Document that _repr_*_() can return metadata

docs/source/config/integrating.rst

@@ -1,88 +1,95 b''
 .. _integrating:
 
 =====================================
 Integrating your objects with IPython
 =====================================
 
 Tab completion
 ==============
 
 To change the attributes displayed by tab-completing your object, define a
 ``__dir__(self)`` method for it. For more details, see the documentation of the
 built-in `dir() function <http://docs.python.org/library/functions.html#dir>`_.
 
 You can also customise key completions for your objects, e.g. pressing tab after
 ``obj["a``. To do so, define a method ``_ipython_key_completions_()``, which
 returns a list of objects which are possible keys in a subscript expression
 ``obj[key]``.
 
 .. versionadded:: 5.0
    Custom key completions
 
 .. _integrating_rich_display:
 
 Rich display
 ============
 
 The notebook and the Qt console can display richer representations of objects.
-To use this, you can define any of a number of ``_repr_*_()`` methods. Note that
+To use this, you can define any number of ``_repr_*_()`` methods. Note that
 these are surrounded by single, not double underscores.
 
 Both the notebook and the Qt console can display ``svg``, ``png`` and ``jpeg``
 representations. The notebook can also display ``html``, ``javascript``,
 ``markdown`` and ``latex``. If the methods don't exist, or return ``None``, it
 falls back to a standard ``repr()``.
 
 For example::
 
     class Shout(object):
         def __init__(self, text):
             self.text = text
         
         def _repr_html_(self):
             return "<h1>" + self.text + "</h1>"
 
+We often want to provide frontends with guidance on how to display the data. To
+support this, ``_repr_*_()`` methods can also return a data, metadata tuple where
+metadata is a dictionary containing arbitrary key, value pairs for the frontend
+to interpret. An example use case is ``_repr_jpeg()``, which can be set to
+return a jpeg image and a ``{'height': 400, 'width': 600}`` dictionary to inform
+the frontend how to size the image.
+
 There are also two more powerful display methods:
 
 .. class:: MyObject
 
    .. method:: _repr_mimebundle_(include=None, exclude=None)
 
      Should return a dictionary of multiple formats, keyed by mimetype, or a tuple
      of two dictionaries: *data, metadata*. If this returns something, other
      ``_repr_*_`` methods are ignored. The method should take keyword arguments
      ``include`` and ``exclude``, though it is not required to respect them.
 
    .. method:: _ipython_display_()
 
       Displays the object as a side effect; the return value is ignored. If this
       is defined, all other display methods are ignored.
 
 Formatters for third-party types
 --------------------------------
 
 The user can also register formatters for types without modifying the class::
 
     from bar import Foo
 
     def foo_html(obj):
         return '<marquee>Foo object %s</marquee>' % obj.name
 
     html_formatter = get_ipython().display_formatter.formatters['text/html']
     html_formatter.for_type(Foo, foo_html)
 
     # Or register a type without importing it - this does the same as above:
     html_formatter.for_type_by_name('bar.Foo', foo_html)
 
 Custom exception tracebacks
 ===========================
 
 Rarely, you might want to display a custom traceback when reporting an
 exception. To do this, define the custom traceback using
 `_render_traceback_(self)` method which returns a list of strings, one string
 for each line of the traceback. For example, the `ipyparallel
 <http://ipyparallel.readthedocs.io/>`__ a parallel computing framework for
 IPython, does this to display errors from multiple engines.
 
 Please be conservative in using this feature; by replacing the default traceback
 you may hide important information from the user.