Re: [Python-Dev] Possible bug in class-init, lookin for mentors

[Mariatta Wijaya]

Justus, welcome.

Consider joining the core-mentorship mailing list. If you have specific
questions on how to contribute or how to get started, we can help you
there. https://mail.python.org/mailman/listinfo/core-mentorship

Thanks, Erik. Yes, CPython has moved to GitHub https://github.com/python/
cpython but we will continue using bugs.python.org as the issue tracker.
We prefer that discussions happen on the issue tracker, and GitHub is for
code reviewing only.

The Dev Guide has been updated with our new GitHub workflow, specifically:
http://cpython-devguide.readthedocs.io/pullrequest.html
http://cpython-devguide.readthedocs.io/gitbootcamp.html

Thanks :)


Mariatta Wijaya

On Tue, May 2, 2017 at 9:01 AM, Erik Bray  wrote:

> On Fri, Apr 21, 2017 at 12:09 PM, Justus Schwabedal
>  wrote:
> > Hi everyone,
> >
> > I possibly found a bug in class initialization and would like to fix it.
> > Because it's my first journey to core-dev, I would really appreciate the
> > help of a mentor that I may ask a few questions to get me up to speed.
> >
> > To my person, I have previously worked on larger projects in python, c,
> and
> > c++ if that information helps, and I'm really curious to learn more about
> > the interiors of the greatest interpreter known to wo-/men.
> >
> > Here comes the bug-producing example:
> >
> > `class Foo:
> > def __init__(self, bar=[]):
> > self.list = bar
> >
> > spam_1 = Foo()
> > spam_2 = Foo()
> >
> > spam_1.list.append(42)
> > print(spam_2.list)`
> >
> > At least I think it's a bug.  Maybe it's a feature..
>
> Sorry to resurrect an old-ish thread; I haven't looked at Python-dev
> in several weeks.  I just wanted to point out that while everyone on
> this thread pointed out how this isn't a bug (clear to anyone who's
> spent enough time with Python), we have here an experienced C/C++
> developer who is interested in helping out on Python core devel, and
> no one took him up on that offer.
>
> Jus, for what it's worth, there are a slew of *actual* Python bugs to
> be worked on--many languishing for years--due to lack of available
> developer time.  You can have a good look at the (daunting) list of
> bugs at the old bugs.python.org [1].  IIUC Python development is
> slowly moving over to GitHub, but the issue list hasn't migrated yet
> so that would still be the place to start.
>
> If you find a bug that looks worth your time to try to fix, you should
> probably follow up on the issue for that bug itself.  I can't make any
> promises anyone will have time for mentorship, but I'd be willing to
> point you in the right direction.  I'm not a core developer but I know
> Python internals reasonably well and might be able to help *depending*
> on the issue.
>
> Best,
> Erik
>
> [1] http://bugs.python.org/
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>
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Re: [Python-Dev] Possible bug in class-init, lookin for mentors

[Erik Bray]

On Fri, Apr 21, 2017 at 12:09 PM, Justus Schwabedal
 wrote:
> Hi everyone,
>
> I possibly found a bug in class initialization and would like to fix it.
> Because it's my first journey to core-dev, I would really appreciate the
> help of a mentor that I may ask a few questions to get me up to speed.
>
> To my person, I have previously worked on larger projects in python, c, and
> c++ if that information helps, and I'm really curious to learn more about
> the interiors of the greatest interpreter known to wo-/men.
>
> Here comes the bug-producing example:
>
> `class Foo:
> def __init__(self, bar=[]):
> self.list = bar
>
> spam_1 = Foo()
> spam_2 = Foo()
>
> spam_1.list.append(42)
> print(spam_2.list)`
>
> At least I think it's a bug.  Maybe it's a feature..

Sorry to resurrect an old-ish thread; I haven't looked at Python-dev
in several weeks.  I just wanted to point out that while everyone on
this thread pointed out how this isn't a bug (clear to anyone who's
spent enough time with Python), we have here an experienced C/C++
developer who is interested in helping out on Python core devel, and
no one took him up on that offer.

