Doc/howto/pyporting.rst

.. _pyporting-howto:

*********************************
Porting Python 2 Code to Python 3
*********************************

:author: Brett Cannon

.. topic:: Abstract

   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 choose which strategy works best
   for your project to support both Python 2 & 3 along with how to execute
   that strategy.

   If you are looking to port an extension module instead of pure Python code,
   please see :ref:`cporting-howto`.


Choosing a Strategy
===================
When a project makes the decision that it's time to support both Python 2 & 3,
a decision needs to be made as to how to go about accomplishing that goal.
Which strategy goes with will depend on how large the project's existing
codebase is and how much divergence you want from your Python 2 codebase from
your Python 3 one (e.g., starting a new version with Python 3).

If your project is brand-new or does not have a large codebase, then you may
want to consider writing/porting :ref:`all of your code for Python 3
and use 3to2 <use_3to2>` to port your code for Python 2.

If your project has a pre-existing Python 2 codebase and you would like Python
3 support to start off a new branch or version of your project, then you will
most likely want to :ref:`port using 2to3 <use_2to3>`. This will allow you port
your Python 2 code to Python 3 in a semi-automated fashion and begin to
maintain it separately from your Python 2 code. This approach can also work if
your codebase is small and/or simple enough for the translation to occur
quickly.

Finally, if you want to maintain Python 2 and Python 3 versions of your project
simultaneously and with no differences, then you can write :ref:`Python 2/3
source-compatible code <use_same_source>`. While the code is not quite as
idiomatic as it would be written just for Python 3 or automating the port from
Python 2, it does makes it easier to continue to do rapid development
regardless of what major version of Python you are developing against at the
time.

Regardless of which approach you choose, porting is probably not as hard or
time-consuming as you might initially think. You can also tackle the problem
piece-meal as a good portion of porting is simply updating your code to follow
current best practices in a Python 2/3 compatible way.


Universal Bits of Advice
------------------------
Regardless of what strategy you pick, there are a few things you should
consider.

One is make sure you have a robust test suite. You need to make sure everything
continues to work, just like when you support a new minor version of Python.
This means making sure your test suite is thorough and is ported properly
between Python 2 & 3. You will also most likely want to use something like tox_
to automate testing between both a Python 2 and Python 3 VM.

Two, once your project has Python 3 support, make sure to add the proper
classifier on the Cheeseshop_ (PyPI_). To have your project listed as Python 3
compatible it must have the
`Python 3 classifier <http://pypi.python.org/pypi?:action=browse&c=533>`_
(from
http://techspot.zzzeek.org/2011/01/24/zzzeek-s-guide-to-python-3-porting/)::

   setup(
     name='Your Library',
     version='1.0',
     classifiers=[
         # make sure to use :: Python *and* :: Python :: 3 so
         # that pypi can list the package on the python 3 page
         'Programming Language :: Python',
         'Programming Language :: Python :: 3'
     ],
     packages=['yourlibrary'],
     # make sure to add custom_fixers to the MANIFEST.in
     include_package_data=True,
     # ...
   )


Doing so will cause your project to show up in the
`Python 3 packages list
<http://pypi.python.org/pypi?:action=browse&c=533&show=all>`_. You will know
you set the classifier properly as visiting your project page on the Cheeseshop
will show a Python 3 logo in the upper-left corner of the page.

Three, the six_ project provides a library which helps iron out differences
between Python 2 & 3. If you find there is a sticky point that is a continual
point of contention in your translation or maintenance of code, consider using
a source-compatible solution relying on six. If you have to create your own
Python 2/3 compatible solution, you can use ``sys.version_info[0] >= 3`` as a
guard.

Four, read all the approaches. Just because some bit of advice applies to one
approach more than another doesn't mean that some advice doesn't apply to other
strategies.

Five, drop support for older Python versions if possible. While not a
requirement, `Python 2.5`_) introduced a lot of useful syntax and libraries
which have become idiomatic in Python 3. `Python 2.6`_ introduced future
statements which makes compatibility much easier if you are going from Python 2
to 3.
`Python 2.7`_ continues the trend in the stdlib. So choose the newest version
of Python for which you believe you believe can be your minimum support version
and work from there.


