.. _tutorial:

======================
Introducing IPython
======================

You don't need to know anything beyond Python to start using IPython – just type
commands as you would at the standard Python prompt. But IPython can do much
more than the standard prompt. Some key features are described here. For more
information, check the :ref:`tips page <tips>`, or look at examples in the
`IPython cookbook <https://github.com/ipython/ipython/wiki/Cookbook%3A-Index>`_.

If you haven't done that yet see :ref:`how to install ipython <install>`.

If you've never used Python before, you might want to look at `the official
tutorial <http://docs.python.org/tutorial/>`_.

Start IPython by issuing the ``ipython`` command from your shell, you should be
greeted by the following::

    Python 3.6.0
    Type 'copyright', 'credits' or 'license' for more information
    IPython 6.0.0.dev -- An enhanced Interactive Python. Type '?' for help.

    In [1]:


Unlike the Python REPL, you will see that the input prompt is ``In [N]:``
instead of ``>>>``. The number ``N`` in the prompt will be used later in this
tutorial but should usually not impact the computation.

You should be able to type single line expressions and press enter to evaluate
them. If an expression is incomplete, IPython will automatically detect this and
add a new line when you press :kbd:`Enter` instead of executing right away.

Feel free to explore multi-line text input. Unlike many other REPLs, with
IPython you can use the up and down arrow keys when editing multi-line
code blocks.

Here is an example of a longer interaction with the IPython REPL,
which we often refer to as an IPython *session* ::

    In [1]: print('Hello IPython')
    Hello IPython

    In [2]: 21 * 2
    Out[2]: 42

    In [3]: def say_hello(name):
       ...:     print('Hello {name}'.format(name=name))
       ...:

We won't get into details right now, but you may notice a few differences to
the standard Python REPL. First, your code should be syntax-highlighted as you
type. Second, you will see that some results will have an ``Out[N]:`` prompt,
while some other do not. We'll come to this later.

Depending on the exact command you are typing you might realize that sometimes
:kbd:`Enter` will add a new line, and sometimes it will execute the current
statement. IPython tries to guess what you are doing, so most of the time you
should not have to care. Though if by any chance IPython does not do the right
thing you can force execution of the current code block by pressing in sequence
:kbd:`Esc` and :kbd:`Enter`. You can also force the insertion of a new line at
the position of the cursor by using :kbd:`Ctrl-o`.

The four most helpful commands
==============================

The four most helpful commands, as well as their brief description, is shown
to you in a banner, every time you start IPython:

==========    =========================================================
command       description
==========    =========================================================
?             Introduction and overview of IPython's features.
%quickref     Quick reference.
help          Python's own help system.
object?       Details about 'object', use 'object??' for extra details.
==========    =========================================================

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>``
to view the object's attributes. Besides Python objects and keywords, tab
completion also works on file and directory names.

Starting with IPython 6.0, if ``jedi`` is installed, IPython will try to pull
completions from Jedi as well. This allows to not only inspect currently
existing objects, but also to infer completion statically without executing
code. There is nothing particular needed to get this to work, simply use tab
completion on more complex expressions like the following::

    >>> data = ['Number of users', 123456]
    ... data[0].<tab>

IPython and Jedi will be able to infer that ``data[0]`` is actually a string
and should show relevant completions like ``upper()``, ``lower()`` and other
string methods. You can use the :kbd:`Tab` key to cycle through completions,
and while a completion is highlighted, its type will be shown as well.
When the type of the completion is a function, the completer will also show the
signature of the function when highlighted.

Exploring 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. To get specific information on an object, you
can use the magic commands ``%pdoc``, ``%pdef``, ``%psource`` and ``%pfile``

.. _magics_explained:

Magic functions
===============

IPython has a set of predefined 'magic functions' that you can call with a
command line style syntax.  There are two kinds of magics, line-oriented and
cell-oriented.  **Line magics** are prefixed with the ``%`` character and work
much like OS command-line calls: they get as an argument the rest of the line,
where arguments are passed without parentheses or quotes. **Line magics** can
return results and can be used in the right hand side of an assignment.  **Cell
magics** are prefixed with a double ``%%``, and they are functions that get as
an argument not only the rest of the line, but also the lines below it in a
separate argument.

Magics are useful as convenient functions where Python syntax is not the most
natural one, or when one want to embed invalid python syntax in their work flow. 