Jus, for what it's worth, there are a slew of *actual* Python bugs to
be worked on--many languishing for years--due to lack of available
developer time.  You can have a good look at the (daunting) list of
bugs at the old bugs.python.org [1].  IIUC Python development is
slowly moving over to GitHub, but the issue list hasn't migrated yet
so that would still be the place to start.

If you find a bug that looks worth your time to try to fix, you should
probably follow up on the issue for that bug itself.  I can't make any
promises anyone will have time for mentorship, but I'd be willing to
point you in the right direction.  I'm not a core developer but I know
Python internals reasonably well and might be able to help *depending*
on the issue.

Best,
Erik

[1] http://bugs.python.org/
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Re: [Python-Dev] Possible bug in class-init, lookin for mentors

[Mark Lawrence via Python-Dev]

On 21/04/2017 16:03, Guyzmo via Python-Dev wrote:

On Fri, Apr 21, 2017 at 11:47:24AM +0200, Justus Schwabedal wrote:

At least I think it's a bug.  Maybe it's a feature..


it's indeed a feature.


I possibly found a bug in class __init__ and would like to fix it


technically, it's a method. More precisely, it's the constructor method.



No, __new__ is the constructor, __init__ is the initializer. It is 
completely impossible to state when a Python object has been initialised 
as you can throw in attributes any time you like.


--
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: [Python-Dev] Possible bug in class-init, lookin for mentors

[Antoine Rozo]

And it is not related to __init__ method.
You have the same behaviour with any other function or method.

>>> def append_to_list(item, l=[]):
... l.append(item)
... return l
...
>>> append_to_list(1)
[1]
>>> append_to_list(2)
[1, 2]

2017-04-21 17:18 GMT+02:00 Manolis Mavrofidis :

> In a nutshell.
> You create two instances and you assign the same list to both of them
> which you instantiate when you run your code.
> >>> id(spam_1.list)
> 4530230984 # <- Here
> >>> id(spam_2.list)
> 4530230984 # <- Here
> >>> id(spam_1)
> 4530231632 # Nice unique instance
> >>> id(spam_2)
> 4530231200 # Nice unique instance as well
>
> Try
> >>> class Foo:
> ... def __init__(self, list=None):
> ... self.list = list
> ...
> >>> spam_1 = Foo()
> >>> spam_2 = Foo([]) <- Cheating.
> >>> spam_1
> <__main__.Foo instance at 0x10e05d9e0>
> >>> spam_2
> <__main__.Foo instance at 0x10e05d950>
> >>> spam_2.list.append(42)
> >>> print(spam_1.list)
> None
> >>> print(spam_2.list)
> [42]
> >>> id(spam_1.list)
> 4527705752
> >>> id(spam_2.list)
> 4530231488
>
> Or something along those lines :)
>
> On 21 April 2017 at 16:03, Guyzmo via Python-Dev 
> wrote:
> > On Fri, Apr 21, 2017 at 11:47:24AM +0200, Justus Schwabedal wrote:
> >> At least I think it's a bug.  Maybe it's a feature..
> >
> > it's indeed a feature.
> >
> >> I possibly found a bug in class __init__ and would like to fix it
> >
> > technically, it's a method. More precisely, it's the constructor method.
> >
> >> So I'm looking for a mentor to help me.
> >>
> >> class Foo:
> >> def __init__(self, bar=[]):
> >> self.list = bar
> >>
> >> spam_1 = Foo()
> >> spam_2 = Foo()
> >>
> >> spam_1.list.append(42)
> >> print(spam_2.list)`
> >
> > the argument `bar` of your method is instanciated at the time you're
> > declaring the method. It's happening once for the the lifetime of the
> > execution of your code.
> >
> > By allocating the `bar` reference into the `self.list` member, you're
> > assigning the same *instance* of that list into the `self.list` member.
> >
> > So everytime you create a new Foo instance, you're actually assigning
> > the same `[]` instance into `self.list` which is why, when you mutate
> > the list, it's happening in all the instances of Foo as well.
> >
> > I hope it makes sense to you !
> >
> > --
> > Guyzmo
> > ___
> > Python-Dev mailing list
> > Python-Dev@python.org
> > https://mail.python.org/mailman/listinfo/python-dev
> > Unsubscribe: https://mail.python.org/mailman/options/python-dev/
> mmavrofides%40gmail.com
>
>
>
> --
> "Only those who will risk going too far
> can possibly find out how far one can go.
>  "T.S. Eliot
> http://0x109.tuxfamily.org
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> Unsubscribe: https://mail.python.org/mailman/options/python-dev/
> antoine.rozo%40gmail.com
>



