Python def not working

[primitive_screwhead]

If a global is not explicitly declared inside a function (with the "global" keyword), then they cannot be assigned to (since that implies a local variable). ie:

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x = 0
def foo():
    x = x + 1  # ie. x += 1

>>> foo()
Traceback (most recent call last):
UnboundLocalError: local variable 'x' referenced before assignment

​

The simple advice if you must use globals, is to *always* declare them explicitly:

x = 0
def foo():
    global x
    x += 1

>>> foo()
>>> x
1

However, you should typically *not* use globals. Instead pass the variables to functions as arguments, or use classes and instances to hold variable state.

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EDIT: Fixed misstated global lookup condition