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Python Coding Style

This page outlines standards for Python code contributed to freeIPA v2 and beyond.

Python version

In the FreeIPA master branch and branches >= 3.0.0, we require at least Python 2.7. This permits the use of set literals, dict and set comprehensions, and many other features.

Python coding style

Although Python's syntax enforces quite a lot of stylistic consistency, there is still much wiggle room, so a style guide will help keep us productive and happy.

Fortunately, thanks to the "Benevolent Dictator For Life" (Guido van Rossum, the author of Python), there is only one coding style for Python, which is defined in PEP-8 and PEP-257:

Python unit testing

All the Python unit testing is being done using nose:

nose was chosen because it allows unit tests to be written very quickly and in a quite natural and flexible manner.

All the unit tests should be located in the tests/ directory in the source tree.

Python API documentation

We are generating API documentation using the Epydoc tool:

The Epydoc generated documentation is an excellent way to learn the v2 code base, particularly because all the cross-references are hyper-linked. Up-to-date documentation is always available on freeipa.org:

Python Internationalized (i18n) Strings

If the string will be internationalized (e.g. is marked with _()) and it has more than one format substitution you MUST use named format specifiers, not positional format specifiers.

Here is an example of incorrect usage with positional substitution:

 _('item %s has %s value') % (name, value)

Here is the correct usage using named substitution:

 _('item %(name)s has %(value)s value') % {'name':name, 'value':value}


 _('item %(name)s has %(value)s value') % dict(name=name, value=value}

Why does this matter? Word ordering is locale dependent. Translators need the flexibility to reorder the words in the string. If you use positional format substitutions the translator can't reorder the wording. However, if you use named format substitutions the translator has the freedom to reorder the wording. Try to pick names for the for format specifiers which will provide hints to the translator as to meaning of the substitution.

Python docstrings

We are using reStructuredText markup in the docstrings, which you can read about here:

There is considerable example code included in the docstrings. All these examples are automatically tested using the Python doctest module:

We are relying on nose to automatically discover all the code examples and then to test them with doctest. This is done using the --with-doctest option, like this:

nosetests --with-doctest

Unused (dummy) variables

We want to avoid unused variables in code as much as possible. In case you need to use a variable that has no further use (iteration, tuple unpacking) then this variable must start with an underscore character ('_'). The Pylint check for unused variables has been enabled and such dummy variables must match the regular expression '_.+' otherwise they are reported as errors.


name, _weight, age = human['John']
print(name, age)