Why does integer division yield a float instead of another integer?

Question

Take a look at PEP-238: Changing the Division Operator

The // operator will be available to request floor division unambiguously.

Oops, immediately found 2//2. This will output an int rather than a float.

Behavior of Division Operator in Python 2.7 and Python 3

In Python 2.7: By default, division operator will return integer output.

To get the result in double multiple 1.0 to "dividend or divisor"

100/35 => 2 #(Expected is 2.857142857142857)
(100*1.0)/35 => 2.857142857142857
100/(35*1.0) => 2.857142857142857

In Python 3

// => used for integer output
/ => used for double output
    
100/35 => 2.857142857142857
100//35 => 2
100.//35 => 2.0    # floating-point result if divsor or dividend real

The accepted answer already mentions PEP 238. I just want to add a quick look behind the scenes for those interested in what's going on without reading the whole PEP.

Python maps operators like +, -, * and / to special functions, such that e.g. a + b is equivalent to

a.__add__(b)

Regarding division in Python 2, there is by default only / which maps to __div__ and the result is dependent on the input types (e.g. int, float).

Python 2.2 introduced the __future__ feature division, which changed the division semantics the following way (TL;DR of PEP 238):

• / maps to __truediv__ which must "return a reasonable approximation of the mathematical result of the division" (quote from PEP 238)
• // maps to __floordiv__, which should return the floored result of /

With Python 3.0, the changes of PEP 238 became the default behaviour and there is no more special method __div__ in Python's object model.

If you want to use the same code in Python 2 and Python 3 use

from __future__ import division

and stick to the PEP 238 semantics of / and //.