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 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 :ref:`cporting-howto`.

   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`_.

   If you prefer to read a (free) book on porting a project to Python 3,
   consider reading `Porting to Python 3`_ by Lennart Regebro which should cover
   much of what is discussed in this HOWTO.

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


Before You Begin
================

If your project is on the Cheeseshop_/PyPI_, make sure it has the proper
`trove classifiers`_ to signify what versions of Python it **currently**
supports. At minimum you should specify the major version(s), e.g.
``Programming Language :: Python :: 2`` if your project currently only supports
Python 2. It is preferrable that you be as specific as possible by listing every
major/minor version of Python that you support, e.g. if your project supports
Python 2.6 and 2.7, then you want the classifiers of::

 Programming Language :: Python :: 2
 Programming Language :: Python :: 2.6
 Programming Language :: Python :: 2.7

Once your project supports Python 3 you will want to go back and add the
appropriate classifiers for Python 3 as well. This is important as setting the
``Programming Language :: Python :: 3`` classifier will lead to your project
being listed under the `Python 3 Packages`_ section of PyPI.

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/feature release of Python. This means making sure your test suite is
thorough and is ported properly between Python 2 & 3 (consider using coverage_
to measure that you have effective test coverage). You will also most likely
want to use something like tox_ to automate testing between all of your
supported versions of Python. You will also want to **port your tests first** so
that you can make sure that you detect breakage during the transition. Tests also
tend to be simpler than the code they are testing so it gives you an idea of how
easy it can be to port code.

Drop support for older Python versions if possible. `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. Choose the newest version
of Python which you believe can be your minimum support version
and work from there.

Target the newest version of Python 3 that you can. Beyond just the usual
bugfixes, compatibility has continued to improve between Python 2 and 3 as time
has passed. E.g. Python 3.3 added back the ``u`` prefix for
strings, making source-compatible Python code easier to write.


Writing Source-Compatible Python 2/3 Code
=========================================

Over the years the Python community has discovered that the easiest way to
support both Python 2 and 3 in parallel is to write Python code that works in
either version. While this might sound counter-intuitive at first, it actually
is not difficult and typically only requires following some select
(non-idiomatic) practices and using some key projects to help make bridging
between Python 2 and 3 easier.

Projects to Consider
--------------------

The lowest level library for suppoting Python 2 & 3 simultaneously is six_.
Reading through its documentation will give you an idea of where exactly the
Python language changed between versions 2 & 3 and thus what you will want the
library to help you continue to support.

To help automate porting your code over to using six, you can use
modernize_. This project will attempt to rewrite your code to be as modern as
possible while using six to smooth out any differences between Python 2 & 3.

If you want to write your compatible code to feel more like Python 3 there is
the future_ project. It tries to provide backports of objects from Python 3 so
that you can use them from Python 2-compatible code, e.g. replacing the
``bytes`` type from Python 2 with the one from Python 3.
It also provides a translation script like modernize (its translation code is
actually partially based on it) to help start working with a pre-existing code
base. It is also unique in that its translation script will also port Python 3
code backwards as well as Python 2 code forwards.


Tips & Tricks
-------------

To help with writing source-compatible code using one of the projects mentioned
in `Projects to Consider`_, consider following the below suggestions. Some of
them are handled by the suggested projects, so if you do use one of them then
read their documentation first to see which suggestions below will taken care of
for you.

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 where compatibility might be an
issue (the ``-3`` flag is in Python 2.6 but Python 2.7 adds more warnings).

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
supporting Python 3. But if you project must keep support for `Python 2.5`_ then
it is still possible to simultaneously support 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 and handle
them appropriately (which is where library help from e.g. six_ comes in handy).


``from __future__ import print_function``
'''''''''''''''''''''''''''''''''''''''''

It will not only get you used to typing ``print()`` as a function instead of a
statement, but it will also give you the various benefits the function has over
the Python 2 statement (six_ provides a function if you support Python 2.5 or
older).


``from __future__ import unicode_literals``
'''''''''''''''''''''''''''''''''''''''''''

If you choose not to use this future statement you should then mark all of your
text strings with a ``u`` prefix and only support Python 3.3 or newer. But you
are **strongly** advised to do one or the other (six_ provides a function in
case you don't want to use the future statement **and** you want to support
Python 3.2 or older).


Bytes literals
''''''''''''''

This is a **very** important one. Prefix Python 2 strings that
are meant to contain bytes with a ``b`` prefix to very clearly delineate
what is and is not a Python 3 text string (six_ provides a function to use for
Python 2.5 compatibility).

This point cannot be stressed enough: make sure you know what all of your string
literals in Python 2 are meant to be in Python 3. Any string literal that
should be treated as bytes should have the ``b`` prefix. Any string literal
that should be Unicode/text in Python 2 should either have the ``u`` literal
(supported, but ignored, in Python 3.3 and later) or you should have
``from __future__ import unicode_literals`` at the top of the file. But the key
point is you should know how Python 3 will treat every one one of your string
literals and you should mark them as appropriate.

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.
See the `Handle Common "Gotchas"`_ section for what to watch out for.

``from __future__ import absolute_import``
''''''''''''''''''''''''''''''''''''''''''
Discussed in more detail below, but you should use this future statement to
prevent yourself from accidentally using implicit relative imports.


Supporting `Python 2.5`_ and Newer Only
///////////////////////////////////////

If you are supporting `Python 2.5`_ and newer there are still some features of
Python that you can utilize.


