Python Division //

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.

// is unconditionally "truncating division", e.g:

>>> 4.0//1.5
2.0

As you see, even though both operands are floats, // still truncates -- so you always know securely what it's gonna do.

Single / may or may not truncate 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 truncate, 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 truncation, always use //, which guarantees it. If and when you know you don't want truncation, slap a float() around other operand and use /. Any other combination, and you're at the mercy of version, imports, and flags!-)

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'
100000000 loops, best of 3: 0.0149 usec per loop
$ python -m timeit '20.5 / 2'
10000000 loops, best of 3: 0.0484 usec per loop
$ python -m timeit '20 / 2'
10000000 loops, best of 3: 0.043 usec per loop
$ python -m timeit '20 // 2'
100000000 loops, best of 3: 0.0144 usec per loop