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Interactive demos with IPython

IPython ships with a basic system for running scripts interactively in sections, useful when presenting code to audiences. A few tags embedded in comments (so that the script remains valid Python code) divide a file into separate blocks, and the demo can be run one block at a time, with IPython printing (with syntax highlighting) the block before executing it, and returning to the interactive prompt after each block. The interactive namespace is updated after each block is run with the contents of the demo's namespace.

This allows you to show a piece of code, run it and then execute interactively commands based on the variables just created. Once you want to continue, you simply execute the next block of the demo. The following listing shows the markup necessary for dividing a script into sections for execution as a demo.

    """A simple interactive demo to illustrate the use of IPython's Demo class.

    Any python script can be run as a demo, but that does little more than showing
    it on-screen, syntax-highlighted in one shot.  If you add a little simple
    markup, you can stop at specified intervals and return to the ipython prompt,
    resuming execution later.
    """

    print 'Hello, welcome to an interactive IPython demo.'
    print 'Executing this block should require confirmation before proceeding,'
    print 'unless auto_all has been set to true in the demo object'

    # The mark below defines a block boundary, which is a point where IPython will
    # stop execution and return to the interactive prompt.
    # Note that in actual interactive execution,
    # <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!'

In order to run a file as a demo, you must first make a `Demo` object out of it. If the file is named `myscript.py`, the following code will make a demo:

  from IPython.demo import Demo

  mydemo = Demo('myscript.py')

This creates the `mydemo` object, whose blocks you run one at a time by simply calling the object with no arguments. If you have autocall active in IPython (the default), all you need to do is type

  mydemo

and IPython will call it, executing each block. Demo objects can be restarted, you can move forward or back skipping blocks, re-execute the last block, etc. Simply use the Tab key on a demo object to see its methods, and call ``?'` on them to see their docstrings for more usage details. In addition, the `demo` module itself contains a comprehensive docstring, which you can access via

  from IPython import demo

  demo?

Limitations: It is important to note that these demos are limited to fairly simple uses. In particular, you can not put division marks in indented code (loops, if statements, function definitions, etc.) Supporting something like this would basically require tracking the internal execution state of the Python interpreter, so only top-level divisions are allowed. If you want to be able to open an IPython instance at an arbitrary point in a program, you can use IPython's embedding facilities, described in detail in Sec. 9.