.. _tox: http://codespeak.net/tox/
.. _Cheeseshop:
.. _PyPI: http://pypi.python.org/
.. _six: http://packages.python.org/six
.. _Python 2.7: http://www.python.org/2.7.x
.. _Python 2.6: http://www.python.org/2.6.x
.. _Python 2.5: http://www.python.org/2.5.x
.. _Python 2.4: http://www.python.org/2.4.x


.. _use_3to2:

Python 3 and 3to2
=================
If you are starting a new project or your codebase is small enough, you may
want to consider writing your code for Python 3 and backporting to Python 2
using 3to2_. Thanks to Python 3 being more strict about things than Python 2
(e.g., bytes vs. strings), the source translation can be easier and more
straightforward than from Python 2 to 3. Plus it gives you more direct
experience developing in Python 3 which, since it is the future of Python, is a
good thing long-term.

A drawback of this approach is that 3to2 is a third-party project. This means
that the Python core developers (and thus this guide) can make no promises
about how well 3to2 works at any time. There is nothing to suggest, though,
that 3to2 is not a high-quality project.


.. _3to2: https://bitbucket.org/amentajo/lib3to2/overview


.. _use_2to3:

Python 2 and 2to3
=================
Included with Python since 2.6, 2to3_ tool (and :mod:`lib2to3` module) helps
with porting Python 2 to Python 3 by performing various source translations.
This is a perfect solution for projects which wish to branch their Python 3
code from their Python 2 codebase and maintain them as independent codebases.
You can even begin preparing to use this approach today by writing
future-compatible Python code which works cleanly in Python 2 in conjunction
with 2to3; all steps outlined below will work with Python 2 code up to the
point when the actual use of 2to3 occurs.

Use of 2to3 as an on-demand translation step at install time is also possible,
preventing the need to maintain a separate Python 3 codebase, but this approach
does come with some drawbacks. While users will only have to pay the
translation cost once at installation, you as a developer will need to pay the
cost regularly during development. If your codebase is sufficiently large
enough then the translation step ends up acting like a compilation step,
robbing you of the rapid development process you are used to with Python.
Obviously the time required to translate a project will vary, so do an
experimental translation just to see how long it takes to evaluate whether you
prefer this approach compared to using :ref:`use_same_source` or simply keeping
a separate Python 3 codebase.

Below are the typical steps taken by a project which uses a 2to3-based approach
to supporting Python 2 & 3.


Support Python 2.7
------------------
As a first step, make sure that your project is compatible with `Python 2.7`_.
This is just good to do as Python 2.7 is the last release of Python 2 and thus
will be used for a rather long time. It also allows for use of the ``-3`` flag
to Python to help discover places in your code which 2to3 cannot handle but are
known to cause issues.

Try to Support Python 2.6 and Newer Only
----------------------------------------
While not possible for all projects, if you can support `Python 2.6`_ and newer
**only**, your life will be much easier. Various future statements, stdlib
additions, etc. exist only in Python 2.6 and later which greatly assist in
porting to Python 3. But if you project must keep support for `Python 2.5`_ (or
even `Python 2.4`_) then it is still possible to port to Python 3.

Below are the benefits you gain if you only have to support Python 2.6 and
newer. Some of these options are personal choice while others are
**strongly** recommended (the ones that are more for personal choice are
labeled as such).  If you continue to support older versions of Python then you
at least need to watch out for situations that these solutions fix.


``from __future__ import division``
'''''''''''''''''''''''''''''''''''
While the exact same outcome can be had by using the ``-Qnew`` argument to
Python, using this future statement lifts the requirement that your users use
the flag to get the expected behavior of division in Python 3 (e.g., ``1/2 ==
0.5; 1//2 == 0``).


``from __future__ import absolute_imports``
'''''''''''''''''''''''''''''''''''''''''''
Implicit relative imports (e.g., importing ``spam.bacon`` from within
``spam.eggs`` with the statement ``import bacon``) does not work in Python 3.
This future statement moves away from that and allows the use of explicit
relative imports (e.g., ``from . import bacon``).


``from __future__ import print_function``
'''''''''''''''''''''''''''''''''''''''''
This is a personal choice. 2to3 handles the translation from the print
statement to the print function rather well so this is an optional step. This
future statement does help, though, with getting used to typing
``print('Hello, World')`` instead of ``print 'Hello, World'``.


``from __future__ import unicode_literals``
'''''''''''''''''''''''''''''''''''''''''''
Another personal choice. You can always mark what you want to be a (unicode)
string with a ``u`` prefix to get the same effect. But regardless of whether
you use this future statement or not, you **must** make sure you know exactly
which Python 2 strings you want to be bytes, and which are to be strings. This
means you should, **at minimum** mark all strings that are meant to be text
strings with a ``u`` prefix if you do not use this future statement.


Bytes literals
''''''''''''''
This is a **very** important one. The ability to prefix Python 2 strings that
are meant to contain bytes with a ``b`` prefix help to very clearly delineate
what is and is not a Python 3 string. When you run 2to3 on code, all Python 2
strings become Python 3 strings **unless** they are prefixed with ``b``.

