Developer's guide
#################

:Author: Ben Finney <ben+python@benfinney.id.au>
:Updated: 2014-11-28


Project layout
==============

::

    ./                  Top level of source tree
        doc/            Project documentation
        bin/            Executable programs
        daemon/         Main ‘daemon’ library
        test/           Unit tests


Code style
==========

Python
------

All Python code should conform to the guidelines in PEP8_. In
particular:

* Indent each level using 4 spaces (``U+0020 SPACE``), and no TABs
  (``U+0008 CHARACTER TABULATION``).

* Name modules in lower case, ``multiplewordslikethis``.

* Name classes in title case, ``MultipleWordsLikeThis``.

* Name functions, instances and other variables in lower case,
  ``multiple_words_like_this``.

* Every module, class, and function has a Python doc string explaining
  its purpose and API.

  *Exception*: Functions whose purpose and API are mandated by Python
  itself (dunder-named methods) do not need a doc string.

* Doc strings are written as triple-quoted strings.

  * The text of the doc string is marked up with reStructuredText.

  * The first line is a one-line synopsis of the object. This summary
    line appears on the same line as the opening triple-quote,
    separated by a single space.
  
  * Further lines, if needed, are separated from the first by one
    blank line.

  * The synopsis is separated by one space from the opening
    triple-quote; this causes it to appear four columns past the
    beginning of the line. All subsequent lines are indented at least
    four columns also.

  * The synopsis is followed by a reStructuredText field list. The
    field names are: “param foo” for each parameter (where “foo” is
    the parameter name), and “return” for the return value. The field
    values describe the purpose of each.
  
  * The closing triple-quote appears on a separate line.

  Example::

    def frobnicate(spam, algorithm="dv"):
        """ Perform frobnication on ``spam``.

            :param spam: A travortionate (as a sequence of strings).
            :param algorithm: The name of the algorithm to use for
                frobnicating the travortionate.
            :return: The frobnicated travortionate, if it is
                non-empty; otherwise None.

            The frobnication is done by the Dietzel-Venkman algorithm,
            and optimises for the case where ``spam`` is freebled and
            agglutinative.

            """
        spagnify(spam)
        # …

* All ``import`` statements appear at the top of the module.

* Each ``import`` statement imports a single module, or multiple names
  from a single module.

  Example::

    import sys
    import os
    from spam import foo, bar, baz

..  _PEP8: http://www.python.org/dev/peps/pep-0008/

Additional style guidelines:

* All text files (including program code) are encoded in UTF-8.

* A page break (``U+000C FORM FEED``) whitespace character is used
  within a module to break up semantically separate areas of the
  module.

* Editor hints for Emacs and Vim appear in a comment block at the
  file's end::

      
      # Local variables:
      # coding: utf-8
      # mode: python
      # End:
      # vim: fileencoding=utf-8 filetype=python :


Unit tests
==========

All code should aim for 100% coverage by unit tests. New code, or
changes to existing code, will only be considered for inclusion in the
development tree when accompanied by corresponding additions or
changes to the unit tests.

Test-driven development
-----------------------

Where possible, practice test-driven development to implement program
code.

* During a development session, maintain a separate window or terminal
  with the unit test suite for the project running continuously, or
  automatically every few seconds.

* Any time a test is failing, the only valid change is to make all
  tests pass.

* Develop new interface features (changes to the program unit's
  behaviour) only when all current tests pass.

* Refactor as needed, but only when all tests pass.

  * Refactoring is any change to the code which does not alter its
    interface or expected behaviour, such as performance
    optimisations, readability improvements, modularisation
    improvements etc.

* Develop new or changed program behaviour by:

  * *First* write a single, specific test case for that new behaviour,
    then watch the test fail in the absence of the desired behaviour.

  * Implement the minimum necessary change to satisfy the failing
    test. Continue until all tests pass again, then stop making
    functional changes.

  * Once all tests (including the new test) pass, consider refactoring
    the code and the tests immediately, then ensure all the tests pass
    again after any changes.

  * Iterate for each incremental change in interface or behaviour.

Test-driven development is not absolutely necessary, but is the
simplest, most direct way to generate the kind of program changes
accompanied by unit tests that are necessary for inclusion in the
project.


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