Python version Check (major attribute)

Question

What does sys.version_info.major do?

I understand that sys.version_info returns the version of interpreter we are using. What role does major have to do in this?

How does it work in the following code?

import sys 

if sys.version_info.major < 3:
    #code  here   
else:
    #code here

Answer 1

Python 3.5.1 (v3.5.1:37a07cee5969, Dec  5 2015, 21:12:44)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.version_info
sys.version_info(major=3, minor=5, micro=1, releaselevel='final', serial=0)

major can tell you whether it's python2 or python3. I guess it imports different libraries depends on the specific python versions.

I am using python 3.5.1, you can see the version_info about it.

Answer 2

For example, I have Python 3.5.3 installed on my computer. sys_version stores the following information:

sys.version_info(major=3, minor=5, micro=3, releaselevel='final', serial=0)

The reason version is split into major, minor and micro is that programmers writing code for multiple versions of Python may need to differentiate between them somehow. Most differences are between major versions 2 and 3 of Python and that explains the context of major usage in your code.

Answer 3

Here's a trite example:

from __future__ import print_function
import sys 
  if sys.version_info.major<3:
           input = raw_input  # Python2 ~== eval(input())  
  else:
            #code here

This makes Python 2.x behave a bit more like Python 3 ... the future import cannot be in the if suite because it must appear before other imports. But the changes in semantics for the input() builtin function between Python 2.x and Python 3 can handled in a conditional.

For the most part it's a bad idea to go much beyond this in your efforts to support Python2 and Python3 in the same file.

The more challenging changes between Python2 and Python3 relate to strings as bytes vs. the newer strings as Unicode; and thus the need to explicitly specify encodings and use bytes objects for many networking protocols and APIs.