Variable scope with dictionaries

Question

This executes without error:

def myfunction():
    global a_dict
    a_dict['s'] = 'whatevs'
    b_dict['s'] = 'lalala'
    a_dict = b_dict

a_dict = {'s':[], 'm':[]}
b_dict = {'s':[], 'm':[]}
myfunction()

Why does this not throw an error without also adding global b_dict to the function - as I'm writing to both dictionaries?

It sure does if I comment out the global a_dict ("'a_dict' referenced before assignment") - as I would expect.

You can reduce your chances of getting downvoted by spelling "I" correctly. — mkrieger1, Jan 17 '21 at 17:31

score 1 · Accepted Answer · answered Jan 17 '21 at 17:39

You only need global foo when you want to assign to the global name foo. You are not assigning to b_dict; you are assigning to one of its keys, equivalent to b_dict.__setitem__('s', 'lalala'). Both uses of b_dict treat it as a free variable, since there is no local variable named b_dict defined.

The value of a free variable is taken from the closest enclosing scope (in this case, the global scope) where it is defined.

score 0 · Answer 2 · answered Jan 17 '21 at 17:36

0

The fact of declaring a variable as global in a function allows you to use a variable outside that function. If there is no variable b declared inside it, Python is going to assume it is a global variable declared before the function call.

answered Jan 17 '21 at 17:36

Janska

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Variable scope with dictionaries

2 Answers2