Same code in Python 3.x and Python 2.x gives different results

Question

I've built a function to calculate median (as the name indicates).

The function receives a list of ints, for instances [4,5,5,4] and should output 4,5.

The code:

def median(lst):
    lst.sort()
    a=int(len(lst)/2)
    if len(lst) % 2 == 0:
        med=float((lst[a]+lst[a-1])/2)
    else:
        med=lst[a]
    return med
print(median([4,5,5,4]))

This works fine when using Python 3.x but not in Python 2.x. What am i doing wrong?

Answer 1

The problem is you are assuming the division operator is the same in both Python 2 and 3. Instead (adapted from mhlester):

• in Python 2, / is integer division (int inputs);

• in Python 3, / is float division;

• in Python 2 and 3, // is integer division.

In order to achieve what you are looking for, there's different workarounds. For instances:

1. You can make sure to have at least one operand of your division in float. Eg:

   def median(lst):
       lst.sort()
       a=int(len(lst)/float(2))
       if len(lst) % 2 == 0:
           med=float((lst[a]+lst[a-1])/float(2))
       else:
           med=lst[a]
       return med
   print(median([4,5,5,4])) 
   

2. You can use the import from __future__ import division (this needs to be done before any other imports). Eg:

   from __future__ import division
   
   def median(lst):
       lst.sort()
       a=int(len(lst)/2)
       if len(lst) % 2 == 0:
           med=float((lst[a]+lst[a-1])/2)
       else:
           med=lst[a]
       return med
   print(median([4,5,5,4]))