There are some differences between byte literals in Python 2 and those in
Python 3 thanks to the bytes type just being an alias to ``str`` in Python 2.
Probably the biggest "gotcha" is that indexing results in different values. In
Python 2, the value of ``b'py'[1]`` is ``'y'``, while in Python 3 it's ``121``.
You can avoid this disparity by always slicing at the size of a single element:
``b'py'[1:2]`` is ``'y'`` in Python 2 and ``b'y'`` in Python 3 (i.e., close
enough).

You cannot concatenate bytes and strings in Python 3. But since in Python
2 has bytes aliased to ``str``, it will succeed: ``b'a' + u'b'`` works in
Python 2, but ``b'a' + 'b'`` in Python 3 is a :exc:`TypeError`. A similar issue
also comes about when doing comparisons between bytes and strings.


Handle Common "Gotchas"
-----------------------
There are a few things that just consistently come up as sticking points for
people which 2to3 cannot handle automatically or can easily be done in Python 2
to help modernize your code.


Specify when opening a file as binary
'''''''''''''''''''''''''''''''''''''

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 :mod:`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 bytes data) or text access
(allowing to read and/or write unicode data).

Text files
''''''''''

Text files created using ``open()`` under Python 2 return byte strings,
while under Python 3 they return unicode strings.  Depending on your porting
strategy, this can be an issue.

If you want text files to return unicode strings in Python 2, you have two
possibilities:

* Under Python 2.6 and higher, use :func:`io.open`.  Since :func:`io.open`
  is essentially the same function in both Python 2 and Python 3, it will
  help iron out any issues that might arise.

* If pre-2.6 compatibility is needed, then you should use :func:`codecs.open`
  instead.  This will make sure that you get back unicode strings in Python 2.

Subclass ``object``
'''''''''''''''''''
New-style classes have been around since Python 2.2. You need to make sure you
are subclassing from ``object`` to avoid odd edge cases involving method
resolution order, etc. This continues to be totally valid in Python 3 (although
unneeded as all classes implicitly inherit from ``object``).


Deal With the Bytes/String Dichotomy
''''''''''''''''''''''''''''''''''''
One of the biggest issues people have when porting code to Python 3 is handling
the bytes/string dichotomy. Because Python 2 allowed the ``str`` type to hold
textual data, people have over the years been rather loose in their delineation
of what ``str`` instances held text compared to bytes. In Python 3 you cannot
be so care-free anymore and need to properly handle the difference. The key
handling this issue to to make sure that **every** string literal in your
Python 2 code is either syntactically of functionally marked as either bytes or
text data. After this is done you then need to make sure your APIs are designed
to either handle a specific type or made to be properly polymorphic.


Mark Up Python 2 String Literals
********************************

First thing you must do is designate every single string literal in Python 2
as either textual or bytes data. If you are only supporting Python 2.6 or
newer, this can be accomplished by marking bytes literals with a ``b`` prefix
and then designating textual data with a ``u`` prefix or using the
``unicode_literals`` future statement.

If your project supports versions of Python pre-dating 2.6, then you should use
the six_ project and its ``b()`` function to denote bytes literals. For text
literals you can either use six's ``u()`` function or use a ``u`` prefix.


Decide what APIs Will Accept
****************************
In Python 2 it was very easy to accidentally create an API that accepted both
bytes and textual data. But in Python 3, thanks to the more strict handling of
disparate types, this loose usage of bytes and text together tends to fail.

Take the dict ``{b'a': 'bytes', u'a': 'text'}`` in Python 2.6. It creates the
dict ``{u'a': 'text'}`` since ``b'a' == u'a'``. But in Python 3 the equivalent
dict creates ``{b'a': 'bytes', 'a': 'text'}``, i.e., no lost data. Similar
issues can crop up when transitioning Python 2 code to Python 3.

This means you need to choose what an API is going to accept and create and
consistently stick to that API in both Python 2 and 3.


