395 lines
20 KiB
ReStructuredText
395 lines
20 KiB
ReStructuredText
.. _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`_.
|
|
|
|
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:
|
|
|
|
#. Only worry about supporting Python 2.7
|
|
#. Make sure you have good test coverage (coverage.py_ can help;
|
|
``pip install coverage``)
|
|
#. Learn the differences between Python 2 & 3
|
|
#. Use Modernize_ or Futurize_ to update your code (``pip install modernize`` or
|
|
``pip install future``, respectively)
|
|
#. Use Pylint_ to help make sure you don't regress on your Python 3 support
|
|
(``pip install pylint``)
|
|
#. Use caniusepython3_ to find out which of your dependencies are blocking your
|
|
use of Python 3 (``pip install caniusepython3``)
|
|
#. 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.
|
|
|
|
|
|
Details
|
|
=======
|
|
|
|
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 2.6 and older
|
|
-------------------------------------
|
|
|
|
While you can make Python 2.5 work with Python 3, it is **much** easier 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 skip Python 2.5 and older, 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 you should aim for only supporting Python 2.7. Python 2.6 is no longer
|
|
supported and thus is not receiving bugfixes. This means **you** will have to
|
|
work around any issues you come across with Python 2.6. 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. It will simply be easier
|
|
for you if you only support the versions of Python that you have to support.
|
|
|
|
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.
|
|
|
|
Division
|
|
++++++++
|
|
|
|
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:
|
|
|
|
#. Add ``from __future__ import division`` to your files
|
|
#. 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 a ``__truediv__`` method but not ``__floordiv__`` then your
|
|
code would begin to fail (e.g. a user-defined class that uses ``/`` to
|
|
signify some operation but not ``//`` for the same thing or at all).
|
|
|
|
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). Do
|
|
note that as of Python 3.5 the ``__mod__`` method was added to the bytes type.
|
|
|
|
======================== =====================
|
|
**Text data** **Binary data**
|
|
------------------------ ---------------------
|
|
\ decode
|
|
------------------------ ---------------------
|
|
encode
|
|
------------------------ ---------------------
|
|
format
|
|
------------------------ ---------------------
|
|
isdecimal
|
|
------------------------ ---------------------
|
|
isnumeric
|
|
======================== =====================
|
|
|
|
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 :mod:`io` module for details.
|
|
Therefore, you **must** make a decision of whether a file will be used for
|
|
binary access (allowing binary data to be read and/or written) or text access
|
|
(allowing text data to be read and/or written). You should also use :func:`io.open`
|
|
for opening files instead of the built-in :func:`open` function as the :mod:`io`
|
|
module is consistent from Python 2 to 3 while the built-in :func:`open` function
|
|
is not (in Python 3 it's actually :func:`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:
|
|
|
|
#. Decide which of your APIs take text and which take binary data
|
|
#. 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)
|
|
#. Mark all binary literals with a ``b`` prefix, use a ``u`` prefix or
|
|
:mod:`__future__` import statement for text literals
|
|
#. Decode binary data to text as soon as possible, encode text as binary data as
|
|
late as possible
|
|
#. Open files using :func:`io.open` and make sure to specify the ``b`` mode when
|
|
appropriate
|
|
#. 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 or bytes to an int
|
|
(the latter is available starting in Python 3.5). By default type-differing
|
|
comparisons simply return ``False``, but if you made a mistake in your
|
|
separation of text/binary data handling or indexing on bytes you wouldn't easily
|
|
find the mistake. This flag will raise an exception when these kinds of
|
|
comparisons occur, making the mistake much easier to track down.
|
|
|
|
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.
|
|
|
|
#. Update your code to only support Python 2.7
|
|
#. Make sure you have good test coverage (coverage.py_ can help)
|
|
#. Learn the differences between Python 2 & 3
|
|
#. 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``.
|
|
|
|
|
|
.. _2to3: https://docs.python.org/3/library/2to3.html
|
|
.. _caniusepython3: https://pypi.python.org/pypi/caniusepython3
|
|
.. _cheat sheet: http://python-future.org/compatible_idioms.html
|
|
.. _coverage.py: https://pypi.python.org/pypi/coverage
|
|
.. _Futurize: http://python-future.org/automatic_conversion.html
|
|
.. _Modernize: http://python-modernize.readthedocs.org/en/latest/
|
|
.. _Porting to Python 3: http://python3porting.com/
|
|
.. _Pylint: https://pypi.python.org/pypi/pylint
|
|
.. _Python 3 Q & A: http://ncoghlan-devs-python-notes.readthedocs.org/en/latest/python3/questions_and_answers.html
|
|
|
|
.. _python-future: http://python-future.org/
|
|
.. _python-porting: https://mail.python.org/mailman/listinfo/python-porting
|
|
.. _six: https://pypi.python.org/pypi/six
|
|
.. _tox: https://pypi.python.org/pypi/tox
|
|
.. _trove classifier: https://pypi.python.org/pypi?%3Aaction=list_classifiers
|
|
.. _"What's New": https://docs.python.org/3/whatsnew/index.html
|