docs/source/interactive/tutorial.txt

.. _tutorial:

======================
Quick IPython tutorial
======================

.. contents::

IPython can be used as an improved replacement for the Python prompt,
and for that you don't really need to read any more of this manual. But
in this section we'll try to summarize a few tips on how to make the
most effective use of it for everyday Python development, highlighting
things you might miss in the rest of the manual (which is getting long).
We'll give references to parts in the manual which provide more detail
when appropriate.

The following article by Jeremy Jones provides an introductory tutorial
about IPython: http://www.onlamp.com/pub/a/python/2005/01/27/ipython.html

Highlights
==========

Tab completion
--------------

TAB-completion, especially for attributes, is a convenient way to explore the
structure of any object you're dealing with. Simply type object_name.<TAB>
and a list of the object's attributes will be printed (see readline_ for
more). Tab completion also works on file and directory names, which combined
with IPython's alias system allows you to do from within IPython many of the
things you normally would need the system shell for.

Explore your objects
--------------------

Typing object_name? will print all sorts of details about any object,
including docstrings, function definition lines (for call arguments) and
constructor details for classes. The magic commands %pdoc, %pdef, %psource
and %pfile will respectively print the docstring, function definition line,
full source code and the complete file for any object (when they can be
found). If automagic is on (it is by default), you don't need to type the '%'
explicitly. See sec. `dynamic object information`_ for more.

The `%run` magic command
------------------------

The %run magic command allows you to run any python script and load all of
its data directly into the interactive namespace. Since the file is re-read
from disk each time, changes you make to it are reflected immediately (in
contrast to the behavior of import). I rarely use import for code I am
testing, relying on %run instead. See magic_ section for more on this and
other magic commands, or type the name of any magic command and ? to get
details on it. See also sec. dreload_ for a recursive reload command. %run
also has special flags for timing the execution of your scripts (-t) and for
executing them under the control of either Python's pdb debugger (-d) or
profiler (-p). With all of these, %run can be used as the main tool for
efficient interactive development of code which you write in your editor of
choice.

Debug a Python script
---------------------

Use the Python debugger, pdb. The %pdb command allows you to toggle on and
off the automatic invocation of an IPython-enhanced pdb debugger (with
coloring, tab completion and more) at any uncaught exception. The advantage
of this is that pdb starts inside the function where the exception occurred,
with all data still available. You can print variables, see code, execute
statements and even walk up and down the call stack to track down the true
source of the problem (which often is many layers in the stack above where
the exception gets triggered). Running programs with %run and pdb active can
be an efficient to develop and debug code, in many cases eliminating the need
for print statements or external debugging tools. I often simply put a 1/0 in
a place where I want to take a look so that pdb gets called, quickly view
whatever variables I need to or test various pieces of code and then remove
the 1/0. Note also that '%run -d' activates pdb and automatically sets
initial breakpoints for you to step through your code, watch variables, etc.
See Sec. `Output caching`_ for details.

Use the output cache
--------------------

All output results are automatically stored in a global dictionary named Out
and variables named _1, _2, etc. alias them. For example, the result of input
line 4 is available either as Out[4] or as _4. Additionally, three variables
named _, __ and ___ are always kept updated with the for the last three
results. This allows you to recall any previous result and further use it for
new calculations. See Sec. `Output caching`_ for more.

Suppress output
---------------

Put a ';' at the end of a line to suppress the printing of output. This is
useful when doing calculations which generate long output you are not
interested in seeing. The _* variables and the Out[] list do get updated with
the contents of the output, even if it is not printed. You can thus still
access the generated results this way for further processing.

Input cache
-----------

A similar system exists for caching input. All input is stored in a global
list called In , so you can re-execute lines 22 through 28 plus line 34 by
typing 'exec In[22:29]+In[34]' (using Python slicing notation). If you need
to execute the same set of lines often, you can assign them to a macro with
the %macro function. See sec. `Input caching`_ for more.

Use your input history
----------------------

The %hist command can show you all previous input, without line numbers if
desired (option -n) so you can directly copy and paste code either back in
IPython or in a text editor. You can also save all your history by turning on
logging via %logstart; these logs can later be either reloaded as IPython
sessions or used as code for your programs.

Define your own system aliases
------------------------------

Even though IPython gives you access to your system shell via the ! prefix,
it is convenient to have aliases to the system commands you use most often.
This allows you to work seamlessly from inside IPython with the same commands
you are used to in your system shell. IPython comes with some pre-defined
aliases and a complete system for changing directories, both via a stack (see
%pushd, %popd and %dhist) and via direct %cd. The latter keeps a history of
visited directories and allows you to go to any previously visited one.

Call system shell commands
--------------------------

Use Python to manipulate the results of system commands. The '!!' special
syntax, and the %sc and %sx magic commands allow you to capture system output
into Python variables.

Use Python variables when calling the shell
-------------------------------------------

Expand python variables when calling the shell (either via '!' and '!!' or
via aliases) by prepending a $ in front of them. You can also expand complete
python expressions. See `System shell access`_ for more.

Use profiles
------------

Use profiles to maintain different configurations (modules to load, function
definitions, option settings) for particular tasks. You can then have
customized versions of IPython for specific purposes. See sec. profiles_ for
more.


