Have you used the variable sum anywhere else? That would explain it.
>>> sum = 1
>>> numbers = [1, 2, 3]
>>> numsum = (sum(numbers))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'int' object is not callable
The name sum doesn't point to the function anymore now, it points to an integer.
Solution: Don't call your variable sum, call it total or something similar.