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From: Chris Angelico <rosuav@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: What is a function parameter =[] for?
Date: Thu, 19 Nov 2015 11:14:49 +1100
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On Thu, Nov 19, 2015 at 11:02 AM, Ian Kelly <ian.g.kelly@gmail.com> wrote:
> On Wed, Nov 18, 2015 at 4:22 PM, Chris Angelico <rosuav@gmail.com> wrote:
>> On Thu, Nov 19, 2015 at 10:14 AM, BartC <bc@freeuk.com> wrote:
>>> So, looking at some source code, a default value for certain types is only
>>> certain to be that value for the very first call of that function?
>>
>> On the contrary, it is certain always to be that exact object.
>
> "Certain" may be a bit overly strong.
>
>>>> def f(x=42):
> ...   return x
> ...
>>>> f()
> 42
>>>> f.__defaults__ = (43,)
>>>> f()
> 43

I'll raise you one.

>>> def f(x=42):
...     return x
...
>>> f()
42
>>> import ctypes
>>> ctypes.c_int.from_address(id(43)+ ctypes.sizeof(ctypes.c_size_t) + ctypes.sizeof(ctypes.c_voidp)).value=42
>>> f.__defaults__ = (43,)
>>> f()
42
>>>

Nothing is certain in Python. And two wrongs can make a... hmm... no,
this is not a right. It is not a privilege either. It is a dastardly
trick played on people's minds.

ChrisA