Bytes / unicode comparison
**************************

In Python 3, mixing bytes and unicode is forbidden in most situations; it
will raise a :class:`TypeError` where Python 2 would have attempted an implicit
coercion between types.  However, there is one case where it doesn't and
it can be very misleading::

   >>> b"" == ""
   False

This is because comparison for equality is required by the language to always
succeed (and return ``False`` for incompatible types).  However, this also
means that code incorrectly ported to Python 3 can display buggy behaviour
if such comparisons are silently executed.  To detect such situations,
Python 3 has a ``-b`` flag that will display a warning::

   $ python3 -b
   >>> b"" == ""
   __main__:1: BytesWarning: Comparison between bytes and string
   False

To turn the warning into an exception, use the ``-bb`` flag instead::

   $ python3 -bb
   >>> b"" == ""
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   BytesWarning: Comparison between bytes and string


Indexing bytes objects
''''''''''''''''''''''

Another potentially surprising change is the indexing behaviour of bytes
objects in Python 3::

   >>> b"xyz"[0]
   120

Indeed, Python 3 bytes objects (as well as :class:`bytearray` objects)
are sequences of integers.  But code converted from Python 2 will often
assume that indexing a bytestring produces another bytestring, not an
integer.  To reconcile both behaviours, use slicing::

   >>> b"xyz"[0:1]
   b'x'
   >>> n = 1
   >>> b"xyz"[n:n+1]
   b'y'

The only remaining gotcha is that an out-of-bounds slice returns an empty
bytes object instead of raising ``IndexError``:

   >>> b"xyz"[3]
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   IndexError: index out of range
   >>> b"xyz"[3:4]
   b''


``__str__()``/``__unicode__()``
'''''''''''''''''''''''''''''''
In Python 2, objects can specify both a string and unicode representation of
themselves. In Python 3, though, there is only a string representation. This
becomes an issue as people can inadvertantly do things in their ``__str__()``
methods which have unpredictable results (e.g., infinite recursion if you
happen to use the ``unicode(self).encode('utf8')`` idiom as the body of your
``__str__()`` method).

There are two ways to solve this issue. One is to use a custom 2to3 fixer. The
blog post at http://lucumr.pocoo.org/2011/1/22/forwards-compatible-python/
specifies how to do this. That will allow 2to3 to change all instances of ``def
__unicode(self): ...`` to ``def __str__(self): ...``. This does require you
define your ``__str__()`` method in Python 2 before your ``__unicode__()``
method.

The other option is to use a mixin class. This allows you to only define a
``__unicode__()`` method for your class and let the mixin derive
``__str__()`` for you (code from
http://lucumr.pocoo.org/2011/1/22/forwards-compatible-python/)::

   import sys

   class UnicodeMixin(object):

     """Mixin class to handle defining the proper __str__/__unicode__
     methods in Python 2 or 3."""

     if sys.version_info[0] >= 3: # Python 3
         def __str__(self):
             return self.__unicode__()
     else:  # Python 2
         def __str__(self):
             return self.__unicode__().encode('utf8')


   class Spam(UnicodeMixin):

     def __unicode__(self):
         return u'spam-spam-bacon-spam'  # 2to3 will remove the 'u' prefix


Don't Index on Exceptions
'''''''''''''''''''''''''

In Python 2, the following worked::

   >>> exc = Exception(1, 2, 3)
   >>> exc.args[1]
   2
   >>> exc[1]  # Python 2 only!
   2

But in Python 3, indexing directly off of an exception is an error. You need to
make sure to only index on :attr:`BaseException.args` attribute which is a
sequence containing all arguments passed to the :meth:`__init__` method.

Even better is to use documented attributes the exception provides.

Don't use ``__getslice__`` & Friends
''''''''''''''''''''''''''''''''''''

Been deprecated for a while, but Python 3 finally drops support for
``__getslice__()``, etc. Move completely over to :meth:`__getitem__` and
friends.


Stop Using :mod:`doctest`
'''''''''''''''''''''''''
While 2to3 tries to port doctests properly, it's a rather tough thing to do. It
is probably best to simply convert your critical doctests to :mod:`unittest`.


Eliminate ``-3`` Warnings
-------------------------
When you run your application's test suite, run it using the ``-3`` flag passed
to Python. This will cause various warnings to be raised during execution about
things that 2to3 cannot handle automatically (e.g., modules that have been
removed). Try to eliminate those warnings to make your code even more portable
to Python 3.


Run 2to3
--------
Once you have made your Python 2 code future-compatible with Python 3, it's
time to use 2to3_ to actually port your code.


Manually
''''''''
To manually convert source code using 2to3_, you use the ``2to3`` script that
is installed with Python 2.6 and later.::

   2to3 <directory or file to convert>

This will cause 2to3 to write out a diff with all of the fixers applied for the
converted source code. If you would like 2to3 to go ahead and apply the changes
you can pass it the ``-w`` flag::

   2to3 -w <stuff to convert>

There are other flags available to control exactly which fixers are applied,
etc.


