docs/source/development/development.txt

.. _development:

==================================
IPython development guidelines
==================================

.. contents::


Overview
========

IPython is the next generation of IPython.  It is named such for two reasons:

- Eventually, IPython will become IPython version 1.0.
- This new code base needs to be able to co-exist with the existing IPython until
  it is a full replacement for it.  Thus we needed a different name.  We couldn't
  use ``ipython`` (lowercase) as some files systems are case insensitive.

There are two, no three, main goals of the IPython effort:

1. Clean up the existing codebase and write lots of tests.
2. Separate the core functionality of IPython from the terminal to enable IPython
   to be used from within a variety of GUI applications.
3. Implement a system for interactive parallel computing.

While the third goal may seem a bit unrelated to the main focus of IPython, it turns
out that the technologies required for this goal are nearly identical with those
required for goal two. This is the main reason the interactive parallel computing
capabilities are being put into IPython proper. Currently the third of these goals is
furthest along.

This document describes IPython from the perspective of developers.  


Project organization
====================

Subpackages
-----------

IPython is organized into semi self-contained subpackages.  Each of the subpackages will have its own:

- **Dependencies**.  One of the most important things to keep in mind in
  partitioning code amongst subpackages, is that they should be used to cleanly
  encapsulate dependencies.  
- **Tests**.  Each subpackage shoud have its own ``tests`` subdirectory that
  contains all of the tests for that package.  For information about writing tests
  for IPython, see the `Testing System`_ section of this document.
- **Configuration**.  Each subpackage should have its own ``config`` subdirectory
  that contains the configuration information for the components of the 
  subpackage.  For information about how the IPython configuration system
  works, see the `Configuration System`_ section of this document.
- **Scripts**.  Each subpackage should have its own ``scripts`` subdirectory that
  contains all of the command line scripts associated with the subpackage.

Installation and dependencies
-----------------------------
   
IPython will not use `setuptools`_ for installation. Instead, we will use standard
``setup.py`` scripts that use `distutils`_. While there are a number a extremely nice
features that `setuptools`_ has (like namespace packages), the current implementation
of `setuptools`_ has performance problems, particularly on shared file systems. In
particular, when Python packages are installed on NSF file systems, import times
become much too long (up towards 10 seconds). 

Because IPython is being used extensively in the context of high performance
computing, where performance is critical but shared file systems are common, we feel
these performance hits are not acceptable. Thus, until the performance problems
associated with `setuptools`_ are addressed, we will stick with plain `distutils`_. We
are hopeful that these problems will be addressed and that we will eventually begin
using `setuptools`_. Because of this, we are trying to organize IPython in a way that
will make the eventual transition to `setuptools`_ as painless as possible.

Because we will be using `distutils`_, there will be no method for automatically installing dependencies.  Instead, we are following the approach of `Matplotlib`_ which can be summarized as follows:

- Distinguish between required and optional dependencies.  However, the required
  dependencies for IPython should be only the Python standard library.
- Upon installation check to see which optional dependencies are present and tell
  the user which parts of IPython need which optional dependencies.

It is absolutely critical that each subpackage of IPython has a clearly specified set
of dependencies and that dependencies are not carelessly inherited from other IPython
subpackages. Furthermore, tests that have certain dependencies should not fail if
those dependencies are not present. Instead they should be skipped and print a
message.

.. _setuptools: http://peak.telecommunity.com/DevCenter/setuptools
.. _distutils: http://docs.python.org/lib/module-distutils.html
.. _Matplotlib: http://matplotlib.sourceforge.net/

Specific subpackages
--------------------

``core``
	This is the core functionality of IPython that is independent of the
	terminal, network and GUIs.  Most of the code that is in the current
	IPython trunk will be refactored, cleaned up and moved here.

``kernel``
	The enables the IPython core to be expose to a the network.  This is
	also where all of the parallel computing capabilities are to be found.
	
``config``
	The configuration package used by IPython.

``frontends``
	The various frontends for IPython.  A frontend is the end-user application
	that exposes the capabilities of IPython to the user.  The most basic frontend
	will simply be a terminal based application that looks just like today 's 
	IPython.  Other frontends will likely be more powerful and based on GUI toolkits.

``notebook``
	An application that allows users to work with IPython notebooks.

``tools``
	This is where general utilities go.


