Static variables [was Re: syntax difference]

[Steven D'Aprano]

From: "Steven D'Aprano" <steven.d'aprano at 1:261/38.remove-r7u-this>

From: Steven D'Aprano <steve+comp.lang.python at pearwood.info>

On Wed, 20 Jun 2018 14:18:19 +1000, Chris Angelico wrote:

> Ah. Yeah, that would be a plausible feature to add to Python. But in C,
> a static variable is basically the same thing as a global variable,
> except that its name is scoped to the function. There is only one of it.
> What happens in Python? For instance:
>
> def f():
>     def g():
>         static x = 0
>         x += 1
>         return x
>     return g
>
> Does the static variable exist once for each instance of g()? If so,
> it'll behave like a closure variable; if not, it'll behave like a
> global. Either way, I'm pretty much certain that people will expect the
> other.

Yes, but given the normal execution model of Python, only one solution is
valid. Since the function g is created fresh each time f is called, each one
gets a fresh static x.

If you want all the g's to share the same x, you would write:

def f():
    static x = 0
    def g():
        x += 1
        return x
    return g


In this case, every invocation of f shares the same static x, and all the g's
refer to that same x, using the ordinary closure mechanism. In the earlier
case, each invocation of f creates a brand new g with its own x.

Simple and elegant.

This could at last get rid of that useful but ugly idiom:

    def function(real, arguments, len=len, int=int, str=str):
        ...

if we allowed the "static" declaration to access the values from the
surrounding scope:

    def function(real, arguments):
        static len=len, int=int, str=str

But I think nicer than that would be a decorator:

    @static(len=len, int=int, str=str)
    def function(real, arguments):
        ...

which adds local variables len, int, str to the function, with the given
values, and transforms all the bytecode LOAD_NAME len to LOAD_FAST len (or
whatever).

(We might need a new bytecode to SET_STATIC.)

That would be a nice bytecode hack to prove the usefulness of the concept!


--
Steven D'Aprano
"Ever since I learned about confirmation bias, I've been seeing it everywhere."
 -- Jon Ronson

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