MatrixSynapse/docs/code_style.rst

Code Style
==========

Formatting tools
----------------

The Synapse codebase uses a number of code formatting tools in order to
quickly and automatically check for formatting (and sometimes logical) errors
in code.

The necessary tools are detailed below.

- **black**

  The Synapse codebase uses `black <https://pypi.org/project/black/>`_ as an
  opinionated code formatter, ensuring all comitted code is properly
  formatted.

  First install ``black`` with::

    pip install --upgrade black

  Have ``black`` auto-format your code (it shouldn't change any functionality)
  with::

    black . --exclude="\.tox|build|env"

- **flake8**

  ``flake8`` is a code checking tool. We require code to pass ``flake8`` before being merged into the codebase.

  Install ``flake8`` with::

    pip install --upgrade flake8

  Check all application and test code with::

    flake8 synapse tests

- **isort**

  ``isort`` ensures imports are nicely formatted, and can suggest and
  auto-fix issues such as double-importing.

  Install ``isort`` with::

    pip install --upgrade isort

  Auto-fix imports with::

    isort -rc synapse tests

  ``-rc`` means to recursively search the given directories.

It's worth noting that modern IDEs and text editors can run these tools
automatically on save. It may be worth looking into whether this
functionality is supported in your editor for a more convenient development
workflow. It is not, however, recommended to run ``flake8`` on save as it
takes a while and is very resource intensive.

General rules
-------------

- **Naming**:

  - Use camel case for class and type names
  - Use underscores for functions and variables.

- **Docstrings**: should follow the `google code style
  <https://google.github.io/styleguide/pyguide.html#38-comments-and-docstrings>`_.
  This is so that we can generate documentation with `sphinx
  <http://sphinxcontrib-napoleon.readthedocs.org/en/latest/>`_. See the
  `examples
  <http://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_google.html>`_
  in the sphinx documentation.

- **Imports**:

  - Imports should be sorted by ``isort`` as described above.

  - Prefer to import classes and functions rather than packages or modules.

    Example::

      from synapse.types import UserID
      ...
      user_id = UserID(local, server)

    is preferred over::

      from synapse import types
      ...
      user_id = types.UserID(local, server)

    (or any other variant).

    This goes against the advice in the Google style guide, but it means that
    errors in the name are caught early (at import time).

  - Avoid wildcard imports (``from synapse.types import *``) and relative
    imports (``from .types import UserID``).

Configuration file format
-------------------------

The `sample configuration file <./sample_config.yaml>`_ acts as a reference to
Synapse's configuration options for server administrators. Remember that many
readers will be unfamiliar with YAML and server administration in general, so
that it is important that the file be as easy to understand as possible, which
includes following a consistent format.

Some guidelines follow:

* Sections should be separated with a heading consisting of a single line
  prefixed and suffixed with ``##``. There should be **two** blank lines
  before the section header, and **one** after.

* Each option should be listed in the file with the following format:

  * A comment describing the setting. Each line of this comment should be
    prefixed with a hash (``#``) and a space.

    The comment should describe the default behaviour (ie, what happens if
    the setting is omitted), as well as what the effect will be if the
    setting is changed.

    Often, the comment end with something like "uncomment the
    following to \<do action>".

  * A line consisting of only ``#``.

  * A commented-out example setting, prefixed with only ``#``.

    For boolean (on/off) options, convention is that this example should be
    the *opposite* to the default (so the comment will end with "Uncomment
    the following to enable [or disable] \<feature\>." For other options,
    the example should give some non-default value which is likely to be
    useful to the reader.

* There should be a blank line between each option.

* Where several settings are grouped into a single dict, *avoid* the
  convention where the whole block is commented out, resulting in comment
  lines starting ``# #``, as this is hard to read and confusing to
  edit. Instead, leave the top-level config option uncommented, and follow
  the conventions above for sub-options. Ensure that your code correctly
  handles the top-level option being set to ``None`` (as it will be if no
  sub-options are enabled).

* Lines should be wrapped at 80 characters.

Example::

    ## Frobnication ##

    # The frobnicator will ensure that all requests are fully frobnicated.
    # To enable it, uncomment the following.
    #
    #frobnicator_enabled: true

    # By default, the frobnicator will frobnicate with the default frobber.
    # The following will make it use an alternative frobber.
    #
    #frobincator_frobber: special_frobber

    # Settings for the frobber
    #
    frobber:
       # frobbing speed. Defaults to 1.
       #
       #speed: 10

       # frobbing distance. Defaults to 1000.
       #
       #distance: 100

Note that the sample configuration is generated from the synapse code and is
maintained by a script, ``scripts-dev/generate_sample_config``. Making sure
that the output from this script matches the desired format is left as an
exercise for the reader!