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Update dev_guide with Python3 porting advice.

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Also update the date that Python2.4 support will be dropped to say
Ansible-2.4 will no longer have python-2.4 support.
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Toshio Kuratomi 2016-10-14 12:18:04 -07:00
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docsite/rst/dev_guide/developing_modules_python3.rst
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@ -1,34 +1,39 @@
===========================
Porting Modules to Python 3
===========================
===============================
Ansible and Porting to Python 3
===============================
Ansible modules are not the usual Python-3 porting exercise. There are two
factors that make it harder to port them than most code:
Ansible can be divided into three overlapping pieces for the purposes of
porting:
1. Many modules need to run on Python-2.4 in addition to Python-3.
2. A lot of mocking has to go into unittesting a Python-3 module. So it's
harder to test that your porting has fixed everything or to make sure that
later commits haven't regressed.
1. Controller-side code. This is the code which runs on the machine where you
invoke /usr/bin/ansible
2. Modules. This is the code which Ansible transmits over the wire and
invokes on the managed machine.
3. module_utils code. This is code whose primary purpose is to be used by the
modules to perform tasks. However, some controller-side code might use
generic functions from here.
Which version of Python-3.x and which version of Python-2.x are our minimums?
=============================================================================
Much of the knowledge of porting code will be usable on all three of these
pieces but there are some special considerations for some of it as well.
The short answer is Python-3.4 and Python-2.4 but please read on for more
information.
Minimum Version of Python-3.x and Python-2.x
============================================
For Python-3 we are currently using Python-3.4 as a minimum. However, no long
term supported Linux distributions currently ship with Python-3. When that
occurs, we will probably take that as our minimum Python-3 version rather than
Python-3.4. Thus far, Python-3 has been adding small changes that make it
more compatible with Python-2 in its newer versions (For instance, Python-3.5
added the ability to use percent-formatted byte strings.) so it should be more
pleasant to use a newer version of Python-3 if it's available. At some point
this will change but we'll just have to cross that bridge when we get to it.
In controller side code, we support Python-3.5 or greater and Python-2.6 or
greater.
Modules (and by extension, module_utils) is more complex. The short answer is
Python-3.5 and Python-2.4 but please read on for more information.
Python-3.5 was chosen as a minimum because it is the earliest Python-3 version
adopted as the default Python by a Long Term Support (LTS) Linux distribution.
In this case, Ubuntu-16.04. Previous LTS Linux distros shipped with
a Python-2 version which users can rely upon instead of the Python-3 version.
For Python-2 the default is for modules to run on Python-2.4. This allows
users with older distributions that are stuck on Python-2.4 to manage their
machines. Modules are allowed to drop support for Python-2.4 when one of
their dependent libraries require a higher version of python. This is not an
their dependent libraries require a higher version of Python. This is not an
invitation to add unnecessary dependent libraries in order to force your
module to be usable only with a newer version of Python. Instead it is an
acknowledgment that some libraries (for instance, boto3 and docker-py) will
@ -38,12 +43,228 @@ only function with newer Python.
The only long term supported distro that we know of with Python-2.4 is
RHEL5 (and its rebuilds like CentOS5) which is supported until April of
2017. We will likely end our support for Python-2.4 in modules in an
Ansible release around that time. We know of no long term supported
distributions with Python-2.5 so the new minimum Python-2 version will
likely be Python-2.6. This will let us take advantage of the
forwards-compat features of Python-2.6 so porting and maintainance of
Python-2/Python-3 code will be easier after that.
2017. For Ansible, that means Ansible-2.3 will be the last major release
that supports Python-2.4 on the module-side. Ansible-2.4 will require
Python-2.6 or greater for modules.
-----------------------------------
Porting Controller Code to Python 3
-----------------------------------
Most of the general tips for porting code to be used on both Python-2 and
Python-3 applies to porting controller code. The best place to start learning
to port code is `Lennart Regebro's book: Porting to Python 3 <http://python3porting.com/>`_.
