This does something different. a // b tries to "fit" b into a as many times as possible. For example, 1.0 // 0.4 == 2.0 because 0.4 fits twice into 1.0 (with a remainder of 0.2). Though as the result should always be an integer, I'd argue that the result should actually be 2, not 2.0. But alas, it's not.
With your change, you calculate 1 // 0 instead, which crashes.
That said, I think checking isinstance(x, float) was always possible. (And nowadays, you can put a type annotation.)
