python global variables

Global variables do not conform to the spirit of parameter passing, so I rarely use them unless I define constants. Today, a colleague asked a question about global variables, and I discovered that there is a way to do it.

The program is roughly like this:

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

The running results are as follows:

UnboundLocalError: local variable 'CONSTANT' referenced before assignment

It seems that the global variable has become a local variable in the function modifyConstant. It seems that the global variable has not taken effect?

Made some modifications:

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

is running normally. It seems that global variables can be accessed inside the function.

So, the problem is that because the variable CONSTANT is modified inside the function, Python considers CONSTANT to be a local variable, and print CONSTANT is before CONSTANT += 1, so of course this error will occur.

So, how to access and modify global variables inside a function? The keyword global should be used to modify variables (a bit like PHP):

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