.. _pluginstutorial:

Writing your own plugins to extend calibre's functionality
====================================================================

calibre has a very modular design. Almost all functionality in calibre comes in the form of plugins. Plugins are used for conversion, for downloading news (though these are called recipes), for various components of the user interface, to connect to different devices, to process files when adding them to calibre and so on. You can get a complete list of all the built-in plugins in calibre by going to :guilabel:`Preferences->Advanced->Plugins`.

Here, we will teach you how to create your own plugins to add new features to calibre.


.. contents:: Contents
  :depth: 2
  :local:

.. note:: This only applies to calibre releases >= 0.8.60

Anatomy of a calibre plugin
---------------------------

A calibre plugin is very simple, it's just a ZIP file that contains some Python code
and any other resources like image files needed by the plugin. Without further ado,
let's see a basic example.

Suppose you have an installation of calibre that you are using to self publish various e-documents in EPUB and MOBI
formats. You would like all files generated by calibre to have their publisher set as "Hello world", here's how to do it.
Create a file named :file:`__init__.py` (this is a special name and must always be used for the main file of your plugin)
and enter the following Python code into it:

.. literalinclude:: plugin_examples/helloworld/__init__.py
    :lines: 10-

That's all. To add this code to calibre as a plugin, simply run the following in
the folder in which you created :file:`__init__.py`::

    calibre-customize -b .

.. note::
    On macOS, the command line tools are inside the calibre bundle, for example,
    if you installed calibre in :file:`/Applications` the command line tools
    are in :file:`/Applications/calibre.app/Contents/MacOS/`.

You can download the Hello World plugin from
:download_file:`helloworld_plugin.zip`.

Every time you use calibre to convert a book, the plugin's :meth:`run` method will be called and the
converted book will have its publisher set to "Hello World". This is a trivial plugin, let's move on to
a more complex example that actually adds a component to the user interface.

A User Interface plugin
-------------------------

This plugin will be spread over a few files (to keep the code clean). It will show you how to get resources
(images or data files) from the plugin ZIP file, allow users to configure your plugin,
how to create elements in the calibre user interface and how to access
and query the books database in calibre.

You can download this plugin from :download_file:`interface_demo_plugin.zip`

.. _import_name_txt:

The first thing to note is that this ZIP file has a lot more files in it, explained below, pay particular attention to
``plugin-import-name-interface_demo.txt``.

    **plugin-import-name-interface_demo.txt**
        An empty text file used to enable the multi-file plugin magic. This file must be present in all plugins that use
        more than one .py file. It should be empty and its filename must be of the form: ``plugin-import-name-**some_name**.txt``.
        The presence of this file allows you to import code from the .py files present inside the ZIP file, using a statement like::

            from calibre_plugins.some_name.some_module import some_object

        The prefix ``calibre_plugins`` must always be present. ``some_name`` comes from the filename of the empty text file.
        ``some_module`` refers to :file:`some_module.py` file inside the ZIP file. Note that this importing is just as
        powerful as regular Python imports. You can create packages and subpackages of .py modules inside the ZIP file,
        just like you would normally (by defining __init__.py in each sub-folder), and everything should "just work".

        The name you use for ``some_name`` enters a global namespace shared by all plugins, **so make it as unique as possible**.
        But remember that it must be a valid Python identifier (only alphabets, numbers and the underscore).

    **__init__.py**
        As before, the file that defines the plugin class

    **main.py**
        This file contains the actual code that does something useful

    **ui.py**
        This file defines the interface part of the plugin

    **images/icon.png**
        The icon for this plugin

    **about.txt**
        A text file with information about the plugin

    **translations**
        A folder containing .mo files with the translations of the user
        interface of your plugin into different languages. See below for
        details.

Now let's look at the code.

__init__.py
^^^^^^^^^^^^^

First, the obligatory ``__init__.py`` to define the plugin metadata:

.. literalinclude:: plugin_examples/interface_demo/__init__.py
    :lines: 10-

The only noteworthy feature is the field :attr:`actual_plugin`. Since calibre has both command line and GUI interfaces,
GUI plugins like this one should not load any GUI libraries in __init__.py. The actual_plugin field does this for you,
by telling calibre that the actual plugin is to be found in another file inside your ZIP archive, which will only be loaded
in a GUI context.

