Update porting HOWTO to drop unicode_literals and mention static type checking

brettcannon · brettcannon · commit 5866719510b6 · 2016-12-17T12:37:20.000-08:00
diff --git a/Doc/howto/pyporting.rst b/Doc/howto/pyporting.rst
@@ -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.
@@ -32,19 +33,17 @@ are:
 #. 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
    (``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
@@ -54,7 +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
@@ -82,12 +81,13 @@ 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
+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
 ++++++++
 
@@ -173,6 +178,7 @@ 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
 +++++++++++++++++++++++
 
@@ -189,7 +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.
@@ -198,15 +204,15 @@ 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
+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.
 
@@ -232,22 +238,25 @@ 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
 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
@@ -274,21 +283,22 @@ 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 is
+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.
 
@@ -340,14 +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_
@@ -364,22 +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
 ---------------------------------------------
@@ -404,20 +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
-====================================
-
-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
+Consider using optional static type checking
+--------------------------------------------
 
-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
@@ -428,13 +434,19 @@ supported by Python 2. You should also update the classifiers in your
 .. _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
