Example analysis of Python global variables usage

This example describes the Python global variable usage. Share to everyone for your reference, as follows:

Global variables do not conform to the spirit of parameter passing, so, usually I seldom use unless I define constants. Today, a colleague asked a question about global variables and found that there were doorways.

The procedure is broadly like this:

CONSTANT = 0def modifyconstant ():    print CONSTANT    CONSTANT + = 1    returnif __name__ = = ' __main__ ':    modifyc Onstant ()    print CONSTANT

The results of the operation are as follows:

Unboundlocalerror:local variable ' CONSTANT ' referenced before assignment

It appears that the global variable is a local variable in the function modifyconstant, and it seems that the global variable is not in effect?

Make some changes:

CONSTANT = 0def modifyconstant ():    print CONSTANT    #constant + = 1    returnif __name__ = = ' __main__ ':    Modify Constant ()    print Constant

Running normally, it seems that the function can access the global variables inside.

So, the problem is, because the variable Constant,python inside the function thinks constant is a local variable, and the print constant is before 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 the variable (a bit like PHP):

CONSTANT = 0def modifyconstant ():    global CONSTANT    print CONSTANT    CONSTANT + = 1    returnif __name__ = = ' __m Ain__ ':    modifyconstant ()    print CONSTANT

It's so easy!

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I hope this article is helpful for Python program design.

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