docs/source/development/inputhook_app.rst

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IPython GUI Support Notes
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IPython allows GUI event loops to be run in an interactive IPython session.
This is done using Python's PyOS_InputHook hook which Python calls
when the :func:`raw_input` function is called and waiting for user input.
IPython has versions of this hook for wx, pyqt4 and pygtk.

When a GUI program is used interactively within IPython, the event loop of
the GUI should *not* be started. This is because, the PyOS_Inputhook itself
is responsible for iterating the GUI event loop.

IPython has facilities for installing the needed input hook for each GUI 
toolkit and for creating the needed main GUI application object. Usually,
these main application objects should be created only once and for some
GUI toolkits, special options have to be passed to the application object
to enable it to function properly in IPython.

We need to answer the following questions:

* Who is responsible for creating the main GUI application object, IPython
  or third parties (matplotlib, enthought.traits, etc.)?

* What is the proper way for third party code to detect if a GUI application
  object has already been created?  If one has been created, how should
  the existing instance be retrieved?

* In a GUI application object has been created, how should third party code
  detect if the GUI event loop is running. It is not sufficient to call the
  relevant function methods in the GUI toolkits (like ``IsMainLoopRunning``)
  because those don't know if the GUI event loop is running through the
  input hook.

* We might need a way for third party code to determine if it is running
  in IPython or not.  Currently, the only way of running GUI code in IPython
  is by using the input hook, but eventually, GUI based versions of IPython
  will allow the GUI event loop in the more traditional manner. We will need
  a way for third party code to distinguish between these two cases.

Here is some sample code I have been using to debug this issue::

    from matplotlib import pyplot as plt

    from enthought.traits import api as traits

    class Foo(traits.HasTraits):
        a = traits.Float()

    f = Foo()
    f.configure_traits()

    plt.plot(range(10))