During Installation
'''''''''''''''''''
When a user installs your project for Python 3, you can have either
:mod:`distutils` or Distribute_ run 2to3_ on your behalf.
For distutils, use the following idiom::

   try:  # Python 3
     from distutils.command.build_py import build_py_2to3 as build_py
   except ImportError:  # Python 2
     from distutils.command.build_py import build_py

   setup(cmdclass = {'build_py':build_py},
     # ...
   )

   For Distribute::

   setup(use_2to3=True,
     # ...
   )

This will allow you to not have to distribute a separate Python 3 version of
your project. It does require, though, that when you perform development that
you at least build your project and use the built Python 3 source for testing.


Verify & Test
-------------
At this point you should (hopefully) have your project converted in such a way
that it works in Python 3. Verify it by running your unit tests and making sure
nothing has gone awry. If you miss something then figure out how to fix it in
Python 3, backport to your Python 2 code, and run your code through 2to3 again
to verify the fix transforms properly.


.. _2to3: http://docs.python.org/py3k/library/2to3.html
.. _Distribute: http://packages.python.org/distribute/


.. _use_same_source:

Python 2/3 Compatible Source
============================
While it may seem counter-intuitive, you can write Python code which is
source-compatible between Python 2 & 3. It does lead to code that is not
entirely idiomatic Python (e.g., having to extract the currently raised
exception from ``sys.exc_info()[1]``), but it can be run under Python 2
**and** Python 3 without using 2to3_ as a translation step. This allows you to
continue to have a rapid development process regardless of whether you are
developing under Python 2 or Python 3. Whether this approach or using
:ref:`use_2to3` works best for you will be a per-project decision.

To get a complete idea of what issues you will need to deal with, see the
`What's New in Python 3.0`_. Others have reorganized the data in other formats
such as http://docs.pythonsprints.com/python3_porting/py-porting.html .

The following are some steps to take to try to support both Python 2 & 3 from
the same source code.


.. _What's New in Python 3.0: http://docs.python.org/release/3.0/whatsnew/3.0.html


Follow The Steps for Using 2to3_ (sans 2to3)
--------------------------------------------
All of the steps outlined in how to
:ref:`port Python 2 code with 2to3 <use_2to3>` apply
to creating a Python 2/3 codebase. This includes trying only support Python 2.6
or newer (the :mod:`__future__` statements work in Python 3 without issue),
eliminating warnings that are triggered by ``-3``, etc.

Essentially you should cover all of the steps short of running 2to3 itself.


Use six_
--------
The six_ project contains many things to help you write portable Python code.
You should make sure to read its documentation from beginning to end and use
any and all features it provides. That way you will minimize any mistakes you
might make in writing cross-version code.


Capturing the Currently Raised Exception
----------------------------------------
One change between Python 2 and 3 that will require changing how you code is
accessing the currently raised exception.  In Python 2 the syntax to access the
current exception is::

   try:
     raise Exception()
   except Exception, exc:
     # Current exception is 'exc'
     pass

This syntax changed in Python 3 to::

   try:
     raise Exception()
   except Exception as exc:
     # Current exception is 'exc'
     pass

Because of this syntax change you must change to capturing the current
exception to::

   try:
     raise Exception()
   except Exception:
     import sys
     exc = sys.exc_info()[1]
     # Current exception is 'exc'
     pass

You can get more information about the raised exception from
:func:`sys.exc_info` than simply the current exception instance, but you most
likely don't need it.

.. note::
   In Python 3, the traceback is attached to the exception instance
   through the **__traceback__** attribute.  If the instance is saved in
   a local variable that persists outside of the ``except`` block, the
   traceback will create a reference cycle with the current frame and its
   dictionary of local variables.  This will delay reclaiming dead
   resources until the next cyclic :term:`garbage collection` pass.

   In Python 2, this problem only occurs if you save the traceback itself
   (e.g. the third element of the tuple returned by :func:`sys.exc_info`)
   in a variable.

Other Resources
===============
The authors of the following blogs posts and wiki pages deserve special thanks
for making public their tips for porting Python 2 code to Python 3 (and thus
helping provide information for this document):

* http://docs.pythonsprints.com/python3_porting/py-porting.html
* http://techspot.zzzeek.org/2011/01/24/zzzeek-s-guide-to-python-3-porting/
* http://dabeaz.blogspot.com/2011/01/porting-py65-and-my-superboard-to.html
* http://lucumr.pocoo.org/2011/1/22/forwards-compatible-python/
* http://lucumr.pocoo.org/2010/2/11/porting-to-python-3-a-guide/
* http://wiki.python.org/moin/PortingPythonToPy3k

If you feel there is something missing from this document that should be added,
please email the python-porting_ mailing list.

.. _python-porting: http://mail.python.org/mailman/listinfo/python-porting