docs/source/config/overview.txt

.. _config_overview:

============================================
Overview of the IPython configuration system
============================================

This section describes the IPython configuration system. Starting with version
0.11, IPython has a completely new configuration system that is quite
different from the older :file:`ipythonrc` or :file:`ipy_user_conf.py`
approaches. The new configuration system was designed from scratch to address
the particular configuration needs of IPython. While there are many
other excellent configuration systems out there, we found that none of them
met our requirements.

.. warning::

    If you are upgrading to version 0.11 of IPython, you will need to migrate
    your old :file:`ipythonrc` or :file:`ipy_user_conf.py` configuration files
    to the new system.  Read on for information on how to do this.

The discussion that follows is focused on teaching user's how to configure
IPython to their liking.  Developer's who want to know more about how they
can enable their objects to take advantage of the configuration system
should consult our :ref:`developer guide <developer_guide>`

The main concepts
=================

There are a number of abstractions that the IPython configuration system uses.
Each of these abstractions is represented by a Python class.

Configuration object: :class:`~IPython.config.loader.Config`
    A configuration object is a simple dictionary-like class that holds
    configuration attributes and sub-configuration objects. These classes
    support dotted attribute style access (``Foo.bar``) in addition to the
    regular dictionary style access (``Foo['bar']``). Configuration objects
    are smart. They know how to merge themselves with other configuration
    objects and they automatically create sub-configuration objects.

Application: :class:`~IPython.core.application.Application`
    An application is a process that does a specific job. The most obvious
    application is the :command:`ipython` command line program. Each
    application reads a *single* configuration file and command line options
    and then produces a master configuration object for the application. This
    configuration object is then passed to the configurable objects that the
    application creates. These configurable objects implement the actual logic
    of the application and know how to configure themselves given the
    configuration object.

Component: :class:`~IPython.config.configurable.Configurable`
    A configurable is a regular Python class that serves as a base class for
    all main classes in an application. The
    :class:`~IPython.config.configurable.Configurable` base class is
    lightweight and only does one things.

    This :class:`~IPython.config.configurable.Configurable` is a subclass
    of :class:`~IPython.utils.traitlets.HasTraits` that knows how to configure
    itself. Class level traits with the metadata ``config=True`` become
    values that can be configured from the command line and configuration
    files.
    
    Developers create :class:`~IPython.config.configurable.Configurable`
    subclasses that implement all of the logic in the application. Each of
    these subclasses has its own configuration information that controls how
    instances are created.

Having described these main concepts, we can now state the main idea in our
configuration system: *"configuration" allows the default values of class
attributes to be controlled on a class by class basis*. Thus all instances of
a given class are configured in the same way. Furthermore, if two instances
need to be configured differently, they need to be instances of two different
classes. While this model may seem a bit restrictive, we have found that it
expresses most things that need to be configured extremely well. However, it
is possible to create two instances of the same class that have different
trait values. This is done by overriding the configuration.

Now, we show what our configuration objects and files look like.

Configuration objects and files
===============================

A configuration file is simply a pure Python file that sets the attributes
of a global, pre-created configuration object.  This configuration object is a 
:class:`~IPython.config.loader.Config` instance.  While in a configuration
file, to get a reference to this object, simply call the :func:`get_config`
function.  We inject this function into the global namespace that the 
configuration file is executed in.

Here is an example of a super simple configuration file that does nothing::

    c = get_config()

Once you get a reference to the configuration object, you simply set
attributes on it.  All you have to know is:

* The name of each attribute.
* The type of each attribute.

The answers to these two questions are provided by the various
:class:`~IPython.config.configurable.Configurable` subclasses that an
application uses. Let's look at how this would work for a simple component
subclass::

    # Sample component that can be configured.
    from IPython.config.configurable import Configurable
    from IPython.utils.traitlets import Int, Float, Str, Bool
    
    class MyClass(Configurable):
        name = Str('defaultname', config=True)
        ranking = Int(0, config=True)
        value = Float(99.0)
        # The rest of the class implementation would go here..

