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Update the porting HOWTO

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Brett Cannon 2016-12-17 12:38:54 -08:00
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@ -17,7 +17,8 @@ Porting Python 2 Code to Python 3
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`_.
came into existence, you can read Nick Coghlan's `Python 3 Q & A`_ or
Brett Cannon's `Why Python 3 exists`_.
For help with porting, you can email the python-porting_ mailing list with
questions.
@ -28,24 +29,21 @@ The Short Explanation
To make your project be single-source Python 2/3 compatible, the basic steps
are:
#. Update your code to drop support for Python 2.5 or older (supporting only
Python 2.7 is ideal)
#. 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 Futurize_ (or Modernize_) to update your code (e.g. ``pip install future``)
#. Use Pylint_ to help make sure you don't regress on your Python 3 support
(if only supporting Python 2.7/3.4 or newer; ``pip install pylint``)
(``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.
#. Consider using optional static type checking to make sure your type usage
works in both Python 2 & 3 (e.g. use mypy_ to check your typing under both
Python 2 & Python 3).
Details
@ -55,7 +53,7 @@ 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.
Python 2 code.
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
@ -67,26 +65,28 @@ Keep those key points in mind while you read on about the details of porting
your code to support Python 2 & 3 simultaneously.
Drop support for Python 2.5 and older (at least)
------------------------------------------------
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 2.6 or newer (and easier still if you only have
to work with Python 2.7). If dropping Python 2.5 is not an option then the six_
project can help you support Python 2.5 & 3 simultaneously
only have to work with Python 2.7. If dropping Python 2.5 is not an
option then the six_ project can help you support Python 2.5 & 3 simultaneously
(``pip install six``). Do realize, though, that nearly all the projects listed
in this HOWTO will not be available to you.
If you are able to only support Python 2.6 or newer, then the required changes
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 please aim for Python 2.7. Bugfixes for that version of Python will continue
until 2020 while Python 2.6 is no longer supported. There are also some tools
mentioned in this HOWTO which do not support Python 2.6 (e.g., Pylint_), and
this will become more commonplace as time goes on.
But you should aim for only supporting Python 2.7. Python 2.6 is no longer
freely upported 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
--------------------------------------------------------------------------
@ -98,6 +98,7 @@ Python 3 yet you should at least have
also specify each major/minor version of Python that you do support, e.g.
``Programming Language :: Python :: 2.7``.
Have good test coverage
-----------------------
@ -106,10 +107,11 @@ 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
coverage (and don't feel bad if you can't easily get passed 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
-------------------------------------------
@ -127,13 +129,15 @@ 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
your code automatically: Futurize_ and Modernize_. 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.
Python, directly relying on six_ to help provide compatibility. As Python 3 is
the future, it might be best to consider Futurize to begin adjusting to any new
practices that Python 3 introduces which you are not accustomed to yet.
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.
@ -153,6 +157,7 @@ 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
++++++++
@ -169,8 +174,10 @@ things:
division or continue using ``/`` and expect a float
The reason that ``/`` isn't simply translated to ``//`` automatically is that if
an object defines its own ``__div__`` method but not ``__floordiv__`` then your
code would begin to fail.
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
+++++++++++++++++++++++
@ -188,7 +195,7 @@ 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
that deals only 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.
@ -197,22 +204,21 @@ 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
with ``unicode`` and those that work with binary data work with the
``bytes`` type from Python 3 (which is a subset of ``str`` in Python 2 and acts
as an alias for ``bytes`` type in Python 2). Usually the biggest issue is
realizing which methods exist on 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).
either Python 2 or 3, but it can't be used by the textual 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**
------------------------ ---------------------
__mod__ (``%`` operator)
------------------------ ---------------------
\ decode
------------------------ ---------------------
encode
@ -232,10 +238,11 @@ 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.
represent text or binary data. You should add a ``b`` prefix to any
literal that presents binary data. For text you should add a ``u`` prefix to
the text literal. (there is a :mod:`__future__` import to force all unspecified
literals to be Unicode, but usage has shown it isn't as effective as adding a
``b`` or ``u`` prefix to all literals explicitly)
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
@ -243,11 +250,13 @@ 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
binary access (allowing binary data to be read and/or written) or textual 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`).
is not (in Python 3 it's actually :func:`io.open`). Do not bother with the
outdated practice of using :func:`codecs.open` as that's only necessary for
keeping compatibility with Python 2.5.
