That is interesting. See these examples. I'm starting a new interpreter each time:

>>> import inspect
>>> def abc(): pass
...
>>> inspect.getclosurevars(abc)
ClosureVars(nonlocals={}, globals={}, builtins={}, unbound=set())
>>>


>>> import inspect
>>> def abc(): a
...
>>> inspect.getclosurevars(abc)
ClosureVars(nonlocals={}, globals={}, builtins={}, unbound={'a'})
>>>

>>> import inspect
>>> a=0
>>> def abc(): a
...
>>> inspect.getclosurevars(abc)
ClosureVars(nonlocals={}, globals={'a': 0}, builtins={}, unbound=set())
>>>

>>> import inspect
>>> a=0
>>> def abc(): a.b
...
>>> inspect.getclosurevars(abc)
ClosureVars(nonlocals={}, globals={'a': 0}, builtins={}, unbound={'b'})
>>>

>>> import inspect
>>> a=0
>>> b=1
>>> def abc(): a.b
...
>>> inspect.getclosurevars(abc)
ClosureVars(nonlocals={}, globals={'a': 0, 'b': 1}, builtins={}, unbound=set())
>>> 

It's odd.