Defining Global Variables

Question

I was just wondering, in python 3, why do some global variables have to be defined as global every singe function you use (that you're defining), but others only need to be be defined once? Probably a question because I haven't understood scope properly, please could someone shed some light?

Answer 1

Variables in a scope can be read but not modified by functions/objects declared within that scope. If you also wish to change their value, you have to declare them as global.

UPDATE: However, please note, that "changing the value" may or may not mean what you are eventually used to if comming from other programming languages, as assignment in Python means giving an object a name. The keyword global allows you to let the given name to point to (being associated with) a different object. Look at the following example:

aa = 1
bb = 2
cc = [ 1, 2, 3 ]

def test():
    global aa
    aa = -1
    bb = -1
    cc[2] = -1

test()
print "aa:", aa
print "bb:", bb
print "cc:", cc

would result in:

aa: -1
bb: 2
cc: [1, 2, -1]

• In the function test() the name aa is declared as global, so aa = -1 in the function would cause to point the global name aa to a new object, namely the integer -1.

• The name bb in test() is not declared as global, therefore the assignment bb = -1 would create the name bb locally within the function and point it to -1, while leaving the name bb in the global scope still pointing to the value 2.

• Variable cc is also not declared as global, therefore, you can't change the association of the global cc in the function test(). However, since in the global scope cc is associated with a mutable object, you can change the content of this object within the function.

Note, that the formulation here is maybe not the official Python way of explaining the differences, but it may help to understand things if you have some background in other programming languages.