Division in Python 3 gives different result than in Python 2

Question

In the following code, I want to calculate the percent of G and C characters in a sequence. In Python 3 I correctly get 0.5, but on Python 2 I get 0

Answer 1

/ is a different operator in Python 3; in Python 2 / alters behaviour when applied to 2 integer operands and returns the result of a floor-division instead:

>>> 3/2   # two integer operands
1
>>> 3/2.0 # one operand is not an integer, float division is used
1.5

Add:

from __future__ import division

to the top of your code to make / use float division in Python 2, or use // to force Python 3 to use integer division:

>>> from __future__ import division
>>> 3/2    # even when using integers, true division is used
1.5
>>> 3//2.0 # explicit floor division
1.0

Using either of these techniques works in Python 2.2 or newer. See PEP 238 for the nitty-gritty details of why this was changed.