Porting Python 2 Code to Python 3

author:Brett Cannon


With Python 3 being the future of Python while Python 2 is still in active use, it is good to have your project available for both major releases of Python. This guide is meant to help you figure out how best to support both Python 2 & 3 simultaneously.

If you are looking to port an extension module instead of pure Python code, please see Porting Extension Modules to Python 3.

If you would like to read one core Python developer’s take on why Python 3 came into existence, you can read Nick Coghlan’s Python 3 Q & A.

For help with porting, you can email the python-porting mailing list with questions.

The Short Explanation

To make your project be single-source Python 2/3 compatible, the basic steps are:

  1. Update your code to drop support for Python 2.5 or older (supporting only Python 2.7 is ideal)
  2. Make sure you have good test coverage (coverage.py can help; pip install coverage)
  3. Learn the differences between Python 2 & 3
  4. Use Modernize or Futurize to update your code (pip install modernize or pip install future, respectively)
  5. Use Pylint to help make sure you don’t regress on your Python 3 support (if only supporting Python 2.7/3.4 or newer; pip install pylint)
  6. Use caniusepython3 to find out which of your dependencies are blocking your use of Python 3 (pip install caniusepython3)
  7. Once your dependencies are no longer blocking you, use continuous integration to make sure you stay compatible with Python 2 & 3 (tox can help test against multiple versions of Python; pip install tox)

If you are dropping support for Python 2 entirely, then after you learn the differences between Python 2 & 3 you can run 2to3 over your code and skip the rest of the steps outlined above.


A key point about supporting Python 2 & 3 simultaneously is that you can start today! Even if your dependencies are not supporting Python 3 yet that does not mean you can’t modernize your code now to support Python 3. Most changes required to support Python 3 lead to cleaner code using newer practices even in Python 2.

Another key point is that modernizing your Python 2 code to also support Python 3 is largely automated for you. While you might have to make some API decisions thanks to Python 3 clarifying text data versus binary data, the lower-level work is now mostly done for you and thus can at least benefit from the automated changes immediately.

Keep those key points in mind while you read on about the details of porting your code to support Python 2 & 3 simultaneously.

Drop support for Python 2.5 and older (at least)

While you can make Python 2.5 work with Python 3, it is much easier if you only have to work with Python 2.6 or newer (and easier still if you only have to work with Python 2.7). If dropping Python 2.5 is not an option then the six project can help you support Python 2.5 & 3 simultaneously (pip install six). Do realize, though, that nearly all the projects listed in this HOWTO will not be available to you.

If you are able to only support Python 2.6 or newer, then the required changes to your code should continue to look and feel like idiomatic Python code. At worst you will have to use a function instead of a method in some instances or have to import a function instead of using a built-in one, but otherwise the overall transformation should not feel foreign to you.

But please aim for Python 2.7. Bugfixes for that version of Python will continue until 2020 while Python 2.6 is no longer supported. There are also some tools mentioned in this HOWTO which do not support Python 2.6 (e.g., Pylint), and this will become more commonplace as time goes on.

Make sure you specify the proper version support in your setup.py file

In your setup.py file you should have the proper trove classifier specifying what versions of Python you support. As your project does not support Python 3 yet you should at least have Programming Language :: Python :: 2 :: Only specified. Ideally you should also specify each major/minor version of Python that you do support, e.g. Programming Language :: Python :: 2.7.

Have good test coverage

Once you have your code supporting the oldest version of Python 2 you want it to, you will want to make sure your test suite has good coverage. A good rule of thumb is that if you want to be confident enough in your test suite that any failures that appear after having tools rewrite your code are actual bugs in the tools and not in your code. If you want a number to aim for, try to get over 80% coverage (and don’t feel bad if you can’t easily get past 90%). If you don’t already have a tool to measure test coverage then coverage.py is recommended.

Learn the differences between Python 2 & 3

Once you have your code well-tested you are ready to begin porting your code to Python 3! But to fully understand how your code is going to change and what you want to look out for while you code, you will want to learn what changes Python 3 makes in terms of Python 2. Typically the two best ways of doing that is reading the “What’s New” doc for each release of Python 3 and the Porting to Python 3 book (which is free online). There is also a handy cheat sheet from the Python-Future project.

Update your code

Once you feel like you know what is different in Python 3 compared to Python 2, it’s time to update your code! You have a choice between two tools in porting your code automatically: Modernize and Futurize. Which tool you choose will depend on how much like Python 3 you want your code to be. Futurize does its best to make Python 3 idioms and practices exist in Python 2, e.g. backporting the bytes type from Python 3 so that you have semantic parity between the major versions of Python. Modernize, on the other hand, is more conservative and targets a Python 2/3 subset of Python, relying on six to help provide compatibility.

