=======================
Python coding standards
=======================

For Python code style follow `PEP 8`_ plus the guidelines below.

.. _PEP 8: http://www.python.org/dev/peps/pep-0008/

Some good links about Python code style:

- `Python Coding Standards <http://lists.osafoundation.org/pipermail/dev/2003-March/000479.html>`_ from Yahoo
- `Google Python Style Guide <http://google-styleguide.googlecode.com/svn/trunk/pyguide.html>`_

.. seealso::

   :doc:`string-i18n`
     How to mark strings for translation.


Use single quotes
-----------------

Use single-quotes for string literals, e.g. ``'my-identifier'``, *but* use
double-quotes for strings that are likely to contain single-quote characters as
part of the string itself (such as error messages, or any strings containing
natural language), e.g.  ``"You've got an error!"``.

Single-quotes are easier to read and to type, but if a string contains
single-quote characters then double-quotes are better than escaping the
single-quote characters or wrapping the string in double single-quotes.

We also use triple single-quotes for docstrings, see `Docstrings`_.

.. _imports:

Imports
-------

- Avoid creating circular imports by only importing modules more
  specialized than the one you are editing.

  CKAN often uses code imported into a data structure instead of
  importing names directly. For example CKAN controllers only use
  ``get_action`` to access logic functions. This allows
  customization by CKAN plugins.

  .. image:: /images/ckan_importing_diagram.png
     :alt: CKAN importing diagram, general modules import
           more specific modules

- Don't use ``from module import *``. Instead list the names you
  need explicitly::

    from module import name1, name2

  Use parenthesis around the names if they are longer than one line::

    from module import (name1, name2, ...
        name12, name13)

  Most of the current CKAN code base imports just the modules and
  then accesses names with ``module.name``. This allows circular
  imports in some cases and may still be necessary for exsiting
  code, but is not recommended for new code.

- Make all imports at the start of the file, after the module docstring.
  Imports should be grouped in the following order:

  1. Standard library imports
  2. Third-party imports
  3. CKAN imports


Logging
-------

We use `the Python standard library's logging module <http://docs.python.org/2.6/library/logging.html>`_
to log messages in CKAN, e.g.::

    import logging
    ...
    logger = logging.getLogger(__name__)
    ...
    logger.debug('some debug message')

When logging:

- Keep log messages short.

- Don't include object representations in the log message.  It *is* useful
  to include a domain model identifier where appropriate.

- Choose an appropriate log-level (DEBUG, INFO, ERROR, WARNING or CRITICAL,
  see `Python's Logging HOWTO`_).

.. _Python's Logging HOWTO: http://docs.python.org/2/howto/logging.html

String formatting
------------------

Don't use the old ``%s`` style string formatting, e.g. ``"i am a %s" % sub``.
This kind of string formatting is not helpful for internationalization.

Use the `new .format() method`_ instead, and give meaningful names to each
replacement field, for example::

  _(' ... {foo} ... {bar} ...').format(foo='foo-value', bar='bar-value')

.. _new .format() method: http://docs.python.org/2/library/stdtypes.html#str.format

.. _docstrings:

Docstrings
----------

.. _PEP 257: http://www.python.org/dev/peps/pep-0257/

We want CKAN's docstrings to be clear and easy to read for programmers who are
smart and competent but who may not know a lot of CKAN technical jargon and
whose first language may not be English. We also want it to be easy to maintain
the docstrings and keep them up to date with the actual behaviour of the code
as it changes over time. So:

- All modules and all public functions, classes and methods exported by a
  module should normally have docstrings (see `PEP 257`_).
- Keep docstrings short, describe only what's necessary and no more.
- Keep docstrings simple: use plain, concise English.
- Try to avoid repetition.


