How do I use global variables in python functions? [duplicate]

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To use global variables inside a function, you need to do global <varName> inside the function, like so.

testVar = 0

def testFunc():
    global testVar
    testVar += 1

print testVar
testFunc()
print testVar

gives the output

>>> 
0
1

Keep in mind, that you only need to declare them global inside the function if you want to do assignments / change them. global is not needed for printing and accessing.

You can do,

def testFunc2():
    print testVar

without declaring it global as we did in the first function and it’ll still give the value all right.

Using a list as an example, you cannot assign a list without declaring it global but you can call it’s methods and change the list. Like follows.

testVar = []
def testFunc1():
    testVar = [2] # Will create a local testVar and assign it [2], but will not change the global variable.

def testFunc2():
    global testVar
    testVar = [2] # Will change the global variable.

def testFunc3():
    testVar.append(2) # Will change the global variable.

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