Coding style and standards

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

Documentation Style Guide

Documentation contributions should follow the Pylons Project Documentation Style Guide. Every project needs to have documentation built with Sphinx using either the Pylons or Pyramid Sphinx Theme for consistency.

PDF output

Set the following values for latex_documents in docs/

                # Grouping the document tree into LaTeX files. List of tuples
# (source start file, target name, title, author, document class [howto/manual]).
latex_documents = [
    ('latexindex', 'pyramid_<project name>.tex', 'Pyramid\_<project name>', 'Author', 'manual'),

It is important to use \_ to escape the underscore in the document title to prevent a failure in LaTeX.

Comment the following line:

                  #latex_logo = '_static/pylons_small.png'

Copy the folder pyramid/docs/_static (contains two .png files) and the file pyramid/docs/ into your docs/ folder.

ePub output

Make sure you have the following value for epub_exclude_files in docs/

                # A list of files that should not be packed into the epub file.
  epub_exclude_files = ['_static/opensearch.xml', '_static/doctools.js',
      '_static/jquery.js', '_static/searchtools.js', '_static/underscore.js',
      '_static/basic.css', 'search.html', '_static/websupport.js']

New Feature Code Requirements

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

  • The feature must be documented in both the API docstrings and the narrative documentation in docs/.
  • The feature must work fully on CPython 2.7 on both UNIX and Windows and PyPy on UNIX. Most Pylons Project packages now run on Python 3. If you're working on such a package, and it already runs on Python 3, it must continue to run under Python 3 after your change. Some packages explicitly list Python 2.4, 2.5, or 2.6 support. Such support should be maintained if it exists. The tox.ini of most Pylons Project packages indicates which versions the package is tested under.
  • 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).

Testing code in a consistent manner can be difficult. To help developers learn our style ("dogma") of testing we've made available a set of testing notes at Unit Testing Guidelines.

Coding Style


All Python code should follow a derivation of PEP-8 style guidelines. Whitespace rules and other rules are relaxed (for example, it is not necessary to put two newlines between classes, though that's just fine if you do). Other rules are relaxed too, such as spaces around operators. 79-column lines, however, are mandatory.

If you do use the pep8 tool for automated checking, here is an invocation that is close to what is required by the project (subject to change):

pep8 --ignore=E302,E261,E231,E123,E301,E226,E262,E225,E303,E125,E251,E201,E202,E128,E122,E701,E203,E222,W293,W291,W391 *.py

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.

If you need to import lots of names from a single package, use:

              from thepackage import (

It may look unusual, but it has advantages:

  • It allows one to swap out the higher-level package foo for something else that provides the similar API. An example would be swapping out one database for another (for example, graduating from SQLite to PostgreSQL).
  • Looks more tidy in cases where a large number of objects get imported from that package.
  • Adding or removing imported objects from the package is quicker, and results in simpler diffs.

Import Order

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

  1. Python standard library
  2. Third party packages
  3. Other modules from the current package

Wildcard Imports

Do not import all the names from a package (in other words, never use from package import *); import just the ones that are needed. Single-line imports apply 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 = {}

Causing others to need and rely on import-time side effects is highly discouraged.

Creating code that requires someone to import a module or package for the singular purpose of causing some module-scoped code to be run is highly discouraged. It is only permissible to add such code to the core in paster templates, where it might be required by some other framework (for example, SQLAlchemy "declarative base" classes must be imported to be registered).