Python tutorial global variable usage

The examples in this article describe the usage of Python global variables. Share it with everyone for your reference, the details are as follows:

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 procedure is roughly as follows:

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 becomes a local variable in the function modifyConstant. 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

It runs 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? Variables should be modified using the keyword global (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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I hope this article will be helpful to everyone in Python programming.