What is a function parameter =[] for?

[Antoon Pardon]

Op 23-11-15 om 14:58 schreef Steven D'Aprano:
> On Mon, 23 Nov 2015 09:40 pm, BartC wrote:
> 
>> On 23/11/2015 07:47, Steven D'Aprano wrote:
>>
>>> I think it would be cleaner and better if Python had dedicated syntax for
>>> declaring static local variables:
>>
>> Interesting. So why is it that when /I/ said:
>>
>>  > On Mon, 23 Nov 2015 12:21 am, BartC wrote:
>>  >
>>  >> But if it's used for static storage, then why not just use static
>>  >> storage?
>>
>> You replied with the insulting:
>>
>>  > /head-desk
>>
>> ?
>>
>> Maybe it's my turn to bang my head on the desk.
> 
> Let me steal^W borrow an idea from Galileo, and present the explanation in
> the form of a dialogue between two philosophers of computer science,
> Salviati and Simplicio, and a third, intelligent layman, Sagredo.
> 
> https://en.wikipedia.org/wiki/Dialogue_Concerning_the_Two_Chief_World_Systems
> 
> 
> Salviati: Function defaults can also be used for static storage.
> 
> Simplicio: If you want static storage, why not use static storage?
> 
> Salviati: Function defaults in Python *are* static storage.
> 
>     Although they not the only way to get static storage, as closures can
> also be used for that purpose, they are surely the simplest way to get
> static storage in Python.
> 
> Sagredo: Simplest though it might be, surely a reasonable person would
> consider that using function parameters for static storage is abuse of the
> feature and a global variable would be better?
> 
> Salviati: Global variables have serious disadvantages. I will agree that
> using function parameters for static storage is something of a code smell,
> but good enough for rough and ready code. Nevertheless, it would be good if
> Python had dedicated syntax for static storage.
> 
> Simplicio: Ah-ha! Gotcha!
> 
> Salviati: No, perhaps you missed that I was referring to a hypothetical
> future addition to Python, not a current feature. But even if it did exist
> today, your statement misses the point that by using function defaults I
> *am* using static storage. In effect, you are telling me that rather than
> using static storage I should instead use static storage.

If you really want mutable static storage, you can simulate it more cleanly with
a closure than using a default argument.

def factory():

  static_dict = {}

  def myfunc(a, b):
    mutate static_dict as much as you like

  return myfunc

myfunc = factory()

Or you can use a class of course.