-- 
Antoine Rozo
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Re: [Python-Dev] Possible bug in class-init, lookin for mentors

[Manolis Mavrofidis]

In a nutshell.
You create two instances and you assign the same list to both of them
which you instantiate when you run your code.
>>> id(spam_1.list)
4530230984 # <- Here
>>> id(spam_2.list)
4530230984 # <- Here
>>> id(spam_1)
4530231632 # Nice unique instance
>>> id(spam_2)
4530231200 # Nice unique instance as well

Try
>>> class Foo:
... def __init__(self, list=None):
... self.list = list
...
>>> spam_1 = Foo()
>>> spam_2 = Foo([]) <- Cheating.
>>> spam_1
<__main__.Foo instance at 0x10e05d9e0>
>>> spam_2
<__main__.Foo instance at 0x10e05d950>
>>> spam_2.list.append(42)
>>> print(spam_1.list)
None
>>> print(spam_2.list)
[42]
>>> id(spam_1.list)
4527705752
>>> id(spam_2.list)
4530231488

Or something along those lines :)

On 21 April 2017 at 16:03, Guyzmo via Python-Dev  wrote:
> On Fri, Apr 21, 2017 at 11:47:24AM +0200, Justus Schwabedal wrote:
>> At least I think it's a bug.  Maybe it's a feature..
>
> it's indeed a feature.
>
>> I possibly found a bug in class __init__ and would like to fix it
>
> technically, it's a method. More precisely, it's the constructor method.
>
>> So I'm looking for a mentor to help me.
>>
>> class Foo:
>> def __init__(self, bar=[]):
>> self.list = bar
>>
>> spam_1 = Foo()
>> spam_2 = Foo()
>>
>> spam_1.list.append(42)
>> print(spam_2.list)`
>
> the argument `bar` of your method is instanciated at the time you're
> declaring the method. It's happening once for the the lifetime of the
> execution of your code.
>
> By allocating the `bar` reference into the `self.list` member, you're
> assigning the same *instance* of that list into the `self.list` member.
>
> So everytime you create a new Foo instance, you're actually assigning
> the same `[]` instance into `self.list` which is why, when you mutate
> the list, it's happening in all the instances of Foo as well.
>
> I hope it makes sense to you !
>
> --
> Guyzmo
> ___
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> Unsubscribe: 
> https://mail.python.org/mailman/options/python-dev/mmavrofides%40gmail.com



-- 
"Only those who will risk going too far
can possibly find out how far one can go.
 "T.S. Eliot
http://0x109.tuxfamily.org
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Re: [Python-Dev] Possible bug in class-init, lookin for mentors

[Max Zettlmeißl]

On 21 April 2017 at 12:09, Justus Schwabedal  wrote:
> I possibly found a bug in class initialization and would like to fix it.
>
> Here comes the bug-producing example:
>
> `class Foo:
> def __init__(self, bar=[]):
> self.list = bar
>
> spam_1 = Foo()
> spam_2 = Foo()
>
> spam_1.list.append(42)
> print(spam_2.list)`
>
> At least I think it's a bug.  Maybe it's a feature..
>

It is not a bug.