Regardless of which tool you choose, they will update your code to run under Python 3 while staying compatible with the version of Python 2 you started with. Depending on how conservative you want to be, you may want to run the tool over your test suite first and visually inspect the diff to make sure the transformation is accurate. After you have transformed your test suite and verified that all the tests still pass as expected, then you can transform your application code knowing that any tests which fail is a translation failure.

Unfortunately the tools can’t automate everything to make your code work under Python 3 and so there are a handful of things you will need to update manually to get full Python 3 support (which of these steps are necessary vary between the tools). Read the documentation for the tool you choose to use to see what it fixes by default and what it can do optionally to know what will (not) be fixed for you and what you may have to fix on your own (e.g. using io.open() over the built-in open() function is off by default in Modernize). Luckily, though, there are only a couple of things to watch out for which can be considered large issues that may be hard to debug if not watched for.


In Python 3, 5 / 2 == 2.5 and not 2; all division between int values result in a float. This change has actually been planned since Python 2.2 which was released in 2002. Since then users have been encouraged to add from __future__ import division to any and all files which use the / and // operators or to be running the interpreter with the -Q flag. If you have not been doing this then you will need to go through your code and do two things:

  1. Add from __future__ import division to your files
  2. Update any division operator as necessary to either use // to use floor division or continue using / and expect a float

The reason that / isn’t simply translated to // automatically is that if an object defines its own __div__ method but not __floordiv__ then your code would begin to fail.

Text versus binary data

In Python 2 you could use the str type for both text and binary data. Unfortunately this confluence of two different concepts could lead to brittle code which sometimes worked for either kind of data, sometimes not. It also could lead to confusing APIs if people didn’t explicitly state that something that accepted str accepted either text or binary data instead of one specific type. This complicated the situation especially for anyone supporting multiple languages as APIs wouldn’t bother explicitly supporting unicode when they claimed text data support.

To make the distinction between text and binary data clearer and more pronounced, Python 3 did what most languages created in the age of the internet have done and made text and binary data distinct types that cannot blindly be mixed together (Python predates widespread access to the internet). For any code that only deals with text or only binary data, this separation doesn’t pose an issue. But for code that has to deal with both, it does mean you might have to now care about when you are using text compared to binary data, which is why this cannot be entirely automated.

To start, you will need to decide which APIs take text and which take binary (it is highly recommended you don’t design APIs that can take both due to the difficulty of keeping the code working; as stated earlier it is difficult to do well). In Python 2 this means making sure the APIs that take text can work with unicode in Python 2 and those that work with binary data work with the bytes type from Python 3 and thus a subset of str in Python 2 (which the bytes type in Python 2 is an alias for). Usually the biggest issue is realizing which methods exist for which types in Python 2 & 3 simultaneously (for text that’s unicode in Python 2 and str in Python 3, for binary that’s str/bytes in Python 2 and bytes in Python 3). The following table lists the unique methods of each data type across Python 2 & 3 (e.g., the decode() method is usable on the equivalent binary data type in either Python 2 or 3, but it can’t be used by the text data type consistently between Python 2 and 3 because str in Python 3 doesn’t have the method).

Text data Binary data
__mod__ (% operator)  

Making the distinction easier to handle can be accomplished by encoding and decoding between binary data and text at the edge of your code. This means that when you receive text in binary data, you should immediately decode it. And if your code needs to send text as binary data then encode it as late as possible. This allows your code to work with only text internally and thus eliminates having to keep track of what type of data you are working with.

The next issue is making sure you know whether the string literals in your code represent text or binary data. At minimum you should add a b prefix to any literal that presents binary data. For text you should either use the from __future__ import unicode_literals statement or add a u prefix to the text literal.

As part of this dichotomy you also need to be careful about opening files. Unless you have been working on Windows, there is a chance you have not always bothered to add the b mode when opening a binary file (e.g., rb for binary reading). Under Python 3, binary files and text files are clearly distinct and mutually incompatible; see the io module for details. Therefore, you must make a decision of whether a file will be used for binary access (allowing to read and/or write binary data) or text access (allowing to read and/or write text data). You should also use io.open() for opening files instead of the built-in open() function as the io module is consistent from Python 2 to 3 while the built-in open() function is not (in Python 3 it’s actually io.open()).

