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 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.
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.
