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Only the Python object None has type NoneType.

Usually a return value of None means the function did not return anything.

This frequently happens when one defines a function which does a computation but forgets to return the result, as in:

def list_cartesian_product(F, n):
    K = list(cartesian_product([F]*n))

instead of

def list_cartesian_product(F, n):
    K = list(cartesian_product([F]*n))
    return K