The constructors of both str and bytes have different semantics for the same arguments between Python 2 & 3. Passing an integer to bytes in Python 2 will give you the string representation of the integer: bytes(3) == '3'. But in Python 3, an integer argument to bytes will give you a bytes object as long as the integer specified, filled with null bytes: bytes(3) == b'\x00\x00\x00'. A similar worry is necessary when passing a bytes object to str. In Python 2 you just get the bytes object back: str(b'3') == b'3'. But in Python 3 you get the string representation of the bytes object: str(b'3') == "b'3'".

Finally, the indexing of binary data requires careful handling (slicing does not require any special handling). In Python 2, b'123'[1] == b'2' while in Python 3 b'123'[1] == 50. Because binary data is simply a collection of binary numbers, Python 3 returns the integer value for the byte you index on. But in Python 2 because bytes == str, indexing returns a one-item slice of bytes. The six project has a function named six.indexbytes() which will return an integer like in Python 3: six.indexbytes(b'123', 1).

To summarize:

  1. Decide which of your APIs take text and which take binary data
  2. Make sure that your code that works with text also works with unicode and code for binary data works with bytes in Python 2 (see the table above for what methods you cannot use for each type)
  3. Mark all binary literals with a b prefix, use a u prefix or __future__ import statement for text literals
  4. Decode binary data to text as soon as possible, encode text as binary data as late as possible
  5. Open files using io.open() and make sure to specify the b mode when appropriate
  6. Be careful when indexing binary data

Prevent compatibility regressions

Once you have fully translated your code to be compatible with Python 3, you will want to make sure your code doesn’t regress and stop working under Python 3. This is especially true if you have a dependency which is blocking you from actually running under Python 3 at the moment.

To help with staying compatible, any new modules you create should have at least the following block of code at the top of it:

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
from __future__ import unicode_literals

You can also run Python 2 with the -3 flag to be warned about various compatibility issues your code triggers during execution. If you turn warnings into errors with -Werror then you can make sure that you don’t accidentally miss a warning.

You can also use the Pylint project and its --py3k flag to lint your code to receive warnings when your code begins to deviate from Python 3 compatibility. This also prevents you from having to run Modernize or Futurize over your code regularly to catch compatibility regressions. This does require you only support Python 2.7 and Python 3.4 or newer as that is Pylint’s minimum Python version support.

Check which dependencies block your transition

After you have made your code compatible with Python 3 you should begin to care about whether your dependencies have also been ported. The caniusepython3 project was created to help you determine which projects – directly or indirectly – are blocking you from supporting Python 3. There is both a command-line tool as well as a web interface at https://caniusepython3.com .

The project also provides code which you can integrate into your test suite so that you will have a failing test when you no longer have dependencies blocking you from using Python 3. This allows you to avoid having to manually check your dependencies and to be notified quickly when you can start running on Python 3.

Update your setup.py file to denote Python 3 compatibility

Once your code works under Python 3, you should update the classifiers in your setup.py to contain Programming Language :: Python :: 3 and to not specify sole Python 2 support. This will tell anyone using your code that you support Python 2 and 3. Ideally you will also want to add classifiers for each major/minor version of Python you now support.

Use continuous integration to stay compatible

Once you are able to fully run under Python 3 you will want to make sure your code always works under both Python 2 & 3. Probably the best tool for running your tests under multiple Python interpreters is tox. You can then integrate tox with your continuous integration system so that you never accidentally break Python 2 or 3 support.

You may also want to use use the -bb flag with the Python 3 interpreter to trigger an exception when you are comparing bytes to strings. Usually it’s simply False, but if you made a mistake in your separation of text/binary data handling you may be accidentally comparing text and binary data. This flag will raise an exception when that occurs to help track down such cases.

And that’s mostly it! At this point your code base is compatible with both Python 2 and 3 simultaneously. Your testing will also be set up so that you don’t accidentally break Python 2 or 3 compatibility regardless of which version you typically run your tests under while developing.

Dropping Python 2 support completely

If you are able to fully drop support for Python 2, then the steps required to transition to Python 3 simplify greatly.

  1. Update your code to only support Python 2.7
  2. Make sure you have good test coverage (coverage.py can help)
  3. Learn the differences between Python 2 & 3
  4. Use 2to3 to rewrite your code to run only under Python 3

After this your code will be fully Python 3 compliant but in a way that is not supported by Python 2. You should also update the classifiers in your setup.py to contain Programming Language :: Python :: 3 :: Only.