Why does Python floor division return a float when the divisor and/or dividend is a float?

Question

If I understand correctly, the return value for floor division is always a whole number, even if the dividend and/or divisor are not whole numbers, so why does it not always return an integer.

It's detrimental in my case because converting from a large float to an int instead of having the return value as an arbitrary-precision integer obviously loses precision.

I can't see any function that does float floor division to return an integer. Obviously I could make a function to do so, e.g. by multiplying both values by the same amount so that they're both integers, but it would be a lot slower than a C implementation.

Here's an example: 5.2 // 2 is 2.0 not 2.

Answer 1

In answer to your question why?, it is by design, and the rationale for this is in PEP 238:

Floor division will be implemented in all the Python numeric types, and will have the semantics of:

a // b == floor(a/b)

except that the result type will be the common type into which a and b are coerced before the operation.

Specifically, if a and b are of the same type, a//b will be of that type too. If the inputs are of different types, they are first coerced to a common type using the same rules used for all other arithmetic operators.

...

For floating point inputs, the result is a float. For example:

3.5//2.0 == 1.0

For complex numbers, // raises an exception, since floor() of a complex number is not allowed.

This PEP dates back to Python 2.2. I've suppressed a paragraph that discusses the now obsolete distinction between int and long.