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Coding Style and Standards

Projects under the Pylons Project scope have rigorous standards for both coding style, testing, and documentation.

Documentation Styling

Every project needs to have documentation built with sphinx using the pylons sphinx theme for consistency.

To build documentation using the Pylons theme, first checkout the theme from github with:

git clone git://

Then in the project that needs documentation built with it, under the docs dir symlink in the pylons theme:

ln -s /path/to/pylons_sphinx_theme _themes

This will then allow you to build the project utilizing the theme, and when updates are made to the theme the docs can be rebuilt easily.

New Feature Code Requirements

In order to add a feature to any Pylons Project package:

  • The feature must be documented in both the API and narrative documentation (in docs/).
  • The feature must work fully on the following CPython versions: 2.4, 2.5, 2.6, and 2.7 on both UNIX and Windows.
  • The feature must not cause installation or runtime failure on Jython or App Engine. If it doesn't cause installation or runtime failure, but doesn't actually work on these platforms, that caveat should be spelled out in the documentation.
  • The feature must not depend on any particular persistence layer (filesystem, SQL, etc).
  • The feature must not add unnecessary dependencies (where "unnecessary" is of course subjective, but new dependencies should be discussed).

The above requirements are relaxed for paster template dependencies. If a paster template has an install-time dependency on something that doesn't work on a particular platform, that caveat should be spelled out clearly in its documentation (within its docs/ directory).

To determine if a feature should be added to an existing package, or deserves a package of its own, feel free to talk to one of the developer teams.

Documentation Coverage

If you fix a bug, and the bug requires an API or behavior modification, all documentation in the package which references that API or behavior must change to reflect the bug fix, ideally in the same commit that fixes the bug or adds the feature.

Change Log

Feature additions and bugfixes must be added to the CHANGES.txt file in the prevailing style. Changelog entries should be long and descriptive, not cryptic. Other developers should be able to know what your changelog entry means.

Test Coverage

The codebase must have 100% test statement coverage after each commit. You can test coverage via python nosetests --with-coverage (requires the nose and coverage packages).

Coding Style

All Python code should follow PEP-8 style guide-lines. Whitespace rules are relaxed and it is not necessary to put 2 newlines between classes (though that's just fine if you do). 80-column lines, in particular, are mandatory.

  • Single-line imports

    Do this:

    import os
    import sys

    Do not do this:

    import os, sys

    Importing a single item per line makes it easier to read patches and commit diffs.

  • Import Order

    Imports should be ordered by their origin. Names should be imported in this order:

    1. Python standard library
    2. 3rd party packages
    3. Other modules from the current package
  • Wildcard Imports

    Do not import all the names from a package, import just the ones that are needed. Single-line imports applies here as well, each name from the other package should be imported on its own line.

  • No mutable objects as default arguments

    Remember that since Python only parses the default argument for a function/method just once, they cannot be safely used as default arguments.

    Do not do this:

    def somefunc(default={}):
        if default.get(...):

    Either of these is fine:

    def somefunc(default=None):
        default = default or {}
    def somefunc(default=None):
        if default is None:
            default = {}
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