[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 <python-dev at python.org> 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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