# division – What is the reason for having ‘//’ in Python?

## The Question :

439 people think this question is useful

I saw this in someone’s code:

y = img_index // num_images



where img_index is a running index and num_images is 3.

When I mess around with // in IPython, it seems to act just like a division sign (i.e. one forward slash). I was just wondering if there is any reason for having double forward slashes?

544 people think this answer is useful

In Python 3, they made the / operator do a floating-point division, and added the // operator to do integer division (i.e., quotient without remainder); whereas in Python 2, the / operator was simply integer division, unless one of the operands was already a floating point number.

In Python 2.X:

>>> 10/3
3
>>> # To get a floating point number from integer division:
>>> 10.0/3
3.3333333333333335
>>> float(10)/3
3.3333333333333335



In Python 3:

>>> 10/3
3.3333333333333335
>>> 10//3
3



For further reference, see PEP238.

187 people think this answer is useful

// is unconditionally “flooring division”, e.g:

>>> 4.0//1.5
2.0



As you see, even though both operands are floats, // still floors — so you always know securely what it’s going to do.

Single / may or may not floor depending on Python release, future imports, and even flags on which Python’s run, e.g.:

$python2.6 -Qold -c 'print 2/3' 0$ python2.6 -Qnew -c 'print 2/3'
0.666666666667



As you see, single / may floor, or it may return a float, based on completely non-local issues, up to and including the value of the -Q flag…;-).

So, if and when you know you want flooring, always use //, which guarantees it. If and when you know you don’t want flooring, slap a float() around other operand and use /. Any other combination, and you’re at the mercy of version, imports, and flags!-)

22 people think this answer is useful

To complement these other answers, the // operator also offers significant (3x) performance benefits over /, presuming you want integer division.

$python -m timeit '20.5 // 2' 100,000,000 loops, best of 3: 14.9 nsec per loop$ python -m timeit '20.5 / 2'
10,000,000 loops, best of 3: 48.4 nsec per loop

$python -m timeit '20 / 2' 10,000,000 loops, best of 3: 43.0 nsec per loop$ python -m timeit '20 // 2'
100,000,000 loops, best of 3: 14.4 nsec per loop



21 people think this answer is useful

To complement Alex’s response, I would add that starting from Python 2.2.0a2, from __future__ import division is a convenient alternative to using lots of float(…)/…. All divisions perform float divisions, except those with //. This works with all versions from 2.2.0a2 on.

9 people think this answer is useful

// can be considered an alias to math.floor() for divisions with return value of type float. It operates as no-op for divisions with return value of type int.

import math
# let's examine float returns
# -------------------------------------
# divide
>>> 1.0 / 2
0.5
# divide and round down
>>> math.floor(1.0/2)
0.0
# divide and round down
>>> 1.0 // 2
0.0

# now let's examine integer returns
# -------------------------------------
>>> 1/2
0
>>> 1//2
0