IPython Documentation

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This documentation is for an old version of IPython. You can find docs for newer versions here.

Module: lib.demo

Module for interactive demos using IPython.

This module implements a few classes for running Python scripts interactively in IPython for demonstrations. With very simple markup (a few tags in comments), you can control points where the script stops executing and returns control to IPython.

Provided classes

The classes are (see their docstrings for further details):

  • Demo: pure python demos
  • IPythonDemo: demos with input to be processed by IPython as if it had been

typed interactively (so magics work, as well as any other special syntax you may have added via input prefilters).

  • LineDemo: single-line version of the Demo class. These demos are executed

one line at a time, and require no markup.

  • IPythonLineDemo: IPython version of the LineDemo class (the demo is

executed a line at a time, but processed via IPython).

  • ClearMixin: mixin to make Demo classes with less visual clutter. It declares an empty marquee and a pre_cmd that clears the screen before each block (see Subclassing below).
  • ClearDemo, ClearIPDemo: mixin-enabled versions of the Demo and IPythonDemo classes.

Inheritance diagram:

Inheritance diagram of IPython.lib.demo


The classes here all include a few methods meant to make customization by subclassing more convenient. Their docstrings below have some more details:

  • marquee(): generates a marquee to provide visible on-screen markers at each block start and end.
  • pre_cmd(): run right before the execution of each block.
  • post_cmd(): run right after the execution of each block. If the block raises an exception, this is NOT called.


The file is run in its own empty namespace (though you can pass it a string of arguments as if in a command line environment, and it will see those as sys.argv). But at each stop, the global IPython namespace is updated with the current internal demo namespace, so you can work interactively with the data accumulated so far.

By default, each block of code is printed (with syntax highlighting) before executing it and you have to confirm execution. This is intended to show the code to an audience first so you can discuss it, and only proceed with execution once you agree. There are a few tags which allow you to modify this behavior.

The supported tags are:

# <demo> stop

Defines block boundaries, the points where IPython stops execution of the file and returns to the interactive prompt.

You can optionally mark the stop tag with extra dashes before and after the word ‘stop’, to help visually distinguish the blocks in a text editor:

# <demo> — stop —

# <demo> silent

Make a block execute silently (and hence automatically). Typically used in cases where you have some boilerplate or initialization code which you need executed but do not want to be seen in the demo.

# <demo> auto

Make a block execute automatically, but still being printed. Useful for simple code which does not warrant discussion, since it avoids the extra manual confirmation.

# <demo> auto_all

This tag can _only_ be in the first block, and if given it overrides the individual auto tags to make the whole demo fully automatic (no block asks for confirmation). It can also be given at creation time (or the attribute set later) to override what’s in the file.

While _any_ python file can be run as a Demo instance, if there are no stop tags the whole file will run in a single block (no different that calling first %pycat and then %run). The minimal markup to make this useful is to place a set of stop tags; the other tags are only there to let you fine-tune the execution.

This is probably best explained with the simple example file below. You can copy this into a file named ex_demo.py, and try running it via:

from IPython.demo import Demo
d = Demo('ex_demo.py')

Each time you call the demo object, it runs the next block. The demo object has a few useful methods for navigation, like again(), edit(), jump(), seek() and back(). It can be reset for a new run via reset() or reloaded from disk (in case you’ve edited the source) via reload(). See their docstrings below.

Note: To make this simpler to explore, a file called “demo-exercizer.py” has been added to the “docs/examples/core” directory. Just cd to this directory in an IPython session, and type:

%run demo-exercizer.py

and then follow the directions.


The following is a very simple example of a valid demo file.

#################### EXAMPLE DEMO <ex_demo.py> ###############################
'''A simple interactive demo to illustrate the use of IPython's Demo class.'''

print 'Hello, welcome to an interactive IPython demo.'

# The mark below defines a block boundary, which is a point where IPython will
# stop execution and return to the interactive prompt. The dashes are actually
# optional and used only as a visual aid to clearly separate blocks while
# editing the demo code.
# <demo> stop

x = 1
y = 2

# <demo> stop

# the mark below makes this block as silent
# <demo> silent

print 'This is a silent block, which gets executed but not printed.'