Version control
===============

In the past, IPython development has been done using `Subversion`__.  Recently, we made the transition to using `Bazaar`__ and `Launchpad`__.  This makes it much easier for people
to contribute code to IPython.  Here is a sketch of how to use Bazaar for IPython
development.  First, you should install Bazaar.  After you have done that, make
sure that it is working by getting the latest main branch of IPython::

	$ bzr branch lp:ipython

Now you can create a new branch for you to do your work in::

	$ bzr branch ipython ipython-mybranch

The typical work cycle in this branch will be to make changes in `ipython-mybranch`
and then commit those changes using the commit command::

	$ ...do work in ipython-mybranch...
	$ bzr ci -m "the commit message goes here"

Please note that since we now don't use an old-style linear ChangeLog
(that tends to cause problems with distributed version control
systems), you should ensure that your log messages are reasonably
detailed.  Use a docstring-like approach in the commit messages
(including the second line being left *blank*)::

  Single line summary of  changes being committed.

  - more details when warranted ...
  - including crediting outside contributors if they sent the
    code/bug/idea!

If we couple this with a policy of making single commits for each
reasonably atomic change, the bzr log should give an excellent view of
the project, and the `--short` log option becomes a nice summary.

While working with this branch, it is a good idea to merge in changes that have been
made upstream in the parent branch.  This can be done by doing::

	$ bzr pull
	
If this command shows that the branches have diverged, then you should do a merge
instead::

	$ bzr merge lp:ipython

If you want others to be able to see your branch, you can create an account with
launchpad and push the branch to your own workspace::

	$ bzr push bzr+ssh://<me>@bazaar.launchpad.net/~<me>/+junk/ipython-mybranch

Finally, once the work in your branch is done, you can merge your changes back into
the `ipython` branch by using merge::

	$ cd ipython
	$ merge ../ipython-mybranch
	[resolve any conflicts]
	$ bzr ci -m "Fixing that bug"
	$ bzr push

But this will require you to have write permissions to the `ipython` branch.  It you don't
you can tell one of the IPython devs about your branch and they can do the merge for you.

More information about Bazaar workflows can be found `here`__.

.. __: http://subversion.tigris.org/
.. __: http://bazaar-vcs.org/
.. __: http://www.launchpad.net/ipython
.. __: http://doc.bazaar-vcs.org/bzr.dev/en/user-guide/index.html

Documentation
=============

Standalone documentation
------------------------

All standalone documentation should be written in plain text (``.txt``) files using
`reStructuredText`_ for markup and formatting. All such documentation should be placed
in the top level directory ``docs`` of the IPython source tree. Or, when appropriate,
a suitably named subdirectory should be used. The documentation in this location will
serve as the main source for IPython documentation and all existing documentation
should be converted to this format.

In the future, the text files in the ``docs`` directory will be used to generate all
forms of documentation for IPython. This include documentation on the IPython website
as well as *pdf* documentation.

.. _reStructuredText: http://docutils.sourceforge.net/rst.html

Docstring format
----------------

Good docstrings are very important. All new code will use `Epydoc`_ for generating API
docs, so we will follow the `Epydoc`_ conventions. More specifically, we will use
`reStructuredText`_ for markup and formatting, since it is understood by a wide
variety of tools. This means that if in the future we have any reason to change from
`Epydoc`_ to something else, we'll have fewer transition pains.

Details about using `reStructuredText`_ for docstrings can be found `here
<http://epydoc.sourceforge.net/manual-othermarkup.html>`_.

.. _Epydoc: http://epydoc.sourceforge.net/

Additional PEPs of interest regarding documentation of code:

- `Docstring Conventions <http://www.python.org/peps/pep-0257.html>`_
- `Docstring Processing System Framework <http://www.python.org/peps/pep-0256.html>`_
- `Docutils Design Specification <http://www.python.org/peps/pep-0258.html>`_


Coding conventions
==================

General
-------

In general, we'll try to follow the standard Python style conventions as described here:

- `Style Guide for Python Code  <http://www.python.org/peps/pep-0008.html>`_  


Other comments:

- In a large file, top level classes and functions should be 
  separated by 2-3 lines to make it easier to separate them visually.
- Use 4 spaces for indentation.
- Keep the ordering of methods the same in classes that have the same 
  methods.  This is particularly true for classes that implement 
  similar interfaces and for interfaces that are similar.

Naming conventions
------------------

In terms of naming conventions, we'll follow the guidelines from the `Style Guide for
Python Code`_.

For all new IPython code (and much existing code is being refactored), we'll use:

- All ``lowercase`` module names.

- ``CamelCase`` for class names.

- ``lowercase_with_underscores`` for methods, functions, variables and attributes.

