Why does integer division yield a float instead of another integer?
Question:
Consider this division in Python 3:
>>> 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?
In 2.x, the behaviour was indeed reversed; see How can I force division to be floating point? Division keeps rounding down to 0? for the opposite, 2.x-specific problem.
Answers:
Oops, immediately found 2//2. This will output an int rather than a float.
Take a look at PEP-238: Changing the Division Operator
The // operator will be available to request floor division unambiguously.
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 //.
Behavior of the 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, multiply the dividend or divisor by 1.0.
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 the divisor or dividend is real
Consider this division in Python 3:
>>> 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?
In 2.x, the behaviour was indeed reversed; see How can I force division to be floating point? Division keeps rounding down to 0? for the opposite, 2.x-specific problem.
Oops, immediately found 2//2. This will output an int rather than a float.
Take a look at PEP-238: Changing the Division Operator
The // operator will be available to request floor division unambiguously.
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 //.
Behavior of the 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, multiply the dividend or divisor by 1.0.
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 the divisor or dividend is real