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Python Exception: UnboundLocalError: local variable '…' referenced before assignment

The following simple script causes the Exception UnboundLocalError to be raised.

This is because Python assumes a variable to be local to a Python functions if this variable is being assigned to in that function.

a_global_var = 42

def increment_global_var():
    a_global_var += 1

increment_global_var()

In order to be able to assign a value to a global variable in a function, it must be declared as global using the global statement.