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Edits for docs PR #18032

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Scott Butler 8 years ago
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commit 578170a908
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docsite/rst/dev_guide/developing_modules_python3.rst
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@ -22,29 +22,27 @@ Minimum Version of Python-3.x and Python-2.x
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.
For modules (and by extension, module_utils) we support
Python-3.5 and Python-2.4. 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 distributions shipped with a Python-2 version which users can rely upon instead of the
Python-3 version.
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
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 requires 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
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
only function with newer Python.
only function with a newer version of Python.
.. note:: When will we drop support for Python-2.4?
.. note:: Python-2.4 Support:
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
The only long term supported distro that we know of with Python-2.4 support is
RHEL5 (and its rebuilds like CentOS5), which is supported until April of
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
that supports Python-2.4 for modules. Ansible-2.4 will require
Python-2.6 or greater for modules.
-----------------------------------
@ -66,42 +64,41 @@ 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
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
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.
the non-ASCII characters should be in.
Python-3 changes this by making the separation between bytes (:class:`bytes`)
Python-3 changes this behavior 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
mixing the types inappropriately, rather than working until one of their users
causes an exception by entering non-ASCII input. 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
Unicode Sandwich
---------------
In Controller-side code we use a strategy known as the Unicode Sandwich (named
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.
at the border of our code and the outside world (for example, file and network IO,
environment variables, and some library calls) 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.
the outside world we first convert the text back into bytes.
To visual this, imagine a 'sandwich' consisting of a top and bottom layer of bytes, a layer of
conversion between, and all text type in the center.
Common Borders
--------------
@ -114,7 +111,7 @@ 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::
to yield text but instead do the conversion explicitly ourselves. For example::
from ansible.module_utils._text import to_text
@ -128,7 +125,7 @@ yield text but do the conversion explicitly ourselves::
# 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
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::
@ -145,12 +142,12 @@ 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
Dealing with filenames often involves dropping back to bytes because 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
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
the current locale, but it's still good to be explicit and have code which
works on both versions::
import os.path
@ -167,18 +164,18 @@ works on both versions::
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::
filename and also talk to the filesystem, it can be more convenient to
transform to bytes right away and manipulate in bytes. For example::
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.
.. warn:: Make sure all variables passed to 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
all the types are the same (either all bytes or all text). Mixing
bytes and text will cause tracebacks.
Interacting with Other Programs
@ -200,7 +197,7 @@ transform the output into text strings.
Tips, tricks, and idioms to adopt
=================================
Forwards Compat Boilerplate
Forwards Compatability Boilerplate
---------------------------
Use the following boilerplate code at the top of all controller-side modules
@ -240,8 +237,8 @@ prefixing any variable holding bytes with ``b_``. For instance::
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
We do not recommend prefixing the text strings instead because we only operate
on byte strings at the borders, so there are fewer variables that need bytes
than text.
---------------------------
@ -252,8 +249,8 @@ 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
2. A lot of mocking has to go into unit testing a Python-3 module, so it's
harder to test that your porting has fixed everything or to to make sure that
later commits haven't regressed.
Module String Strategy
@ -263,56 +260,18 @@ 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
text and bytes at the borders; instead, we're using a native string strategy
for now.
Supporting only Python-2 or only Python-3
=========================================
Sometimes a module's dependent libraries only run on Python-2 or only run on
Python-3. We do not yet have a strategy for these modules but we'll need to
come up with one. I see three possibilities:
1. We treat these libraries like any other libraries that may not be installed
on the system. When we import them we check if the import was successful.
If so, then we continue. If not we return an error about the library being
missing. Users will have to find out that the library is unavailable on
their version of Python either by searching for the library on their own or
reading the requirements section in :command:`ansible-doc`.
2. The shebang line is the only metadata that Ansible extracts from a module
so we may end up using that to specify what we mean. Something like
``#!/usr/bin/python`` means the module will run on both Python-2 and
Python-3, ``#!/usr/bin/python2`` means the module will only run on
Python-2, and ``#!/usr/bin/python3`` means the module will only run on
Python-3. Ansible's code will need to be modified to accommodate this.
For :command:`python2`, if ``ansible_python2_interpreter`` is not set, it
will have to fallback to `` ansible_python_interpreter`` and if that's not
set, fallback to ``/usr/bin/python``. For :command:`python3`, Ansible
will have to first try ``ansible_python3_interpreter`` and then fallback to
``/usr/bin/python3`` as normal.
3. We add a way for Ansible to retrieve metadata about modules. The metadata
will include the version of Python that is required.
Methods 2 and 3 will both require that we modify modules or otherwise add this
additional information somewhere. 2 needs only a little code changes in
executor/module_common.py to parse. 3 will require a lot of work. This is
probably not worthwhile if this is the only change but could be worthwhile if
there's other things as well. 1 requires that we port all modules to work
with python3 syntax but only the code path to get to the library import being
attempted and then a fail_json() being called because the libraries are
unavailable needs to actually work.
Tips, tricks, and idioms to adopt
=================================
Exceptions
----------
In code which already needs Python-2.6+ (For instance, because a library it
In code which already needs Python-2.6+ (for instance, because a library it
depends on only runs on Python >= 2.6) it is okay to port directly to the new
exception catching syntax::
exception-catching syntax::
try:
a = 2/0

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