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

248
votes
Python 3.1 (r31:73574, Jun 26 2009, 20:21:35) [MSC v.1500 32 bit (Intel)] on win32
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>>> 2/2
1.0

Is this intended? I strongly remember earlier versions returning int/int=int? What should I do, is there a new division operator or must I always cast?

5
pythonintegerpython-3.xdivision
Yes, that's the way division works in 3.x. – hughdbrown
Here's a post by Python's creator talking about how the rounding works, it's orthogonal to your question but I found it interesting: python-history.blogspot.com/2010/08/… – Aaron D
@hughdbrown so that means for all python 3 version essentially? – Charlie Parker
@hughdbrown: yes, PEP is final, meaning both accepted and implemented. – Jonas Byström
If you are doing anything other than simple division, then casting the float result is a better method. For example, calculating 480 // 640 * 320 results in 0 – brianlmerritt

5 Answers

312
votes

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

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

63
votes

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

52
votes

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
22
votes

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 //.

0
votes

According to Python 3 documentation, Python when divided by integer, will generate float despite expected to be integer.

For exclusively printing integer,use floor division method. Floor division is rounding off zero and removing decimal point. Represented by //

Hence, instead of 2/2 ,use 2//2

You can also import division from __future__ irrespective of using Python 2 or Python 3.