In this example, we see that :class:`MyClass` has three attributes, two
of whom (``name``, ``ranking``) can be configured.  All of the attributes
are given types and default values.  If a :class:`MyClass` is instantiated,
but not configured, these default values will be used.  But let's see how
to configure this class in a configuration file::

    # Sample config file
    c = get_config()
    
    c.MyClass.name = 'coolname'
    c.MyClass.ranking = 10

After this configuration file is loaded, the values set in it will override
the class defaults anytime a :class:`MyClass` is created.  Furthermore,
these attributes will be type checked and validated anytime they are set.
This type checking is handled by the :mod:`IPython.utils.traitlets` module,
which provides the :class:`Str`, :class:`Int` and :class:`Float` types.  In 
addition to these traitlets, the :mod:`IPython.utils.traitlets` provides
traitlets for a number of other types.

.. note::

    Underneath the hood, the :class:`Configurable` base class is a subclass of
    :class:`IPython.utils.traitlets.HasTraits`. The
    :mod:`IPython.utils.traitlets` module is a lightweight version of
    :mod:`enthought.traits`. Our implementation is a pure Python subset
    (mostly API compatible) of :mod:`enthought.traits` that does not have any
    of the automatic GUI generation capabilities. Our plan is to achieve 100%
    API compatibility to enable the actual :mod:`enthought.traits` to
    eventually be used instead. Currently, we cannot use
    :mod:`enthought.traits` as we are committed to the core of IPython being
    pure Python.

It should be very clear at this point what the naming convention is for 
configuration attributes::

    c.ClassName.attribute_name = attribute_value

Here, ``ClassName`` is the name of the class whose configuration attribute you
want to set, ``attribute_name`` is the name of the attribute you want to set
and ``attribute_value`` the the value you want it to have. The ``ClassName``
attribute of ``c`` is not the actual class, but instead is another
:class:`~IPython.config.loader.Config` instance.

.. note::

    The careful reader may wonder how the ``ClassName`` (``MyClass`` in
    the above example) attribute of the configuration object ``c`` gets
    created. These attributes are created on the fly by the
    :class:`~IPython.config.loader.Config` instance, using a simple naming
    convention. Any attribute of a :class:`~IPython.config.loader.Config`
    instance whose name begins with an uppercase character is assumed to be a
    sub-configuration and a new empty :class:`~IPython.config.loader.Config`
    instance is dynamically created for that attribute. This allows deeply
    hierarchical information created easily (``c.Foo.Bar.value``) on the fly.

Configuration files inheritance
===============================

Let's say you want to have different configuration files for various purposes.
Our configuration system makes it easy for one configuration file to inherit
the information in another configuration file. The :func:`load_subconfig`
command can be used in a configuration file for this purpose. Here is a simple
example that loads all of the values from the file :file:`base_config.py`::

    # base_config.py
    c = get_config()
    c.MyClass.name = 'coolname'
    c.MyClass.ranking = 100

into the configuration file :file:`main_config.py`::

    # main_config.py
    c = get_config()
    
    # Load everything from base_config.py
    load_subconfig('base_config.py')
    
    # Now override one of the values
    c.MyClass.name = 'bettername'

In a situation like this the :func:`load_subconfig` makes sure that the
search path for sub-configuration files is inherited from that of the parent.
Thus, you can typically put the two in the same directory and everything will
just work.

Class based configuration inheritance
=====================================

There is another aspect of configuration where inheritance comes into play.
Sometimes, your classes will have an inheritance hierarchy that you want
to be reflected in the configuration system.  Here is a simple example::

    from IPython.config.configurable import Configurable
    from IPython.utils.traitlets import Int, Float, Str, Bool
    
    class Foo(Configurable):
        name = Str('fooname', config=True)
        value = Float(100.0, config=True)
    
    class Bar(Foo):
        name = Str('barname', config=True)
        othervalue = Int(0, config=True)

Now, we can create a configuration file to configure instances of :class:`Foo`
and :class:`Bar`::

    # config file
    c = get_config()
    
    c.Foo.name = 'bestname'
    c.Bar.othervalue = 10

This class hierarchy and configuration file accomplishes the following:

* The default value for :attr:`Foo.name` and :attr:`Bar.name` will be
  'bestname'.  Because :class:`Bar` is a :class:`Foo` subclass it also
  picks up the configuration information for :class:`Foo`.
* The default value for :attr:`Foo.value` and :attr:`Bar.value` will be
  ``100.0``, which is the value specified as the class default.
* The default value for :attr:`Bar.othervalue` will be 10 as set in the
  configuration file.  Because :class:`Foo` is the parent of :class:`Bar`
  it doesn't know anything about the :attr:`othervalue` attribute.


