Unbound Variables in Python

Consider the following example

def take_sum(a, b, c):
    return a+b+c

def main():
    print(take_sum)

main()

Outputs

<function take_sum at 0x1031c84c0>

Python looks for take_sum in local scope ie., inside main followed by global scope whereas,

def main():
    if "take_sum" not in globals():
        take_sum = lambda x,y,z: x+y+z
    print(take_sum)

Returns: UnboundLocalError: local variable 'take_sum' referenced before assignment

The function raises an exception because the compiler saw that the code could assign to take_sum as a local variable, and so it makes all the references to take_sum in the code be local. You can no longer look up the global variable take_sum in the normal way once that determination has been made.

To understand how variable names are resolved read Python Namespaces