The book describes several strategies for porting to Python 3. The one we're
using is `to support Python-2 and Python-3 from a single code base
http://python3porting.com/strategies.html#python-2-and-python-3-without-conversion`_
Controller String Strategy
==========================
Background
----------
One of the most essential things to decide upon for porting code to Python-3
is what String Model to use. Strings can be an array of bytes like in C or
they can be an array of text. Text is what we think of as letters, digits,
numbers, other printable symbols, and a small number of unprintable "symbols"
(control codes).
In Python-2, the two types for these (:class:`str` for bytes and
:class:`unicode` for text) are often used interchangably. When dealing only
with ascii characters, the strings can be combined, compared, and converted
from one type to another automatically. When non-ascii characters are
introduced, Python starts throwing exceptions due to not knowing what encoding
the non-ascii characters should be in.
Python-3 changes this by making the separation between bytes (:class:`bytes`)
and text (:class:`str`) more strict. Python will throw an exception when
trying to combine and compare the two types. The programmer has to explicitly
convert from one type to the other to mix values from each.
This change makes it immediately apparent to the programmer when code is
mixing the types inappropriately instead of working until one of their users
starts using non-ascii input and things then breaking. However, it forces the
programmer to proactively define a strategy for working with strings in their
program so that they don't mix text and byte strings unintentionally.
Unicode Sandwch
---------------
In Controller-side code we use a strategy known as the Unicode Sandwich (named
after Python-2's :class:`unicode` text type). For Unicode Sandwich we know that
at the border of our code and the outside world (File and network IO,
environment variables, some library calls, etc) we are going to receive bytes.
We need to transform these bytes into text and use that throughout the
internal portions of our code. When we have to send those strings back out to
the outside world we convert the text back into bytes and then send them on.
Visualizing this, you see is a top and bottom layer of bytes, a layer of
conversion between, and all text type in the center. Thus the sandwich
metaphor.
Common Borders
--------------
This is a partial list of places where we have to convert to and from bytes.
It's not exhaustive but gives you an idea of where to watch for problems.
Reading and writing to files
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In Python-2, reading from files yields bytes. In Python-3, it can yield text.
To make code that's portable to both we don't make use of Python-3's ability
yield text but do the conversion explicitly ourselves::
from ansible.module_utils._text import to_text
with open('filename-with-utf8-data.txt', 'rb') as my_file:
b_data = my_file.read()
try:
data = to_text(b_data, errors='surrogate_or_strict')
except UnicodeError:
# Handle the exception gracefully -- usually by displaying a good
# user-centric error message that can be traced back to this piece
# of code.
.. note:: Much of Ansible assumes that all encoded text is UTF-8. At some
point, if there is demand for other encodings we may change that but for
now it is safe to assume that bytes are UTF-8.
Writing to files is the opposite process::
from ansible.module_utils._text import to_bytes
with open('filename.txt', 'wb') as my_file:
my_file.write(to_bytes(some_text_string))
Note that we don't have to catch :exc:`UnicodeError` here because we're
transforming to UTF-8 and all text strings in Python can be transformed back
to UTF-8.
Filesystem Interaction
~~~~~~~~~~~~~~~~~~~~~~
Dealing with filenames often involves dropping back to bytes as, on UNIX-like
systems, filenames are bytes. On Python-2, if we pass a text string to these
functions, the text string will be converted to a byte string inside of the
function and a traceback will occur if non-ascii characters are present. In
Python-3, a traceback will only occur if the text string can't be decoded in
the current locale but it's still good to be explicit and have code which
works on both versions::
import os.path
from ansible.module_utils._text import to_bytes
filename = u'/var/tmp/くらとみ.txt'
f = open(to_bytes(filename), 'wb')
mtime = os.path.getmtime(to_bytes(filename))
b_filename = os.path.expandvars(to_bytes(filename))
if os.path.exists(to_bytes(filename)):
pass
When you are only manipulating a filename as a string without talking to the
filesystem (or a C library which talks to the filesystem) you can often get
away without converting to bytes. If the code needs to manipulate the
filename and also talk to the filesystem it can be more convenient to
transform to bytes right away and manipulate in bytes, though::
import os.path
os.path.join(u'/var/tmp/café', u'くらとみ')
os.path.split(u'/var/tmp/café/くらとみ')
.. warn:: Make sure all variables input into a function are the same type.
