Defining custom magics

There are two main ways to define your own magic functions: from standalone functions and by inheriting from a base class provided by IPython: :class:`IPython.core.magic.Magics`. Below we show code you can place in a file that you load from your configuration, such as any file in the startup subdirectory of your default IPython profile.

First, let us see the simplest case. The following shows how to create a line magic, a cell one and one that works in both modes, using just plain functions:

from IPython.core.magic import (register_line_magic, register_cell_magic,
                                register_line_cell_magic)

@register_line_magic
def lmagic(line):
    "my line magic"
    return line

@register_cell_magic
def cmagic(line, cell):
    "my cell magic"
    return line, cell

@register_line_cell_magic
def lcmagic(line, cell=None):
    "Magic that works both as %lcmagic and as %%lcmagic"
    if cell is None:
        print("Called as line magic")
        return line
    else:
        print("Called as cell magic")
        return line, cell

# In an interactive session, we need to delete these to avoid
# name conflicts for automagic to work on line magics.
del lmagic, lcmagic

You can also create magics of all three kinds by inheriting from the :class:`IPython.core.magic.Magics` class. This lets you create magics that can potentially hold state in between calls, and that have full access to the main IPython object:

# This code can be put in any Python module, it does not require IPython
# itself to be running already.  It only creates the magics subclass but
# doesn't instantiate it yet.
from __future__ import print_function
from IPython.core.magic import (Magics, magics_class, line_magic,
                                cell_magic, line_cell_magic)

# The class MUST call this class decorator at creation time
@magics_class
class MyMagics(Magics):

    @line_magic
    def lmagic(self, line):
        "my line magic"
        print("Full access to the main IPython object:", self.shell)
        print("Variables in the user namespace:", list(self.shell.user_ns.keys()))
        return line

    @cell_magic
    def cmagic(self, line, cell):
        "my cell magic"
        return line, cell

    @line_cell_magic
    def lcmagic(self, line, cell=None):
        "Magic that works both as %lcmagic and as %%lcmagic"
        if cell is None:
            print("Called as line magic")
            return line
        else:
            print("Called as cell magic")
            return line, cell


# In order to actually use these magics, you must register them with a
# running IPython.

def load_ipython_extension(ipython):
    """
    Any module file that define a function named `load_ipython_extension`
    can be loaded via `%load_ext module.path` or be configured to be
    autoloaded by IPython at startup time.
    """
    # You can register the class itself without instantiating it.  IPython will
    # call the default constructor on it.
    ipython.register_magics(MyMagics)

If you want to create a class with a different constructor that holds additional state, then you should always call the parent constructor and instantiate the class yourself before registration:

@magics_class
class StatefulMagics(Magics):
    "Magics that hold additional state"

    def __init__(self, shell, data):
        # You must call the parent constructor
        super(StatefulMagics, self).__init__(shell)
        self.data = data

    # etc...

def load_ipython_extension(ipython):
    """
    Any module file that define a function named `load_ipython_extension`
    can be loaded via `%load_ext module.path` or be configured to be
    autoloaded by IPython at startup time.
    """
    # This class must then be registered with a manually created instance,
    # since its constructor has different arguments from the default:
    magics = StatefulMagics(ipython, some_data)
    ipython.register_magics(magics)

Accessing user namespace and local scope

When creating line magics, you may need to access surrounding scope to get user variables (e.g when called inside functions). IPython provides the @needs_local_scope decorator that can be imported from IPython.core.magics. When decorated with @needs_local_scope a magic will be passed local_ns as an argument. As a convenience @needs_local_scope can also be applied to cell magics even if cell magics cannot appear at local scope context.

Silencing the magic output

Sometimes it may be useful to define a magic that can be silenced the same way that non-magic expressions can, i.e., by appending a semicolon at the end of the Python code to be executed. That can be achieved by decorating the magic function with the decorator @output_can_be_silenced that can be imported from IPython.core.magics. When this decorator is used, IPython will parse the Python code used by the magic and, if the last token is a ;, the output created by the magic will not show up on the screen. If you want to see an example of this decorator in action, take a look on the time magic defined in IPython.core.magics.execution.py.

Complete Example

Here is a full example of a magic package. You can distribute magics using setuptools, distutils, or any other distribution tools like flit for pure Python packages.

When distributing magics as part of a package, recommended best practice is to execute the registration inside the load_ipython_extension as demonstrated in the example below, instead of directly in the module (as in the initial example with the @register_* decorators). This means a user will need to explicitly choose to load your magic with %load_ext. instead implicitly getting it when importing the module. This is particularly relevant if loading your magic has side effects, if it is slow to load, or if it might override another magic with the same name.

.
├── example_magic
│   ├── __init__.py
│   └── abracadabra.py
└── setup.py

$ cat example_magic/__init__.py
"""An example magic"""
__version__ = '0.0.1'

from .abracadabra import Abracadabra

def load_ipython_extension(ipython):
    ipython.register_magics(Abracadabra)

$ cat example_magic/abracadabra.py
from IPython.core.magic import (Magics, magics_class, line_magic, cell_magic)

@magics_class
class Abracadabra(Magics):

    @line_magic
    def abra(self, line):
        return line

    @cell_magic
    def cadabra(self, line, cell):
        return line, cell