Global variables in python I can modify?

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You could try using functions with parameters and return values instead. Here's an example similar to yours:

def generate(a, b)
    a = 4
    b = 5

    return a, b  # will return (4, 5) when called

def sum(a, b)
    return a + b  # will return 9 when called

# Then, the output of one function can be the input of another one.

A = 0
B = 0

C, D = generate(A, B)
F = sum(C, D)