The following examples show how to call the built-in :magic:`timeit` magic, both
in line and cell mode::

      In [1]: %timeit range(1000)
      179 ns ± 2.66 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

      In [2]: %%timeit x = range(10000)
      ...: max(x)
      ...: 
      264 µs ± 1 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

The built-in magics include:

- Functions that work with code: :magic:`run`, :magic:`edit`, :magic:`save`,
  :magic:`macro`, :magic:`recall`, etc.

- Functions which affect the shell: :magic:`colors`, :magic:`xmode`,
  :magic:`automagic`, etc.

- Other functions such as :magic:`reset`, :magic:`timeit`,
  :cellmagic:`writefile`, :magic:`load`, or :magic:`paste`.

You can always call magics using the ``%`` prefix, and if you're calling a line
magic on a line by itself, as long as the identifier is not defined in your
namespace, you can omit even that::

    run thescript.py

You can toggle this behavior by running the :magic:`automagic` magic.  Cell
magics must always have the ``%%`` prefix.

A more detailed explanation of the magic system can be obtained by calling
``%magic``, and for more details on any magic function, call ``%somemagic?`` to
read its docstring. To see all the available magic functions, call
``%lsmagic``.

.. seealso::

    The :ref:`magic` section of the documentation goes more in depth into how
    the magics works and how to define your own, and :doc:`magics` for a list of
    built-in magics.

    `Cell magics`_ example notebook

Running and Editing
-------------------

The :magic:`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 (unlike
imported modules, which have to be specifically reloaded). IPython also includes
:ref:`dreload <dreload>`, a recursive reload function.

``%run`` has special flags for timing the execution of your scripts (-t), or
for running them under the control of either Python's pdb debugger (-d) or
profiler (-p).

The :magic:`edit` command gives a reasonable approximation of multi-line editing,
by invoking your favorite editor on the spot. IPython will execute the
code you type in there as if it were typed interactively. Note that for
:magic:`edit` to work, the call to startup your editor has to be a blocking
call. In a GUI environment, your editor likely will have such an option.

Debugging
---------

After an exception occurs, you can call :magic:`debug` to jump into the Python
debugger (pdb) and examine the problem. Alternatively, if you call :magic:`pdb`,
IPython will automatically start the debugger on any uncaught exception. 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. This can be an efficient way
to develop and debug code, in many cases eliminating the need for print
statements or external debugging tools.

You can also step through a program from the beginning by calling
``%run -d theprogram.py``.

History
=======

IPython stores both the commands you enter, and the results it produces. You
can easily go through previous commands with the up- and down-arrow keys, or
access your history in more sophisticated ways.

Input and output history are kept in variables called ``In`` and ``Out``, keyed
by the prompt numbers, e.g. ``In[4]``. The last three objects in output history
are also kept in variables named ``_``, ``__`` and ``___``.

You can use the ``%history`` magic function to examine past input and output.
Input history from previous sessions is saved in a database, and IPython can be
configured to save output history.

Several other magic functions can use your input history, including ``%edit``, 
``%rerun``, ``%recall``, ``%macro``, ``%save`` and ``%pastebin``. You can use a
standard format to refer to lines::

    %pastebin 3 18-20 ~1/1-5
    
This will take line 3 and lines 18 to 20 from the current session, and lines
1-5 from the previous session.

System shell commands
=====================

To run any command at the system shell, simply prefix it with ``!``, e.g.::

    !ping www.bbc.co.uk
    
You can capture the output into a Python list, e.g.: ``files = !ls``. To pass
the values of Python variables or expressions to system commands, prefix them
with $: ``!grep -rF $pattern ipython/*`` or wrap in `{braces}`. See :ref:`our
shell section <system_shell_access>` for more details.

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

It's 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 :magic:`pushd`,
:magic:`popd` and :magic:`dhist`) and via direct :magic:`cd`. The latter keeps a
history of visited directories and allows you to go to any previously visited
one.


Configuration
=============

Much of IPython can be tweaked through :doc:`configuration </config/intro>`.
To get started, use the command ``ipython profile create`` to produce the
default config files. These will be placed in
:file:`~/.ipython/profile_default`, and contain comments explaining
what the various options do.

Profiles allow you to use IPython for different tasks, keeping separate config
files and history for each one. More details in :ref:`the profiles section
<profiles>`.

.. _startup_files:

Startup Files
-------------

If you want some code to be run at the beginning of every IPython session, the
easiest way is to add Python (.py) or IPython (.ipy) scripts to your
:file:`profile_default/startup/` directory. Files here will be executed as soon
as the IPython shell is constructed, before any other code or scripts you have
specified. The files will be run in order of their names, so you can control the
ordering with prefixes, like ``10-myimports.py``.

.. include:: ../links.txt
