Add a HOWTO on how to port from Python 2 to Python 3.

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+.. _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 http://docs.python.org/py3k/howto/cporting.html .
+
+
+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.
+
+
+:mod:`io` Module
+''''''''''''''''
+The built-in ``open()`` function in Python 2 always returns a Python 2 string,
+not a unicode string. This is problematic as Python 3's :func:`open` returns a
+string if a file is not opened as binary and bytes if it is.
+
+To help with compatibility, use :func:`io.open` instead of the built-in
+``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.
+
+
+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.
+
+
+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.
+
+
+``__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
+
+
+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 file (e.g., ``
+
+
+Use :func:``codecs.open()``
+'''''''''''''''''''''''''''
+If you are not able to limit your Python 2 compatibility to 2.6 or newer (and
+thus get to use :func:`io.open`), then you should make sure you use
+:func:`codecs.open` over the built-in ``open()`` function. This will make sure
+that you get back unicode strings in Python 2 when reading in text and an
+instance of ``str`` when dealing with bytes.
+
+
+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. One very key point to understand, though, is **do not
+save the traceback to a variable without deleting it**! Because tracebacks
+contain references to the current executing frame you will inadvertently create
+a circular reference, prevent everything in the frame from being garbage
+collected. This can be a massive memory leak if you are not careful. Simply
+index into the returned value from :func:`sys.version_info` instead of
+assigning the tuple it returns to 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