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. Why are the results different?
def gc_content(base_seq): """Return the percentage of G and C characters in base_seq""" seq = base_seq.upper() return (seq.count('G') + seq.count('C')) / len(seq) gc_content('attacgcg') / 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.
float() will work too, but it is basically a hack to avoid the issue. from __future__ import division is the correct way of handling this as Python 3 switches behaviour.from __future__ import division be used when using ipython?In python2.x / performs integers division.
>>> 3/2 1 To get desired result you can change either one of the operands to a float using float():
>>> 3/2. #3/2.0 1.5 >>> 3/float(2) 1.5 or use division from __future__:
>>> from __future__ import division >>> 3/2 1.5 
For Python2 / is integer division when the numerator and denominator are both int, you need to make sure to force floating point division
eg.
return (seq.count('G') + seq.count('C')) / float(len(seq)) alternatively, you can put
from __future__ import division at the top of the file
