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.

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].

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 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:

To customize how the REPL pretty-prints your object, add a _repr_pretty_ method to the class. The method should accept a pretty printer, and a boolean that indicates whether the printer detected a cycle. The method should act on the printer to produce your customized pretty output. Here is an example:

class MyObject(object):

    def _repr_pretty_(self, p, cycle):
        if cycle:
            p.text('MyObject(...)')
        else:
            p.text('MyObject[...]')

For details, see :py:mod:`IPython.lib.pretty`.

from bar.baz 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.baz', '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 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.