py"

and get

"File "", line 1
python comment.py
^
SyntaxError: invalid syntax
"

Sorry, I realize this may not all line up like it does when I'm typing 
this in the email. The caret/error pointer is under the t in comment. I 
am not sure what is causing this error.


If I type

"import comment.py"

the output is

"Comments are very useful!
Traceback (most recent call last):
File "", line 1, in 
ModuleNotFoundError: No module named 'comment.py'; 'comment'
"

If I use IDLE shell:

"python comment.py"

"SyntaxError: invalid syntax"

I know I can open the file in the editor and use F5, but shouldn't I be 
able to use a command from shell? How do I run comment.py from a command 
line without error? Or is that not possible?


cheers,
John
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