It is the way in which Python handles mutable keyword arguments.
If you want to use something in this way you should go with

def __init__(self, bar=None):
if bar is None:
bar = []
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Re: [Python-Dev] Possible bug in class-init, lookin for mentors

[Guyzmo via Python-Dev]

On Fri, Apr 21, 2017 at 11:47:24AM +0200, Justus Schwabedal wrote:
> At least I think it's a bug.  Maybe it's a feature..

it's indeed a feature.

> I possibly found a bug in class __init__ and would like to fix it

technically, it's a method. More precisely, it's the constructor method.

> So I'm looking for a mentor to help me.
> 
> class Foo:
> def __init__(self, bar=[]):
> self.list = bar
> 
> spam_1 = Foo()
> spam_2 = Foo()
> 
> spam_1.list.append(42)
> print(spam_2.list)`

the argument `bar` of your method is instanciated at the time you're
declaring the method. It's happening once for the the lifetime of the
execution of your code.

By allocating the `bar` reference into the `self.list` member, you're
assigning the same *instance* of that list into the `self.list` member.

So everytime you create a new Foo instance, you're actually assigning
the same `[]` instance into `self.list` which is why, when you mutate
the list, it's happening in all the instances of Foo as well.

I hope it makes sense to you !

-- 
Guyzmo
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Re: [Python-Dev] Possible bug in class-init, lookin for mentors

[Paul Moore]

This is correct behaviour. I would suggest that you post this to
python-list for a full discussion of what's going on here, but
basically the default value for argument bar of __init__ is created at
class creation time, and then reused for every instance. This is a
common mistake made by newcomers, using mutable default arguments for
functions (or methods).

A google search for "python mutable default argument" should find you
some useful explanations of what's going on.

Cheers,
Paul

On 21 April 2017 at 10:47, Justus Schwabedal  wrote:
> Hi everyone,
>
> I possibly found a bug in class __init__ and would like to fix it.  So I'm
> looking for a mentor to help me.
>
> `class Foo:
> def __init__(self, bar=[]):
> self.list = bar
>
> spam_1 = Foo()
> spam_2 = Foo()
>
> spam_1.list.append(42)
> print(spam_2.list)`
>
> At least I think it's a bug.  Maybe it's a feature..
>
> Best Regards, Jus
>
>
>
>
>
> --
> Justus Schwabedal
>
> Handy (D): +49 177 939 5281
> email: jschwabe...@googlemail.com
> skype: justus1802
>
> Görlitzer Str. 22
> 01099 Dresden, Sachsen
> Germany
>
> Steinkreuzstr. 23
> 53757 Sankt Augustin, NRW
> Germany
>
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>
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[Python-Dev] Possible bug in class-init, lookin for mentors

[Justus Schwabedal]

Hi everyone,

I possibly found a bug in class initialization and would like to fix it.
Because it's my first journey to core-dev, I would really appreciate the
help of a mentor that I may ask a few questions to get me up to speed.

To my person, I have previously worked on larger projects in python, c, and
c++ if that information helps, and I'm really curious to learn more about
the interiors of the greatest interpreter known to wo-/men.

Here comes the bug-producing example:

`class Foo:
def __init__(self, bar=[]):
self.list = bar

spam_1 = Foo()
spam_2 = Foo()

spam_1.list.append(42)
print(spam_2.list)`

At least I think it's a bug.  Maybe it's a feature..

Best Regards, Jus

---

f*** me  on github :)
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[Python-Dev] Possible bug in class-init, lookin for mentors

[Justus Schwabedal]

Hi everyone,

I possibly found a bug in class __init__ and would like to fix it.  So I'm
looking for a mentor to help me.

`class Foo:
def __init__(self, bar=[]):
self.list = bar

spam_1 = Foo()
spam_2 = Foo()

spam_1.list.append(42)
print(spam_2.list)`

At least I think it's a bug.  Maybe it's a feature..

Best Regards, Jus





-- 
Justus Schwabedal

Handy (D): +49 177 939 5281
email: jschwabe...@googlemail.com
skype: justus1802

Görlitzer Str. 22
01099 Dresden, Sachsen
Germany

Steinkreuzstr. 23
53757 Sankt Augustin, NRW
Germany
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