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
@ -274,13 +283,58 @@ To summarize:
#. 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
#. Mark all binary literals with a ``b`` prefix, textual literals with a ``u``
prefix
#. 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
#. Be careful when indexing into binary data
Use feature detection instead of version detection
++++++++++++++++++++++++++++++++++++++++++++++++++
Inevitably you will have code that has to choose what to do based on what
version of Python is running. The best way to do this is with feature detection
of whether the version of Python you're running under supports what you need.
If for some reason that doesn't work then you should make the version check be
against Python 2 and not Python 3. To help explain this, let's look at an
example.
Let's pretend that you need access to a feature of importlib_ that
is available in Python's standard library since Python 3.3 and available for
Python 2 through importlib2_ on PyPI. You might be tempted to write code to
access e.g. the ``importlib.abc`` module by doing the following::
import sys
if sys.version_info[0] == 3:
from importlib import abc
else:
from importlib2 import abc
The problem with this code is what happens when Python 4 comes out? It would
be better to treat Python 2 as the exceptional case instead of Python 3 and
assume that future Python versions will be more compatible with Python 3 than
Python 2::
import sys
if sys.version_info[0] > 2:
from importlib import abc
else:
from importlib2 import abc
The best solution, though, is to do no version detection at all and instead rely
on feature detection. That avoids any potential issues of getting the version
detection wrong and helps keep you future-compatible::
try:
from importlib import abc
except ImportError:
from importlib2 import abc
Prevent compatibility regressions
---------------------------------
@ -296,14 +350,12 @@ 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_
@ -320,22 +372,23 @@ 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 .
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.
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
---------------------------------------------
@ -347,10 +400,12 @@ tox with your continuous integration system so that you never accidentally break
Python 2 or 3 support.
You may also want to use the ``-bb`` flag with the Python 3 interpreter to
trigger an exception when you are comparing bytes to strings. Usually it's
simply ``False``, but if you made a mistake in your separation of text/binary
data handling you may be accidentally comparing text and binary data. This flag
will raise an exception when that occurs to help track down such cases.
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
@ -358,20 +413,17 @@ 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
====================================
Consider using optional static type checking
--------------------------------------------
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``.
Another way to help port your code is to use a static type checker like
mypy_ or pytype_ on your code. These tools can be used to analyze your code as
if it's being run under Python 2, then you can run the tool a second time as if
your code is running under Python 3. By running a static type checker twice like
this you can discover if you're e.g. misusing binary data type in one version
of Python compared to another. If you add optional type hints to your code you
can also explicitly state whether your APIs use textual or binary data, helping
to make sure everything functions as expected in both versions of Python.
.. _2to3: https://docs.python.org/3/library/2to3.html
@ -379,14 +431,22 @@ supported by Python 2. You should also update the classifiers in your
.. _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
.. _importlib: https://docs.python.org/3/library/importlib.html#module-importlib
.. _importlib2: https://pypi.python.org/pypi/importlib2
.. _Modernize: https://python-modernize.readthedocs.org/en/latest/
.. _mypy: http://mypy-lang.org/
.. _Porting to Python 3: http://python3porting.com/
.. _Pylint: https://pypi.python.org/pypi/pylint
.. _Python 3 Q & A: https://ncoghlan-devs-python-notes.readthedocs.org/en/latest/python3/questions_and_answers.html
.. _pytype: https://github.com/google/pytype
.. _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
.. _Why Python 3 exists: http://www.snarky.ca/why-python-3-exists