PEP 257 (Docstring Conventions)
```````````````````````````````

Generally, follow `PEP 257`_ for docstrings. We'll only describe the ways that
CKAN differs from or extends PEP 257 below.

CKAN docstrings deviate from PEP 257 in a couple of ways:

- We use ``'''triple single quotes'''`` around docstrings, not ``"""triple
  double quotes"""`` (put triple single quotes around one-line docstrings as
  well as multi-line ones, it makes them easier to expand later)
- We use Sphinx domain object cross-references to cross-reference to other
  code objects (see below)
- We use Sphinx directives for documenting parameters, exceptions and return
  values (see below)


.. _Referencing other code objects:

Referencing other code objects with ``:py:``
--------------------------------------------

If you want to refer to another Python or JavaScript module, function or class
etc. in a docstring (or from a ``.rst`` file), use `Sphinx domain object
cross-references
<http://sphinx-doc.org/domains.html#cross-referencing-python-objects>`_, for
example::

    See :py:mod:`ckan.lib.helpers`.

    See :py:func:`ckan.logic.action.create.package_create`.

    See :py:class:`ckan.logic.NotFound`.

For the full list of types of cross-reference,  see the
`Sphinx docs <http://sphinx-doc.org/domains.html#cross-referencing-python-objects>`_.


.. note::

   These kinds of cross-references can also be used to reference other types
   of object besides Python objects, for example `JavaScript objects <http://sphinx-doc.org/domains.html#the-javascript-domain>`_
   or even command-line scripts and options and environment variables. See
   `the Sphinx docs <http://sphinx-doc.org/domains.html>`_ for the full
   details.


Cross-referencing objects like this means that Sphinx will style the reference
with the right CSS, and hyperlink the reference to the docs for the referenced
object. Sphinx can also generate error messages when non-existent objects are
referenced, which helps to keep the docs up to date as the code changes.

.. tip::

   Sphinx will render a cross-reference like
   ``:py:func:`ckan.logic.action.create.package_create``` as the full name of
   the function: :py:func:`ckan.logic.action.create.package_create`. If you want the
   docs to contain only the local name of the function (e.g. just
   :py:func:`~ckan.logic.action.create.package_create`), put a ``~`` at the
   start::

    :py:func:`~ckan.logic.action.create.package_create`

   (But you should always use the fully qualified name in your docstring or
   ``*.rst`` file.)


Documenting exceptions raised with ``:raises``
``````````````````````````````````````````````

There are a few guidelines that CKAN code should follow regarding exceptions:

1. **All public functions that CKAN exports for third-party code to use
   should document any exceptions they raise**. See below for how to document
   exceptions raised.

   For example the template helper functions in :py:mod:`ckan.lib.helpers`,
   anything imported into :py:mod:`ckan.plugins.toolkit`, and all of the
   action API functions defined in :py:mod:`ckan.logic.action`, should list
   exceptions raised in their docstrings.

   This is because CKAN themes, extensions and API clients need to be able to
   call CKAN code without crashing, so they need to know what exceptions they
   should handle (and extension developers shouldn't have to understand the
   CKAN core source code).

2. On the other hand, **internal functions that are only used within CKAN
   shouldn't list exceptions in their docstrings**.

   This is because it would be difficult to keep all the exception lists up to
   date with the actual code behaviour, so the docstrings would become more
   misleading than useful.

3. **Code should only raise exceptions from within its allowed set**.

   Each module in CKAN has a set of zero or more exceptions, defined somewhere
   near the module, that code in that module is allowed to raise. For example
   ``ckan/logic/__init__.py`` defines a number of exception types for code
   in ``ckan/logic/`` to use. CKAN code should never raise exceptions types
   defined elsewhere in CKAN, in third-party code or in the Python standard
   library.

4. **All code should catch any exceptions raised by called functions**, and
   either handle the exception, re-raise the exception (if it's from the code's
   set of allowed exception types), or wrap the exception in an allowed
   exception type and re-raise it.

   This is to make it easy for a CKAN core developer to look at the source code
   of an internal function, scan it for the keyword ``raise``, and see what
   types of exception the function may raise, so they know what exceptions they
   need to catch if they're going to call the function. Developers shouldn't
   have to read the source of all the functions that a function calls (and
   the functions they call...) to find out what exceptions they needs to catch
   to call a function without crashing.

.. todo::

   Insert examples of how to re-raise and how to wrap-and-re-raise an
   exception.

Use ``:raises:`` to document exceptions raised by public functions. The
docstring should say what type of exception is raised and under what
conditions. Use ``:py:class:`` to reference exception types. For example::

    def member_list(context, data_dict=None):
        '''Return the members of a group.

        ... (parameters and return values documented here) ...

