Mutable defaults

[Grant Edwards]

On 2021-02-11, J. Pic <jpic at yourlabs.org> wrote:

> I just meant removing the whole "default value mutating" story, not
> removing mutable variables. Really, I was wondering if there was a use case
> where this actually turns to an advantage,

I've seen people show how it can be used to provide function-scope
persistent storage -- the equivalent of declaring a static variable in
a C function. I don't think I've seen that done in the wild, though.

    class StaticStorage():
        pass
    
    def foo(static=StaticStorage()):
        if not hasattr(static,'x'):
            static.x = 0
        static.x += 1
        return static.x
        
    print(foo())
    print(foo())
    print(foo())
    print(foo())
    print(foo())

Run that, and it prints:

    1
    2
    3
    4
    5