This may be confusing as most of the existing IPython codebase uses a different convention (``lowerCamelCase`` for methods and attributes).  Slowly, we will move IPython over to the new
convention, providing shadow names for backward compatibility in public interfaces. 

There are, however, some important exceptions to these rules.  In some cases, IPython
code will interface with packages (Twisted, Wx, Qt) that use other conventions.  At some level this makes it impossible to adhere to our own standards at all times.  In particular, when subclassing classes that use other naming conventions, you must follow their naming conventions.  To deal with cases like this, we propose the following policy:

- If you are subclassing a class that uses different conventions, use its
  naming conventions throughout your subclass.  Thus, if you are creating a 
  Twisted Protocol class, used Twisted's ``namingSchemeForMethodsAndAttributes.``

- All IPython's official interfaces should use our conventions.  In some cases 
  this will mean that you need to provide shadow names (first implement ``fooBar``
  and then ``foo_bar = fooBar``).  We want to avoid this at all costs, but it 
  will probably be necessary at times.  But, please use this sparingly!

Implementation-specific *private* methods will use ``_single_underscore_prefix``.
Names with a leading double underscore will *only* be used in special cases, as they
makes subclassing difficult (such names are not easily seen by child classes).

Occasionally some run-in lowercase names are used, but mostly for very short names or
where we are implementing methods very similar to existing ones in a base class (like
``runlines()`` where ``runsource()`` and ``runcode()`` had established precedent).

The old IPython codebase has a big mix of classes and modules prefixed with an
explicit ``IP``. In Python this is mostly unnecessary, redundant and frowned upon, as
namespaces offer cleaner prefixing. The only case where this approach is justified is
for classes which are expected to be imported into external namespaces and a very
generic name (like Shell) is too likely to clash with something else. We'll need to
revisit this issue as we clean up and refactor the code, but in general we should
remove as many unnecessary ``IP``/``ip`` prefixes as possible. However, if a prefix
seems absolutely necessary the more specific ``IPY`` or ``ipy`` are preferred.

.. _devel_testing:

Testing system
==============

It is extremely important that all code contributed to IPython has tests. Tests should
be written as unittests, doctests or as entities that the `Nose`_ testing package will
find. Regardless of how the tests are written, we will use `Nose`_ for discovering and
running the tests. `Nose`_ will be required to run the IPython test suite, but will
not be required to simply use IPython.

.. _Nose: http://code.google.com/p/python-nose/

Tests of `Twisted`__ using code should be written by subclassing the ``TestCase`` class
that comes with ``twisted.trial.unittest``. When this is done, `Nose`_ will be able to
run the tests and the twisted reactor will be handled correctly.

.. __: http://www.twistedmatrix.com

Each subpackage in IPython should have its own ``tests`` directory that contains all
of the tests for that subpackage. This allows each subpackage to be self-contained. If
a subpackage has any dependencies beyond the Python standard library, the tests for
that subpackage should be skipped if the dependencies are not found. This is very
important so users don't get tests failing simply because they don't have dependencies.

We also need to look into use Noses ability to tag tests to allow a more modular
approach of running tests.

.. _devel_config:

Configuration system
====================

IPython uses `.ini`_ files for configuration purposes. This represents a huge
improvement over the configuration system used in IPython. IPython works with these
files using the `ConfigObj`_ package, which IPython includes as
``ipython1/external/configobj.py``.

Currently, we are using raw `ConfigObj`_ objects themselves. Each subpackage of IPython
should contain a ``config`` subdirectory that contains all of the configuration
information for the subpackage. To see how configuration information is defined (along
with defaults) see at the examples in ``ipython1/kernel/config`` and
``ipython1/core/config``. Likewise, to see how the configuration information is used,
see examples in ``ipython1/kernel/scripts/ipengine.py``.

Eventually, we will add a new layer on top of the raw `ConfigObj`_ objects. We are
calling this new layer, ``tconfig``, as it will use a `Traits`_-like validation model.
We won't actually use `Traits`_, but will implement something similar in pure Python.
But, even in this new system, we will still use `ConfigObj`_ and `.ini`_ files
underneath the hood. Talk to Fernando if you are interested in working on this part of
IPython. The current prototype of ``tconfig`` is located in the IPython sandbox.

.. _.ini: http://docs.python.org/lib/module-ConfigParser.html
.. _ConfigObj: http://www.voidspace.org.uk/python/configobj.html
.. _Traits: http://code.enthought.com/traits/