        :raises: :py:class:`ckan.logic.NotFound`: if the group doesn't exist

        '''


Sphinx field lists
``````````````````

Use `Sphinx field lists`_ for documenting the parameters, exceptions and
returns of functions:

- Use ``:param`` and ``:type`` to describe each parameter
- Use ``:returns`` and ``:rtype`` to describe each return
- Use ``:raises`` to describe each exception raised

Example of a short docstring:

::

    @property
    def packages(self):
        '''Return a list of all packages that have this tag, sorted by name.

        :rtype: list of ckan.model.package.Package objects

        '''

Example of a longer docstring:

::

    @classmethod
    def search_by_name(cls, search_term, vocab_id_or_name=None):
        '''Return all tags whose names contain a given string.

        By default only free tags (tags which do not belong to any vocabulary)
        are returned. If the optional argument ``vocab_id_or_name`` is given
        then only tags from that vocabulary are returned.

        :param search_term: the string to search for in the tag names
        :type search_term: string
        :param vocab_id_or_name: the id or name of the vocabulary to look in
            (optional, default: None)
        :type vocab_id_or_name: string

        :returns: a list of tags that match the search term
        :rtype: list of ckan.model.tag.Tag objects

        '''

The phrases that follow ``:param foo:``, ``:type foo:``, or ``:returns:``
should not start with capital letters or end with full stops. These should be
short phrases and not full sentences. If more detail is required put it in the
function description instead.

Indicate optional arguments by ending their descriptions with ``(optional)`` in
brackets. Where relevant also indicate the default value: ``(optional, default:
5)``.

.. _Sphinx field lists: http://sphinx.pocoo.org/markup/desc.html#info-field-lists

You can also use a little inline `reStructuredText markup`_ in docstrings, e.g.
``*stars for emphasis*`` or ````double-backticks for literal text````

.. _reStructuredText markup: http://docutils.sourceforge.net/docs/user/rst/quickref.html#inline-markup

.. _Action API Docstrings:

Action API docstrings
`````````````````````

Docstrings from CKAN's action API are processed with `autodoc`_ and
included in the API chapter of CKAN's documentation. The intended audience of
these docstrings is users of the CKAN API and not (just) CKAN core developers.

In the Python source each API function has the same two arguments (``context``
and ``data_dict``), but the docstrings should document the keys that the
functions read from ``data_dict`` and not ``context`` and ``data_dict``
themselves, as this is what the user has to POST in the JSON dict when calling
the API.

Where practical, it's helpful to give examples of param and return values in
API docstrings.

CKAN datasets used to be called packages and the old name still appears in the
source, e.g. in function names like ``package_list()``. When documenting
functions like this write dataset not package, but the first time you do this
put package after it in brackets to avoid any confusion, e.g.

::

    def package_show(context, data_dict):
        '''Return the metadata of a dataset (package) and its resources.

Example of a ckan.logic.action API docstring:

::

    def vocabulary_create(context, data_dict):
        '''Create a new tag vocabulary.

        You must be a sysadmin to create vocabularies.

        :param name: the name of the new vocabulary, e.g. ``'Genre'``
        :type name: string
        :param tags: the new tags to add to the new vocabulary, for the format of
            tag dictionaries see ``tag_create()``
        :type tags: list of tag dictionaries

        :returns: the newly-created vocabulary
        :rtype: dictionary

        '''

.. _Autodoc: http://sphinx.pocoo.org/ext/autodoc.html


Some helpful tools for Python code quality
------------------------------------------

There are various tools that can help you to check your Python code for PEP8
conformance and general code quality. We recommend using them.

* `pep8`_ checks your Python code against some of the style conventions in PEP
  8. As mentioned above, only perform style clean-ups on master to help avoid
  spurious merge conflicts.

* `pylint`_ analyzes Python source code looking for bugs and signs of poor
  quality.

* `pyflakes`_ also analyzes Python programs to detect errors.

* `flake8`_ combines both pep8 and pyflakes into a single tool.

* `Syntastic`_ is a Vim plugin with support for flake8, pyflakes and pylint.

.. _pep8: http://pypi.python.org/pypi/pep8
.. _pylint: http://www.logilab.org/857
.. _pyflakes: http://pypi.python.org/pypi/pyflakes
.. _flake8: http://pypi.python.org/pypi/flake8
.. _Syntastic: https://github.com/scrooloose/syntastic
