Python 3 int division operator is returning a float?

Question

In one of my assignments I came across a weird implementation, and I was curious if it's a bug or the designed behavior.

In Python 3, division by / returns a floating point number, and // means integer division and should return an integer. I've discovered though that if either of the values is a float when doing integer division, it will return a float.

Example:

# These all work as expected
10 / 2
-> 5.0
11 / 2
-> 5.5
10 // 2
-> 5
11 // 2
-> 5
# Here things start to get weird
10.0 // 2
-> 5.0
10 // 2.0
-> 5.0
11.0 // 2
-> 5.0

Is this supposed to behave this way? If so, why does it behave this way?

Answer 1

From PEP-238, which introduced the new division (emphasis mine):

Semantics of Floor Division

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.

In particular, if a and b are both ints or longs, the result has the same type and value as for classic division on these types (including the case of mixed input types; int//long and long//int will both return a long).

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.

For user-defined classes and extension types, all semantics are up to the implementation of the class or type.

So yes, it is supposed to behave that way. "// means integer division and should return an integer" - not quite, it means floor division and should return something equal to an integer (you'd always expect (a // b).is_integer() to be true where either operand is a float).