With the end of support for Python 2 on the horizon (in 2020), many package developers have made their packages compatible with both Python 2 and Python 3 by using constructs such as:

if sys.version_info[0] == 2:
    # Python 2 code
else:
    # Python 3 code

in places where things have changed between Python 2 and 3.

The six package simplifies many of the differences by providing wrappers that behave the same on Python 2 and 3. For instance, iterating over dictionary keys is normally done with:

for item in dictionary.iteritems():
    # code here

in Python 2 and:

for item in dictionary.items():
    # code here

in Python 3. With six, one can simply do:

import six

for item in six.iteritems(dictionary):
    # code here

and this will work seamlessly both with Python 2 and 3. However, there are some more complex cases where one has to resort to the type of if statement shown at the top of this post. The six package again makes this slightly easier by providing PY2 and PY3 boolean constants:

if six.PY2:
    # Python 2 code
else:
    # Python 3 code

So far so good.

This brings me to the main point of this post. We don't really know yet what Python 4 will look like, but we can be pretty sure that the transition from Python 3 to Python 4 will be a lot smoother and will likely not be backward-incompatible in the same way as Python 3 was. If that's the case, we should be able to use packages developed for Python 2 and 3 seamlessly with Python 4. Right?...

Not quite! By searching on GitHub, I found almost 300,000 matches for the following kind of syntax:

if six.PY3:
    # Python 3 code
else:
    # Python 2 code

See the problem? In six, PY3 is defined as:

PY3 = sys.version_info[0] == 3

so that once Python 4 is used, both PY2 and PY3 will be (correctly) False and the above if statement will execute the else statement for Python 2 code. Oops!

Another example of problematic code is the following:

if six.PY2:
    # Python 2 code
elif six.PY3:
    # Python 3 code

In this case, no code will get executed on Python 4 at all!

To avoid this, it's critical that we avoid treating Python 3 as a special case in these kinds of if statements and instead treat Python 2 as the special case, and default to Python 3 code otherwise:

if six.PY2:
    # Python 2 code
else:
    # Python 3 code

It's a small change, but it will save a lot of headaches down the road. So if you develop a Python package, please check now to make sure that your code will be Python 4-compatible!

Update: of course, the same logic applies even if not using the six package. If doing version comparisons using sys.version_info, make sure you don't do:

if sys.version_info[0] == 3:
    # Python 3 code
else:
    # Python 2 code

Either swap the if statement and check for Python 2, or make sure you use >=.