If you're working with something like :func:`os.path.join` which takes
multiple strings and uses them in combination, you need to make sure that
all the types are the same (all bytes type or all text type). Mixing
bytes and text will cause tracebacks.
Interacting with Other Programs
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Interacting with other programs goes through the operating system and
C libraries and operates on things that the UNIX kernel defines. These
interfaces are all byte-oriented so the Python interface is byte oriented as
well. On both Python-2 and Python-3, byte strings should be given to Python's
subprocess library and byte strings should be expected back from it.
One of the main places in Ansible's controller code that we interact with
other programs is the connection plugins' ``exec_command`` methods. These
methods transform any text strings they receive in the command (and arugments
to the command) to execute into bytes and return stdout and stderr as byte strings
Higher level functions (like action plugins' ``_low_level_execute_command``)
transform the output into text strings.
Tips, tricks, and idioms to adopt
=================================
Forwards Compat Boilerplate
---------------------------
Use the following boilerplate code at the top of all controller-side modules
to make certain constructs act the same way on Python-2 and Python-3::
# Make coding more python3-ish
from __future__ import (absolute_import, division, print_function)
__metaclass__ = type
``__metaclass__ = type`` makes all classes defined in the file into new-style
classes without explicitly inheriting from :class:`object`.
The ``__future__`` imports do the following:
:absolute_import: Makes imports look in :attr:`sys.path` for the modules being
imported, skipping the directory in which the module doing the importing
lives. If the code wants to use the directory in which the module doing
the importing, there's a new dot notation to do so.
:division: Makes division of integers always return a float. If you need to
find the quotient use ``x // y`` instead of ``x / y``.
:print_function: Changes :func:`print` from a keyword into a function.
.. seealso::
* `PEP 0328: Absolute Imports <https://www.python.org/dev/peps/pep-0328/#guido-s-decision>`_
* `PEP 0238: Division <https://www.python.org/dev/peps/pep-0238>`_
* `PEP 3105: Print function <https://www.python.org/dev/peps/pep-3105>`_
Prefix byte strings with "b_"
-----------------------------
Since mixing text and bytes types leads to tracebacks we want to be clear
about what variables hold text and what variables hold bytes. We do this by
prefixing any variable holding bytes with ``b_``. For instance::
filename = u'/var/tmp/café.txt'
b_filename = to_bytes(filename)
with open(b_filename) as f:
data = f.read()
Why not prefix the text strings instead? The reason is that we only operate
on byte strings at the borders so there are fewer variables that need bytes
than text.
---------------------------
Porting Modules to Python 3
---------------------------
Ansible modules are not the usual Python-3 porting exercise. There are two
factors that make it harder to port them than most code:
1. Many modules need to run on Python-2.4 in addition to Python-3.
2. A lot of mocking has to go into unittesting a Python-3 module. So it's
harder to test that your porting has fixed everything or to make sure that
later commits haven't regressed.
Module String Strategy
======================
There are a large number of modules in Ansible. Most of those are maintained
by the Ansible community at large, not by a centralized team. To make life
easier on them, it was decided not to break backwards compatibility by
mandating that all strings inside of modules are text and converting between
text and bytes at the borders. Instead, we're using a native string strategy
for now.
Supporting only Python-2 or only Python-3
=========================================
@ -157,3 +378,24 @@ which should not be tested (because we know that they are older modules which
have not yet been ported to pass the Python-3 syntax checks. To get another
old module to compile with Python-3, remove the entry for it from the list.
The goal is to have the LIST be empty.
-------------------------------------
Porting module_utils code to Python 3
-------------------------------------
module_utils code is largely like module code. However, some pieces of it are
used by the controller as well. Because of this, it needs to be usable with
the controller's assumptions. This is most notable in the string strategy.
Module_utils String Strategy
============================
Module_utils **must** use the Native String Strategy. Functions in
module_utils receive either text strings or byte strings and may emit either
the same type as they were given or the native string for the Python version
they are run on depending on which makes the most sense for that function.
Functions which return strings **must** document whether they return text,
byte, or native strings. Module-utils functions are therefore often very
defensive in nature, converting from potential text or bytes at the
beginning of a function and converting to the native string type at the end.