Remember that for this to work, you must have a plugin-import-name-some_name.txt file in your plugin ZIP file,
as discussed above.

Also there are a couple of methods for enabling user configuration of the plugin. These are discussed below.

ui.py
^^^^^^^^

Now let's look at ui.py which defines the actual GUI plugin. The source code is heavily commented and should be self explanatory:

.. literalinclude:: plugin_examples/interface_demo/ui.py
    :lines: 16-

main.py
^^^^^^^^^

The actual logic to implement the Interface Plugin Demo dialog.

.. literalinclude:: plugin_examples/interface_demo/main.py
    :lines: 16-

Getting resources from the plugin ZIP file
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

calibre's plugin loading system defines a couple of built-in functions that allow you to conveniently get files from the plugin ZIP file.

    **get_resources(name_or_list_of_names)**
        This function should be called with a list of paths to files inside the
        ZIP file. For example to access the file :file:`icon.png` in the folder
        images in the ZIP file, you would use: ``images/icon.png``. Always use
        a forward slash as the path separator, even on Windows. When you pass
        in a single name, the function will return the raw bytes of that file
        or None if the name was not found in the ZIP file. If you pass in more
        than one name then it returns a dictionary mapping the names to bytes.  If a
        name is not found, it will not be present in the returned dictionary.

    **get_icons(name_or_list_of_names, plugin_name='')**
        A wrapper for get_resources() that creates QIcon objects
        from the raw bytes returned by get_resources. If a name is not found
        in the ZIP file the corresponding QIcon will be null. In order to
        support icon theme-ing, pass in the human friendly name of your plugin
        as ``plugin_name``. If the user is using an icon theme with icons for
        your plugin, they will be loaded preferentially.


Enabling user configuration of your plugin
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To allow users to configure your plugin, you must define three methods in your base plugin class, **is_customizable**, **config_widget** and **save_settings** as shown below:

.. literalinclude:: plugin_examples/interface_demo/__init__.py
  :pyobject: InterfacePluginDemo.is_customizable

.. literalinclude:: plugin_examples/interface_demo/__init__.py
  :pyobject: InterfacePluginDemo.config_widget

.. literalinclude:: plugin_examples/interface_demo/__init__.py
  :pyobject: InterfacePluginDemo.save_settings

calibre has many different ways to store configuration data (a legacy of its long history). The recommended way is to use the **JSONConfig** class, which stores your configuration information in a .json file.

The code to manage configuration data in the demo plugin is in config.py:

.. literalinclude:: plugin_examples/interface_demo/config.py
    :lines: 10-

The ``prefs`` object is now available throughout the plugin code by a simple::

    from calibre_plugins.interface_demo.config import prefs


You can see the ``prefs`` object being used in main.py:

.. literalinclude:: plugin_examples/interface_demo/main.py
    :lines: 151-


Edit book plugins
------------------------------------------

Now let's change gears for a bit and look at creating a plugin to add tools to
the calibre book editor. The plugin is available here:
:download_file:`editor_demo_plugin.zip`.

The first step, as for all plugins is to create the
import name empty txt file, as described :ref:`above <import_name_txt>`.
We shall name the file ``plugin-import-name-editor_plugin_demo.txt``.

Now we create the mandatory ``__init__.py`` file that contains metadata about
the plugin -- its name, author, version, etc.

.. literalinclude:: plugin_examples/editor_demo/__init__.py
    :lines: 8-

A single editor plugin can provide multiple tools each tool corresponds to a
single button in the toolbar and entry in the :guilabel:`Plugins` menu in the
editor. These can have sub-menus in case the tool has multiple related actions.

The tools must all be defined in the file ``main.py`` in your plugin. Every
tool is a class that inherits from the
:class:`calibre.gui2.tweak_book.plugin.Tool` class. Let's look at ``main.py``
from the demo plugin, the source code is heavily commented and should be
self-explanatory. Read the API documents of the
:class:`calibre.gui2.tweak_book.plugin.Tool` class for more details.

main.py
^^^^^^^^^

Here we will see the definition of a single tool that will multiply all font
sizes in the book by a number provided by the user. This tool demonstrates
various important concepts that you will need in developing your own plugins,
so you should read the (heavily commented) source code carefully.

.. literalinclude:: plugin_examples/editor_demo/main.py
    :lines: 8-

Let's break down ``main.py``. We see that it defines a single tool, named
*Magnify fonts*. This tool will ask the user for a number and multiply all font
sizes in the book by that number.

