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

Question

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?

Answer 1

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.

Answer 2

// 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!-)