Python 3 transition guide

Switching to Python 3 can seem like a daunting task, but this guide will provide some tips and resources to help make it more straightforward.

Changes you can make today to make the transition easier

Any new code you write can easily be Python 3 compatible! This is the first step you can take towards switching. Thankfully, the most disruptive changes to the language have been back-ported to Python 2 so you can ensure that, when writing new code that you intend to execute with Python 2, it will be executable in Python 3 once you switch. To use these back-ported changes, you must import certain utilities from the built-in `__future__ <>`_ package. It is generally recommended that, if you are still writing Python 2 compatible code, you should import the following four modules in any code you write

from __future__ import division, print_function, absolute_import

Copy any paste this into the import block of any new script or piece of code you write in Python 2! Most notably, (1) this will enable you to use the Python 3-style print function:

>>> print("It's only a model")

The Python 2 print statement will now fail:

>>> print "It's only a model"
  File "<stdin>", line 1
    print "It's only a model"
SyntaxError: invalid syntax

And (2), integer division will no longer truncate:

>>> 1/2

will become:

>>> 1/2

For now, don’t worry about absolute_import (but do include it in your import!). If you’d like to know more, this is a good blog post that explains what this does. If you want to know even more, here is a StackOverflow post about it.

Other major changes in Python 3

A slightly more annoying issue is that several of the standard library Python packages have been reorganized or moved. Most of the changes were made to fairly obscure packages (full list here), but there are a few notable changes:

  • cPickle - renamed to pickle
  • cProfile - renamed to profile
  • urllib, urllib2, urlparse - have been combined into subpackages of urllib

To move to Python 3, you can simply replace the imports. To maintain compatibility between Python 2 and 3, an easy solution is to use the six package (see discussion below in Advanced: Writing code that is compatible with Python 2 and 3).

One other major change is that many built-in methods on container classes (e.g., the dictionary) now return iterators instead of list s. For example, in Python 2

>>> airspeed = {'ladened': 2, 'unladened': 11}
>>> airspeed.keys()
['ladened', 'unladened']
>>> airspeed.keys()[0]

In Python 3, the .keys() method instead returns an iterator object

>>> airspeed = {'ladened': 2, 'unladened': 11}
>>> airspeed.keys()
dict_keys(['ladened', 'unladened'])
>>> airspeed.keys()[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'dict_keys' object does not support indexing

These iterator objects support iteration (e.g., you can loop over the dict_keys as you would a list), but as shown above, do not support indexing. To get a list, just wrap any of these methods in a call to list(), e.g.

>>> the_keys = list(airspeed.keys())
>>> the_keys[0]

Upgrading existing Python 2 code

If you have a lot of code with Python 2 print statements and other Python 3-incompatible lines, it can be a huge pain to go file-by-file and modify the code to be compliant. Python provides a tool – 2to3 – to automatically identify code that will error in Python 3 and, optionally, to update it in place. The Python documentation has a page on how to use this tool. For a single file, calling 2to3 on the file without any flags will output a diff showing any invalid code. For example, if we have a script called “” containing a single line print "yo"

% 2to3
--- (original)
+++ (refactored)
@@ -1 +1 @@
-print "yo"
RefactoringTool: Files that need to be modified:

Note that it finds the Python 2-style print statement and even tells you what to change it to, but this did not modify the file to make it compatible. To actually fix the incompatible code, call with the -w flag

% 2to3 -w
--- (original)
+++ (refactored)
@@ -1 +1 @@
-print "yo"
RefactoringTool: Files that were modified:

This can also be run on entire packages or directory trees to update code in bulk.

Give Python 3 a try

To switch to Python 3, you don’t have to permanently leave Python 2 behind and you don’t have to switch in one sitting. Many of us are trying to balance a complicated workflow, deadlines, and software update requirements, so luckily it is possible to try out Python 3 while continuing to work in Python 2 in a separate Python environment. This is most easily done using a virtual environment manager. With virtual environments, you can switch over to Python 3, experiment, see what code runs, what code breaks, but then easily switch back to Python 2 if need be.

There are several possible choices for managing virtual environments in Python but to manage multiple versions of Python we have found the Anaconda package manager to be the best all-in-one option. If you are not using Anaconda, we highly recommend installing it and using it for package and environment management! If you have a separate Python installation and prefer it, you can still install Anaconda to play with switching to Python 3 without messing up your other environment.

If you are using Anaconda for Python 2 or have just installed Anaconda, it’s easy to create a new environment that uses Python 3. If you have never used Anaconda (conda) environments, you should have only one environment. If you type

% conda env list

in your terminal, you should see a single line like

root                  *   /Users/adrian/anaconda

This just tells you that you only have a single (‘root’) environment. It can be useful to have two main installations of Python for testing: one that uses the latest Python 2 version and one that uses the latest Python 3 version. Here we’ll create these two environments and name them two and three. For your main Python 2 environment, you can clone your root environment over (and therefore copy over any packages you’ve installed) by doing

% conda create --name two --clone root

If instead you’d like to create a fresh installation of Python 2 in the new environment, you can do

% conda create --name two python=2

(the python=2 tells conda to install the latest version of Python 2 in the environment named two). We can do the same thing to create a new environment for Python 3

% conda create --name three python=3

Again, the python=3 tells conda to install the latest version of Python 3 in this new environment (named three). To enable an environment, you use

% source activate <name of environment>

So, for each of these you can use

% source activate two


% source activate three

to switch back and forth between Python 2 and 3! After installing Python 3, you may find yourself typing these commands a lot to switch back and forth – you may want to create aliases in your shell profile to make it faster

% alias pytwo="source activate two"
% alias pythree="source activate three"

Once these two environments are set up, you may want to stop using the root environment so you can quickly tell whether you are using Python 2 or 3. But, by default any new shell you open will use the root environment. An easy way to change this is to activate whichever environment you want to use as default in your profile or rc file as well. For example, if you want to move to Python 3 you can add


below the definition of your alias, which will call source activate three whenever the profile or rc file is run.

Advanced: Writing code that is compatible with Python 2 and 3

As mentioned above, a number of standard library packages have been reorganized or renamed, meaning that import statements may fail when executing code in either Python 2 or 3. The pip-installable six package is here to help! six has a subpackage that normalizes the import paths for these cases so you don’t have to write extra code to check whether the code is executed in 2 or 3. For example:

  • cPickle / pickle
>>> from six.moves import cPickle as pickle
  • urllib, urllib2
>>> from six.moves import urllib

This will work in Python 2 or 3. This website contains a number of other useful tips for maintaining code that runs in both 2 and 3.