``from __future__ import absolute_import``
''''''''''''''''''''''''''''''''''''''''''

Implicit relative imports (e.g., importing ``spam.bacon`` from within
``spam.eggs`` with the statement ``import bacon``) do 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``).

In `Python 2.5`_ you must use
the __future__ statement to get to use explicit relative imports and prevent
implicit ones. In `Python 2.6`_ explicit relative imports are available without
the statement, but you still want the __future__ statement to prevent implicit
relative imports. In `Python 2.7`_ the __future__ statement is not needed. In
other words, unless you are only supporting Python 2.7 or a version earlier
than Python 2.5, use this __future__ statement.


Mark all Unicode strings with a ``u`` prefix
'''''''''''''''''''''''''''''''''''''''''''''

While Python 2.6 has a ``__future__`` statement to automatically cause Python 2
to treat all string literals as Unicode, Python 2.5 does not have that shortcut.
This means you should go through and mark all string literals with a ``u``
prefix to turn them explicitly into text strings where appropriate and only
support Python 3.3 or newer. Otherwise use a project like six_ which provides a
function to pass all text string literals through.


Capturing the Currently Raised Exception
''''''''''''''''''''''''''''''''''''''''

In Python 2.5 and earlier 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 (and backported to `Python 2.6`_ and later)
to::

   try:
     raise Exception()
   except Exception as exc:
     # Current exception is 'exc'.
     # In Python 3, 'exc' is restricted to the block; in Python 2.6/2.7 it will "leak".
     pass

Because of this syntax change you must change how you capture the current
exception in Python 2.5 and earlier 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.


Handle Common "Gotchas"
///////////////////////

These are things to watch out for no matter what version of Python 2 you are
supporting which are not syntactic considerations.


``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``).



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 to
handling this issue is to make sure that **every** string literal in your
Python 2 code is either syntactically or 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 predating 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 an equality comparison 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 inadvertently 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).

You can use a mixin class to work around this. 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 on an exception is an error. You need to
make sure to only index on the :attr:`BaseException.args` attribute which is a
sequence containing all arguments passed to the :meth:`__init__` method.

Even better is to use the 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.


Updating doctests
'''''''''''''''''

Don't forget to make them Python 2/3 compatible as well. If you wrote a
monolithic set of doctests (e.g., a single docstring containing all of your
doctests), you should at least consider breaking the doctests up into smaller
pieces to make it more manageable to fix. Otherwise it might very well be worth
your time and effort to port your tests to :mod:`unittest`.


Update ``map`` for imbalanced input sequences
'''''''''''''''''''''''''''''''''''''''''''''

With Python 2, ``map`` would pad input sequences of unequal length with
`None` values, returning a sequence as long as the longest input sequence.

With Python 3, if the input sequences to ``map`` are of unequal length, ``map``
will stop at the termination of the shortest of the sequences. For full
compatibility with ``map`` from Python 2.x, also wrap the sequences in
:func:`itertools.zip_longest`, e.g. ``map(func, *sequences)`` becomes
``list(map(func, itertools.zip_longest(*sequences)))``.

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.


Alternative Approaches
======================

While supporting Python 2 & 3 simultaneously is typically the preferred choice
by people so that they can continue to improve code and have it work for the
most number of users, your life may be easier if you only have to support one
major version of Python going forward.

Supporting Only Python 3 Going Forward From Python 2 Code
---------------------------------------------------------

If you have Python 2 code but going forward only want to improve it as Python 3
code, then you can use 2to3_ to translate your Python 2 code to Python 3 code.
This is only recommended, though, if your current version of your project is
going into maintenance mode and you want all new features to be exclusive to
Python 3.


Backporting Python 3 code to Python 2
-------------------------------------

If you have Python 3 code and have little interest in supporting Python 2 you
can use 3to2_ to translate from Python 3 code to Python 2 code. This is only
recommended if you don't plan to heavily support Python 2 users.


Other Resources
===============

The authors of the following blog posts, wiki pages, and books deserve special
thanks for making public their tips for porting Python 2 code to Python 3 (and
thus helping provide information for this document and its various revisions
over the years):

* http://wiki.python.org/moin/PortingPythonToPy3k
* http://python3porting.com/
* 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/
* https://wiki.ubuntu.com/Python/3

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



.. _2to3: http://docs.python.org/2/library/2to3.html
.. _3to2: https://pypi.python.org/pypi/3to2
.. _Cheeseshop: PyPI_
.. _coverage: https://pypi.python.org/pypi/coverage
.. _future: http://python-future.org/
.. _modernize: https://github.com/mitsuhiko/python-modernize
.. _Porting to Python 3: http://python3porting.com/
.. _PyPI: http://pypi.python.org/
.. _Python 2.2: http://www.python.org/2.2.x
.. _Python 2.5: http://www.python.org/2.5.x
.. _Python 2.6: http://www.python.org/2.6.x
.. _Python 2.7: http://www.python.org/2.7.x
.. _Python 2.5: http://www.python.org/2.5.x
.. _Python 3.3: http://www.python.org/3.3.x
.. _Python 3 Packages: https://pypi.python.org/pypi?:action=browse&c=533&show=all
.. _Python 3 Q & A: http://ncoghlan-devs-python-notes.readthedocs.org/en/latest/python3/questions_and_answers.html
.. _python-porting: http://mail.python.org/mailman/listinfo/python-porting
.. _six: https://pypi.python.org/pypi/six
.. _tox: https://pypi.python.org/pypi/tox
.. _trove classifiers: https://pypi.python.org/pypi?%3Aaction=list_classifiers