Embed IPython in your programs
------------------------------

A few lines of code are enough to load a complete IPython inside your own
programs, giving you the ability to work with your data interactively after
automatic processing has been completed. See sec. embedding_ for more.

Use the Python profiler
-----------------------

When dealing with performance issues, the %run command with a -p option
allows you to run complete programs under the control of the Python profiler.
The %prun command does a similar job for single Python expressions (like
function calls).

Use IPython to present interactive demos
----------------------------------------

Use the IPython.demo.Demo class to load any Python script as an interactive
demo. With a minimal amount of simple markup, you can control the execution
of the script, stopping as needed. See sec. `interactive demos`_ for more.

Run doctests
------------

Run your doctests from within IPython for development and debugging. The
special %doctest_mode command toggles a mode where the prompt, output and
exceptions display matches as closely as possible that of the default Python
interpreter. In addition, this mode allows you to directly paste in code that
contains leading '>>>' prompts, even if they have extra leading whitespace
(as is common in doctest files). This combined with the '%history -tn' call
to see your translated history (with these extra prompts removed and no line
numbers) allows for an easy doctest workflow, where you can go from doctest
to interactive execution to pasting into valid Python code as needed.

Source code handling tips
=========================

IPython is a line-oriented program, without full control of the
terminal. Therefore, it doesn't support true multiline editing. However,
it has a number of useful tools to help you in dealing effectively with
more complex editing.

The %edit command gives a reasonable approximation of multiline editing,
by invoking your favorite editor on the spot. IPython will execute the
code you type in there as if it were typed interactively. Type %edit?
for the full details on the edit command.

If you have typed various commands during a session, which you'd like to
reuse, IPython provides you with a number of tools. Start by using %hist
to see your input history, so you can see the line numbers of all input.
Let us say that you'd like to reuse lines 10 through 20, plus lines 24
and 28. All the commands below can operate on these with the syntax::

    %command 10-20 24 28 

where the command given can be:

    * %macro <macroname>: this stores the lines into a variable which,
      when called at the prompt, re-executes the input. Macros can be
      edited later using '%edit macroname', and they can be stored
      persistently across sessions with '%store macroname' (the storage
      system is per-profile). The combination of quick macros,
      persistent storage and editing, allows you to easily refine
      quick-and-dirty interactive input into permanent utilities, always
      available both in IPython and as files for general reuse.
    * %edit: this will open a text editor with those lines pre-loaded
      for further modification. It will then execute the resulting
      file's contents as if you had typed it at the prompt.
    * %save <filename>: this saves the lines directly to a named file on
      disk.

While %macro saves input lines into memory for interactive re-execution,
sometimes you'd like to save your input directly to a file. The %save
magic does this: its input sytnax is the same as %macro, but it saves
your input directly to a Python file. Note that the %logstart command
also saves input, but it logs all input to disk (though you can
temporarily suspend it and reactivate it with %logoff/%logon); %save
allows you to select which lines of input you need to save.


Lightweight 'version control'
=============================

When you call %edit with no arguments, IPython opens an empty editor
with a temporary file, and it returns the contents of your editing
session as a string variable. Thanks to IPython's output caching
mechanism, this is automatically stored::

    In [1]: %edit

    IPython will make a temporary file named: /tmp/ipython_edit_yR-HCN.py

    Editing... done. Executing edited code...

    hello - this is a temporary file

    Out[1]: "print 'hello - this is a temporary file'\n"

Now, if you call '%edit -p', IPython tries to open an editor with the
same data as the last time you used %edit. So if you haven't used %edit
in the meantime, this same contents will reopen; however, it will be
done in a new file. This means that if you make changes and you later
want to find an old version, you can always retrieve it by using its
output number, via '%edit _NN', where NN is the number of the output
prompt.

Continuing with the example above, this should illustrate this idea::

    In [2]: edit -p

    IPython will make a temporary file named: /tmp/ipython_edit_nA09Qk.py

    Editing... done. Executing edited code...

    hello - now I made some changes

    Out[2]: "print 'hello - now I made some changes'\n"

    In [3]: edit _1

    IPython will make a temporary file named: /tmp/ipython_edit_gy6-zD.py

    Editing... done. Executing edited code...

    hello - this is a temporary file

    IPython version control at work :)

    Out[3]: "print 'hello - this is a temporary file'\nprint 'IPython version control at work :)'\n"


This section was written after a contribution by Alexander Belchenko on
the IPython user list.


Effective logging
=================

A very useful suggestion sent in by Robert Kern follows:

I recently happened on a nifty way to keep tidy per-project log files. I
made a profile for my project (which is called "parkfield")::

    include ipythonrc

    # cancel earlier logfile invocation:

    logfile ''

    execute import time

    execute __cmd = '/Users/kern/research/logfiles/parkfield-%s.log rotate'

    execute __IP.magic_logstart(__cmd % time.strftime('%Y-%m-%d'))

I also added a shell alias for convenience::

    alias parkfield="ipython -pylab -profile parkfield" 

Now I have a nice little directory with everything I ever type in,
organized by project and date.

Contribute your own: If you have your own favorite tip on using IPython
efficiently for a certain task (especially things which can't be done in
the normal Python interpreter), don't hesitate to send it!