The first important thing is the tool name which you must set to some
relatively unique string as it will be used as the key for this tool.

The next important entry point is the
:meth:`calibre.gui2.tweak_book.plugin.Tool.create_action`. This method creates
the QAction objects that appear in the plugins toolbar and plugin menu.
It also, optionally, assigns a keyboard shortcut that the user can customize.
The triggered signal from the QAction is connected to the ask_user() method
that asks the user for the font size multiplier, and then runs the
magnification code.

The magnification code is well commented and fairly simple. The main things to
note are that you get a reference to the editor window as ``self.gui`` and the
editor *Boss* as ``self.boss``. The *Boss* is the object that controls the editor
user interface. It has many useful methods, that are documented in the
:class:`calibre.gui2.tweak_book.boss.Boss` class.

Finally, there is ``self.current_container`` which is a reference to the book
being edited as a :class:`calibre.ebooks.oeb.polish.container.Container`
object. This represents the book as a collection of its constituent
HTML/CSS/image files and has convenience methods for doing many useful things.
The container object and various useful utility functions that can be reused in
your plugin code are documented in :ref:`polish_api`.

Adding translations to your plugin
--------------------------------------

You can have all the user interface strings in your plugin translated and
displayed in whatever language is set for the main calibre user interface.

The first step is to go through your plugin's source code and mark all user
visible strings as translatable, by surrounding them in _(). For example::

    action_spec = (_('My plugin'), None, _('My plugin is cool'), None)

Then use some program to generate .po files from your plugin source code. There
should be one .po file for every language you want to translate into. For
example: de.po for German, fr.po for French and so on. You can use the
`Poedit <https://poedit.net/>`_ program for this.

Send these .po files to your translators. Once you get them back, compile them
into .mo files. You can again use Poedit for that, or just do::

    calibre-debug -c "from calibre.translations.msgfmt import main; main()" filename.po

Put the .mo files into the ``translations`` folder in your plugin.

The last step is to simply call the function `load_translations()` at the top
of your plugin's .py files. For performance reasons you should only call this
function in those .py files that actually have translatable strings. So in a
typical User Interface plugin you would call it at the top of ``ui.py`` but not
``__init__.py``.

You can test the translations of your plugins by changing the user interface
language in calibre under :guilabel:`Preferences->Interface->Look & feel` or by running calibre
with the ``CALIBRE_OVERRIDE_LANG`` environment variable set. For example::

    CALIBRE_OVERRIDE_LANG=de

Replace ``de`` with the language code of the language you want to test.

For translations with plurals, use the ``ngettext()`` function instead of
``_()``. For example::

    ngettext('Delete a book', 'Delete {} books', num_books).format(num_books)

The plugin API
--------------------------------

As you may have noticed above, a plugin in calibre is a class. There are different classes for the different types of plugins in calibre.
Details on each class, including the base class of all plugins can be found in :ref:`plugins`.

Your plugin is almost certainly going to use code from calibre. To learn
how to find various bits of functionality in the
calibre code base, read the section on the calibre :ref:`code_layout`.

Debugging plugins
-------------------

The first, most important step is to run calibre in debug mode. You can do this from the command line with::

    calibre-debug -g

Or from within calibre by right-clicking the :guilabel:`Preferences` button or using the :kbd:`Ctrl+Shift+R` keyboard shortcut.

When running from the command line, debug output will be printed to the console, when running from within calibre the output will go to a txt file.

You can insert print statements anywhere in your plugin code, they will be output in debug mode. Remember, this is Python, you really shouldn't need anything more than print statements to debug ;) I developed all of calibre using just this debugging technique.

You can quickly test changes to your plugin by using the following command
line::

    calibre-debug -s; calibre-customize -b /path/to/your/plugin/folder; calibre

This will shutdown a running calibre, wait for the shutdown to complete, then update your plugin in calibre and relaunch calibre.

More plugin examples
----------------------

You can find a list of many sophisticated calibre plugins `here <https://www.mobileread.com/forums/showthread.php?t=118764>`_.

Sharing your plugins with others
----------------------------------

If you would like to share the plugins you have created with other users of calibre, post your plugin in a new thread in the
`calibre plugins forum <https://www.mobileread.com/forums/forumdisplay.php?f=237>`_.
