default argument value is mutable

[jmp]

On 10/07/2016 03:45 PM, ast wrote:
>
> "jmp" <jeanmichel at sequans.com> a écrit dans le message de
> news:mailman.210.1475844513.30834.python-list at python.org...
>> On 10/07/2016 02:07 PM, ast wrote:
>>>
>>> "jmp" <jeanmichel at sequans.com> a écrit dans le message de
>>> news:mailman.209.1475841371.30834.python-list at python.org...
>>>> On 10/07/2016 01:38 PM, Daiyue Weng wrote:
>>>
>>>>
>>>> So the rule of thumb for default argument value is "No mutable"
>>>>
>>>> Cheers,
>>>
>>>
>>> It can be used to store some variables from one call of
>>> a function to an other one.
>>>
>>> def test( _store={'x':0}):
>>>
>>>     x = _store['x']
>>>     ..... do some stuff
>>>    _store['x'] = x
>>
>> For personal dirty scripts, possibly, for all other situations, never.
>
> not so dirty in my opinion

You made a point, it's not that dirty, it's an "advanced technique" that 
is often actually an error when you don't know what you're doing. See 
the OP's code.

I'm in an environment where people use python as a tool more than an 
effective powerful language to write complex applications. That's 
probably why I'm bias on this issue and prefer the cautious approach.

>> Especially since there's nothing in the code above that cannot be
>> solved using standard idioms .
>
> Yes, putting _store dictionnary outside
>
> _store={'x':0}
>
> def test( ):
>
>     x = _store['x']
>     ..... do some stuff
>     _store['x'] = x
>
>
> or using a global variable, but you pollute in both case
> the global scope unnecessary.

What about

def test():
   if not hasattr(test, '_store'): test._store={'x':0}
   test._store['x'] += 1

Neither you pollute the global namespace, nor you pollute the function 
signature with implementation specifics.

jm