Multiple return values in python

Question

I want to change a python function to return two values. How do I achieve that without affecting any of the previous function calls which only expect one return value?

For eg.

Original Definition:

def foo():
    x = 2
    y = 2
    return (x+y)

sum = foo()

Ne Definition:

def foo():
    x = 2
    y = 2
   return (x+y), (x-y)

sum, diff = foo()

I want to do this in a way that the previous call to foo also remains valid? Is this possible?

Answer 1

def foo(return_2nd=False):

    x = 2
    y = 2
    return (x+y) if not return_2nd else (x+y),(x-y)

then call new version

sum, diff = foo(True)
sum = foo() #old calls still just get sum

Answer 2

By changing the type of return value you are changing the "contract" between this function and any code that calls it. So you probably should change the code that calls it.

However, you could add an optional argument that when set will return the new type. Such a change would preserve the old contract and allow you to have a new one as well. Although it is weird having different kinds of return types. At that point it would probably be cleaner to just create a new function entirely, or fix the calling code as well.