define different function for different versions of python

Question

Is there any way to define a different function (with the same body) for different version of python?

specifically, for python 2.7 define:

def __unicode__(self): 

and for python 3 define:

def __str__(self):

but both will have the same code / body. both must also be a member of class.

Answer 1

While there are compatibility libraries; six and future being the 2 most widely known, sometimes one needs to live without compatibility libs. You can always write your own class decorator, and put it into say mypackage/compat.py. The following works nicely for writing the class in Python 3 format and converting the 3-ready class to Python 2 if needed (the same can be used for next vs __next__, etc:

import sys

if sys.version_info[0] < 3:
    def py2_compat(cls):
        if hasattr(cls, '__str__'):
            cls.__unicode__ = cls.__str__
            del cls.__str__
            # or optionally supply an str that 
            # encodes the output of cls.__unicode__
        return cls
else:
    def py2_compat(cls):
        return cls

@py2_compat
class MyPython3Class(object):
    def __str__(self):
        return u'Here I am!'

(notice that we are using u'' prefix which is PyPy 3, and Python 3.3+ compatible only, so if you need to be compatible with Python 3.2, then you need to adjust accordingly)

---

To supply a __str__ method that encodes the __unicode__ to UTF-8 in Python 2, you can replace the del cls.__str__ with

def __str__(self):
    return unicode(self).encode('UTF-8')
cls.__str__ = __str__