Schrödinger's variable

▶ Schrödinger's variable *

The asterisk at the end of the title indicates the example was not present in the first release and has been recently added.

def make_func():
    for target in ('a', 2, None):
        if isinstance(target, str):
            def f():
                print(f'inside f(), {target=}')
            print(f'just created f(), {target=} at this point!')
    return f

Output (Python version):

>>> f = make_func()
target='a' at this point!

>>> f()
inside f(), target=None

💡 Explanation:

Python doesn't actually bind the value of target in f(). It just creates a function that will look up target in the surrounding context.
Since target will continue to be updated until it is set to None in the last iteration of the for loop, target=None when it prints inside of f()

Output:

>>> f.__code__.co_freevars
('target',)
>>> import inspect
>>> print(inspect.getclosurevars(f).nonlocals['target'])
None

This might be made a bit clearer if we use a global variable rather than a free variable:

for target in ('a', 2, None):
    if isinstance(target, str):
        def f():
            print(f'inside f(), {target=}')
        print(f'just created f(), {target=} at this point!')

Output.

just created f(), target='a' at this point!
>>> print(target)
None
>>> f()
inside f(), target=None
>>> target = "whoa"
inside f(), target='whoa'