Why do you need to state both ‘x’ and ‘y’ before the ‘:’?
Because a lambda is (conceptually) the same as a function, just written inline. Your example is equivalent to
def f(x, y) : return x + y
just without binding it to a name like f.
Also how do you make it return multiple arguments?
The same way like with a function. Preferably, you return a tuple:
lambda x, y: (x+y, x-y)
Or a list, or a class, or whatever.
The thing with self.entry_1.bind should be answered by Demosthenex.