.. _ipython_dir:

Configuration file location
===========================

So where should you put your configuration files?  By default, all IPython
applications look in the so called "IPython directory".  The location of
this directory is determined by the following algorithm:

* If the ``--ipython-dir`` command line flag is given, its value is used.

* If not, the value returned by :func:`IPython.utils.path.get_ipython_dir`
  is used. This function will first look at the :envvar:`IPYTHON_DIR`
  environment variable and then default to a platform-specific default.

On posix systems (Linux, Unix, etc.), IPython respects the ``$XDG_CONFIG_HOME``
part of the `XDG Base Directory`_ specification. If ``$XDG_CONFIG_HOME`` is
defined and exists ( ``XDG_CONFIG_HOME`` has a default interpretation of
:file:`$HOME/.config`), then IPython's config directory will be located in
:file:`$XDG_CONFIG_HOME/ipython`.  If users still have an IPython directory
in :file:`$HOME/.ipython`, then that will be used. in preference to the 
system default.

For most users, the default value will simply be something like 
:file:`$HOME/.config/ipython` on Linux, or :file:`$HOME/.ipython`
elsewhere.

Once the location of the IPython directory has been determined, you need to
know what filename to use for the configuration file. The basic idea is that
each application has its own default configuration filename. The default named
used by the :command:`ipython` command line program is
:file:`ipython_config.py`.  This value can be overriden by the ``-config_file``
command line flag.  A sample :file:`ipython_config.py` file can be found
in :mod:`IPython.config.default.ipython_config.py`.  Simple copy it to your
IPython directory to begin using it.

.. _Profiles:

Profiles
========

A profile is simply a configuration file that follows a simple naming
convention and can be loaded using a simplified syntax.  The idea is
that users often want to maintain a set of configuration files for different
purposes:  one for doing numerical computing with NumPy and SciPy and
another for doing symbolic computing with SymPy.  Profiles make it easy
to keep a separate configuration file for each of these purposes.

Let's start by showing how a profile is used:

.. code-block:: bash

    $ ipython -p sympy

This tells the :command:`ipython` command line program to get its
configuration from the "sympy" profile. The search path for profiles is the
same as that of regular configuration files. The only difference is that
profiles are named in a special way. In the case above, the "sympy" profile
would need to have the name :file:`ipython_config_sympy.py`.

The general pattern is this: simply add ``_profilename`` to the end of the
normal configuration file name. Then load the profile by adding ``-p
profilename`` to your command line options.

IPython ships with some sample profiles in :mod:`IPython.config.profile`.
Simply copy these to your IPython directory to begin using them.

Design requirements
===================

Here are the main requirements we wanted our configuration system to have:

* Support for hierarchical configuration information.

* Full integration with command line option parsers.  Often, you want to read
  a configuration file, but then override some of the values with command line
  options.  Our configuration system automates this process and allows each
  command line option to be linked to a particular attribute in the 
  configuration hierarchy that it will override.

* Configuration files that are themselves valid Python code. This accomplishes
  many things. First, it becomes possible to put logic in your configuration
  files that sets attributes based on your operating system, network setup,
  Python version, etc. Second, Python has a super simple syntax for accessing
  hierarchical data structures, namely regular attribute access
  (``Foo.Bar.Bam.name``). Third, using Python makes it easy for users to
  import configuration attributes from one configuration file to another.
  Forth, even though Python is dynamically typed, it does have types that can
  be checked at runtime. Thus, a ``1`` in a config file is the integer '1',
  while a ``'1'`` is a string.

* A fully automated method for getting the configuration information to the
  classes that need it at runtime. Writing code that walks a configuration
  hierarchy to extract a particular attribute is painful. When you have
  complex configuration information with hundreds of attributes, this makes
  you want to cry.

* Type checking and validation that doesn't require the entire configuration
  hierarchy to be specified statically before runtime. Python is a very
  dynamic language and you don't always know everything that needs to be
  configured when a program starts.


.. _`XDG Base Directory`: http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html