# 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 tips page, or look at examples in the IPython cookbook.

If you’ve never used Python before, you might want to look at the official tutorial or an alternative, Dive into Python.

## 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.

## 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`

## 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. Lines 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 builtin `%timeit` magic, both in line and cell mode:

    In [1]: %timeit range(1000)
    100000 loops, best of 3: 7.76 us per loop

    In [2]: %%timeit x = range(10000)
       ...: max(x)
       ...:
    1000 loops, best of 3: 223 us per loop

The builtin magics include:

- Functions that work with code: `%run`, `%edit`, `%save`, `%macro`, `%recall`, etc.
- Functions which affect the shell: `%colors`, `%xmode`, `%autoindent`, `%automagic`, etc.
- Other functions such as `%reset`, `%timeit`, `%%writefile`, `%load`, or `%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 `%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`.

See also

The Magic command system section of the documentation goes more in depth into how the magics works and how to define your own, and Built-in magic commands for a list of built-in magics.

Cell magics example notebook

### Running and Editing

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 (unlike imported modules, which have to be specifically reloaded). IPython also includes 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 `%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. Note that for `%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 `%debug` to jump into the Python debugger (pdb) and examine the problem. Alternatively, if you call `%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/*`. See our shell section 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 `%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.

## Configuration

Much of IPython can be tweaked through configuration. To get started, use the command `ipython profile create` to produce the default config files. These will be placed in `~/.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 the profiles section.

### 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 `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`.