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:

  • black, a source code formatter;
  • isort, which organises each file's imports;
  • flake8, which can spot common errors; and
  • mypy, a type checker.

Install them with:

pip install -e ".[lint,mypy]"

The easiest way to run the lints is to invoke the linter script as follows.


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 or mypy on save as they take a while and can be very resource intensive.

General rules

  • Naming:
    • Use CamelCase for class and type names
    • Use underscores for function_names and variable_names.
  • Docstrings: should follow the google code style. See the examples 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.


      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 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 ".

    • 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] ." 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.

  • Use two-space indents.

  • true and false are spelt thus (as opposed to True, etc.)

  • Use single quotes (') rather than double-quotes (") or backticks (`) to refer to configuration options.


## 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
  # 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/ Making sure that the output from this script matches the desired format is left as an exercise for the reader!