Check that function returns a value [return]

If a function has a non-None return type, mypy expects that thefunction always explicitly returns a value (or raises an exception).The function should not fall off the end of the function, since thisis often a bug.

Example:

# Error: Missing return statement  [return]
def show(x: int) -> int:
    print(x)
 
# Error: Missing return statement  [return]
def pred1(x: int) -> int:
    if x > 0:
        return x - 1
 
# OK
def pred2(x: int) -> int:
    if x > 0:
        return x - 1
    else:
        raise ValueError('not defined for zero')