# <demo> stop
# <demo> auto
print 'This is an automatic block.'
print 'It is executed without asking for confirmation, but printed.'
z = x+y

print 'z=',x

# <demo> stop
# This is just another normal block.
print 'z is now:', z

print 'bye!'
################### END EXAMPLE DEMO <ex_demo.py> ############################

8 Classes

class IPython.lib.demo.DemoError

Bases: exceptions.Exception

class IPython.lib.demo.Demo(src, title=u'', arg_str=u'', auto_all=None)

Bases: object

__init__(src, title=u'', arg_str=u'', auto_all=None)

Make a new demo object. To run the demo, simply call the object.

See the module docstring for full details and an example (you can use IPython.Demo? in IPython to see it).


  • src is either a file, or file-like object, or a

    string that can be resolved to a filename.

Optional inputs:

  • title: a string to use as the demo name. Of most use when the demo

you are making comes from an object that has no filename, or if you want an alternate denotation distinct from the filename.

  • arg_str(‘’): a string of arguments, internally converted to a list

just like sys.argv, so the demo script can see a similar environment.

  • auto_all(None): global flag to run all blocks automatically without

confirmation. This attribute overrides the block-level tags and applies to the whole demo. It is an attribute of the object, and can be changed at runtime simply by reassigning it to a boolean value.


Move the seek pointer back one block and re-execute.


Move the seek pointer back num blocks (default is 1).


Edit a block.

If no number is given, use the last block executed.

This edits the in-memory copy of the demo, it does NOT modify the original source file. If you want to do that, simply open the file in an editor and use reload() when you make changes to the file. This method is meant to let you change a block during a demonstration for explanatory purposes, without damaging your original script.


Load file object.


Jump a given number of blocks relative to the current one.

The offset can be positive or negative, defaults to 1.

marquee(txt=u'', width=78, mark=u'*')

Return the input string centered in a ‘marquee’.


Method called after executing each block.


Method called before executing each block.


Reload source from disk and initialize state.


Reset the namespace and seek pointer to restart the demo


Execute a string with one or more lines of code


Move the current seek pointer to the given block.

You can use negative indices to seek from the end, with identical semantics to those of Python lists.


Show a single block on screen


Show entire demo on screen, block by block

class IPython.lib.demo.IPythonDemo(src, title=u'', arg_str=u'', auto_all=None)

Bases: IPython.lib.demo.Demo

Class for interactive demos with IPython’s input processing applied.

This subclasses Demo, but instead of executing each block by the Python interpreter (via exec), it actually calls IPython on it, so that any input filters which may be in place are applied to the input block.

If you have an interactive environment which exposes special input processing, you can use this class instead to write demo scripts which operate exactly as if you had typed them interactively. The default Demo class requires the input to be valid, pure Python code.


Execute a string with one or more lines of code

class IPython.lib.demo.LineDemo(src, title=u'', arg_str=u'', auto_all=None)

Bases: IPython.lib.demo.Demo

Demo where each line is executed as a separate block.

The input script should be valid Python code.

This class doesn’t require any markup at all, and it’s meant for simple scripts (with no nesting or any kind of indentation) which consist of multiple lines of input to be executed, one at a time, as if they had been typed in the interactive prompt.

Note: the input can not have any indentation, which means that only single-lines of input are accepted, not even function definitions are valid.


Reload source from disk and initialize state.

class IPython.lib.demo.IPythonLineDemo(src, title=u'', arg_str=u'', auto_all=None)

Bases: IPython.lib.demo.IPythonDemo, IPython.lib.demo.LineDemo

Variant of the LineDemo class whose input is processed by IPython.

class IPython.lib.demo.ClearMixin

Bases: object

Use this mixin to make Demo classes with less visual clutter.

Demos using this mixin will clear the screen before every block and use blank marquees.

Note that in order for the methods defined here to actually override those of the classes it’s mixed with, it must go /first/ in the inheritance tree. For example:

class ClearIPDemo(ClearMixin,IPythonDemo): pass

will provide an IPythonDemo class with the mixin’s features.

marquee(txt=u'', width=78, mark=u'*')

Blank marquee that returns ‘’ no matter what the input.


Method called before executing each block.

This one simply clears the screen.

class IPython.lib.demo.ClearDemo(src, title=u'', arg_str=u'', auto_all=None)

Bases: IPython.lib.demo.ClearMixin, IPython.lib.demo.Demo

class IPython.lib.demo.ClearIPDemo(src, title=u'', arg_str=u'', auto_all=None)

Bases: IPython.lib.demo.ClearMixin, IPython.lib.demo.IPythonDemo

1 Function