Python global variable Usage example analysis _python

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Global variables do not conform to the spirit of parameter passing, so I rarely use them unless we define constants. Today, a colleague asked a question about the global variable, only to find that there was a doorway.

The program is roughly like this:

CONSTANT = 0
def modifyconstant ():
    print CONSTANT
    CONSTANT + + 1 return
if __name__ = ' __main__ ': 
   modifyconstant ()
    print CONSTANT

The results of the operation are as follows:

Unboundlocalerror:local variable ' CONSTANT ' referenced before assignment

It seems that the global variable in the function Modifyconstant edge into a local variable, it seems that the global variable does not take effect?

Make some changes:

CONSTANT = 0
def modifyconstant ():
    print CONSTANT
    #constant + + 1 return
if __name__ = ' __main __ ':
    modifyconstant ()
    print CONSTANT

Functioning normally, it seems that the function is internally accessible to global variables.

So, the problem is, because the variable Constant,python inside the function is considered constant is a local variable, and print constant is constant + 1, so of course this error occurs.

So how do you access and modify global variables inside a function? You should use the keyword global to modify variables (a bit like PHP):

CONSTANT = 0
def modifyconstant ():
    global CONSTANT
    print CONSTANT
    CONSTANT + = 1 return
if __ name__ = = ' __main__ ':
    modifyconstant ()
    print CONSTANT

It's that simple!

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