Why does assigning to my global variables not work in Python?

Question

I'm having terrible trouble trying to understand python scoping rules.

With the following script:

a = 7

def printA():
    print "Value of a is %d" % (a)

def setA(value):
    a = value
    print "Inside setA, a is now %d" %(a)


print "Before setA"
printA()
setA(42)
print "After setA"
printA()

Gives the unexpected (to me) output of:

    Before setA
    Value of a is 7
    Inside setA, a is now 42
    After setA
    Value of a is 7

Where I would expect the last printing of the value of a to be 42, not 7. What am I missing about Python's scope rules for the scoping of global variables?

Answer 1

Global variables are special. If you try to assign to a variable a = value inside of a function, it creates a new local variable inside the function, even if there is a global variable with the same name. To instead access the global variable, add a global statement inside the function:

a = 7
def setA(value):
    global a   # declare a to be a global
    a = value  # this sets the global value of a

See also Naming and binding for a detailed explanation of Python's naming and binding rules.