What is the reason for having '//' in Python? [duplicate]

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:

>>> 10/33>>> # To get a floating point number from integer division:>>> 10.0/33.3333333333333335>>> float(10)/33.3333333333333335

In Python 3:

>>> 10/33.3333333333333335>>> 10//33

For further reference, see PEP238.

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

>>> 4.0//1.52.0

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!-)

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