Python：缓存一个本地 function 变量以供后续调用

Question

In C/C++, a function can declare a local variable as static . When doing so, the value remains in memory and is available to subsequent calls to the function (this variable is no longer local, but that's besides the point).

Is there a way to do something similar in Python, without having to declare any global variable outside of the function?

Use case: a function that (among other things) uses a regex to extract a value from an input string. I want to pre-compile the pattern ( re.compile() ), without having to declare the variable outside of the function scope.

I can inject a variable directly into globals() :

globals()['my_pattern'] = re.compile(...)

But it doesn't look like a good idea.

Answer 1

You could use a function attribute. In Python, functions are first-class objects, so you can abuse of this feature to simulate a static variable:

import re

def find_some_pattern(b):
    if getattr(find_some_pattern, 'a', None) is None:
        find_some_pattern.a = re.compile(r'^[A-Z]+\_(\d{1,2})$')
    m = find_some_pattern.a.match(b)
    if m is not None:
        return m.groups()[0]
    return 'NO MATCH'

Now, you ca try it:

try:
    print(find_some_pattern.a)
except AttributeError:
    print('No attribute a yet!')

for s in ('AAAA_1', 'aaa_1', 'BBBB_3', 'BB_333'):
    print(find_some_pattern(s))

print(find_some_pattern.a)

This is the output:

No attribute a yet!
initialize a!
1
NO MATCH
3
NO MATCH
re.compile('^[A-Z]+\\_(\\d{1,2})$')

It is not the best approach (wrappers or callables are way more elegant, and I suggest you use one of them), but I thought this clearly explains the meaning of:

In Python, functions are first-class objects.