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 Preferences → Advanced → Plugins.

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

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 __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:

from calibre.customize import FileTypePlugin


class HelloWorld(FileTypePlugin):

    name                = 'Hello World Plugin' # Name of the plugin
    description         = 'Set the publisher to Hello World for all new conversions'
    supported_platforms = ['windows', 'osx', 'linux'] # Platforms this plugin will run on
    author              = 'Acme Inc.' # The author of this plugin
    version             = (1, 0, 0)   # The version number of this plugin
    file_types          = {'epub', 'mobi'} # The file types that this plugin will be applied to
    on_postprocess      = True # Run this plugin after conversion is complete
    minimum_calibre_version = (0, 7, 53)

    def run(self, path_to_ebook):
        from calibre.ebooks.metadata.meta import get_metadata, set_metadata
        with open(path_to_ebook, 'r+b') as file:
            ext  = os.path.splitext(path_to_ebook)[-1][1:].lower()
            mi = get_metadata(file, ext)
            mi.publisher = 'Hello World'
            set_metadata(file, mi, ext)
        return path_to_ebook

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

calibre-customize -b .

Note

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

You can download the Hello World plugin from helloworld_plugin.zip.

Every time you use calibre to convert a book, the plugin’s run() method will be called and the converted book will have its publisher set to “Hello World”. This is a trivial plugin, lets 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 interface_demo_plugin.zip

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 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:

from calibre.customize import InterfaceActionBase


class InterfacePluginDemo(InterfaceActionBase):
    '''
    This class is a simple wrapper that provides information about the actual
    plugin class. The actual interface plugin class is called InterfacePlugin
    and is defined in the ui.py file, as specified in the actual_plugin field
    below.

    The reason for having two classes is that it allows the command line
    calibre utilities to run without needing to load the GUI libraries.
    '''
    name                = 'Interface Plugin Demo'
    description         = 'An advanced plugin demo'
    supported_platforms = ['windows', 'osx', 'linux']
    author              = 'Kovid Goyal'
    version             = (1, 0, 0)
    minimum_calibre_version = (0, 7, 53)

    #: This field defines the GUI plugin class that contains all the code
    #: that actually does something. Its format is module_path:class_name
    #: The specified class must be defined in the specified module.
    actual_plugin       = 'calibre_plugins.interface_demo.ui:InterfacePlugin'

    def is_customizable(self):
        '''
        This method must return True to enable customization via
        Preferences->Plugins
        '''
        return True

    def config_widget(self):
        '''
        Implement this method and :meth:`save_settings` in your plugin to
        use a custom configuration dialog.

        This method, if implemented, must return a QWidget. The widget can have
        an optional method validate() that takes no arguments and is called
        immediately after the user clicks OK. Changes are applied if and only
        if the method returns True.

        If for some reason you cannot perform the configuration at this time,
        return a tuple of two strings (message, details), these will be
        displayed as a warning dialog to the user and the process will be
        aborted.

        The base class implementation of this method raises NotImplementedError
        so by default no user configuration is possible.
        '''
        # It is important to put this import statement here rather than at the
        # top of the module as importing the config class will also cause the
        # GUI libraries to be loaded, which we do not want when using calibre
        # from the command line
        from calibre_plugins.interface_demo.config import ConfigWidget
        return ConfigWidget()

    def save_settings(self, config_widget):
        '''
        Save the settings specified by the user with config_widget.

        :param config_widget: The widget returned by :meth:`config_widget`.
        '''
        config_widget.save_settings()

        # Apply the changes
        ac = self.actual_plugin_
        if ac is not None:
            ac.apply_settings()

The only noteworthy feature is the field 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:

from calibre.gui2.actions import InterfaceAction
from calibre_plugins.interface_demo.main import DemoDialog


class InterfacePlugin(InterfaceAction):

    name = 'Interface Plugin Demo'

    # Declare the main action associated with this plugin
    # The keyboard shortcut can be None if you dont want to use a keyboard
    # shortcut. Remember that currently calibre has no central management for
    # keyboard shortcuts, so try to use an unusual/unused shortcut.
    action_spec = ('Interface Plugin Demo', None,
            'Run the Interface Plugin Demo', 'Ctrl+Shift+F1')

    def genesis(self):
        # This method is called once per plugin, do initial setup here

        # Set the icon for this interface action
        # The get_icons function is a builtin function defined for all your
        # plugin code. It loads icons from the plugin zip file. It returns
        # QIcon objects, if you want the actual data, use the analogous
        # get_resources builtin function.
        #
        # Note that if you are loading more than one icon, for performance, you
        # should pass a list of names to get_icons. In this case, get_icons
        # will return a dictionary mapping names to QIcons. Names that
        # are not found in the zip file will result in null QIcons.
        icon = get_icons('images/icon.png', 'Interface Demo Plugin')

        # The qaction is automatically created from the action_spec defined
        # above
        self.qaction.setIcon(icon)
        self.qaction.triggered.connect(self.show_dialog)

    def show_dialog(self):
        # The base plugin object defined in __init__.py
        base_plugin_object = self.interface_action_base_plugin
        # Show the config dialog
        # The config dialog can also be shown from within
        # Preferences->Plugins, which is why the do_user_config
        # method is defined on the base plugin class
        do_user_config = base_plugin_object.do_user_config

        # self.gui is the main calibre GUI. It acts as the gateway to access
        # all the elements of the calibre user interface, it should also be the
        # parent of the dialog
        d = DemoDialog(self.gui, self.qaction.icon(), do_user_config)
        d.show()

    def apply_settings(self):
        from calibre_plugins.interface_demo.config import prefs
        # In an actual non trivial plugin, you would probably need to
        # do something based on the settings in prefs
        prefs

main.py ¶

The actual logic to implement the Interface Plugin Demo dialog.

from calibre_plugins.interface_demo.config import prefs


class DemoDialog(QDialog):

    def __init__(self, gui, icon, do_user_config):
        QDialog.__init__(self, gui)
        self.gui = gui
        self.do_user_config = do_user_config

        # The current database shown in the GUI
        # db is an instance of the class LibraryDatabase from db/legacy.py
        # This class has many, many methods that allow you to do a lot of
        # things. For most purposes you should use db.new_api, which has
        # a much nicer interface from db/cache.py
        self.db = gui.current_db

        self.l = QVBoxLayout()
        self.setLayout(self.l)

        self.label = QLabel(prefs['hello_world_msg'])
        self.l.addWidget(self.label)

        self.setWindowTitle('Interface Plugin Demo')
        self.setWindowIcon(icon)

        self.about_button = QPushButton('About', self)
        self.about_button.clicked.connect(self.about)
        self.l.addWidget(self.about_button)

        self.marked_button = QPushButton(
            'Show books with only one format in the calibre GUI', self)
        self.marked_button.clicked.connect(self.marked)
        self.l.addWidget(self.marked_button)

        self.view_button = QPushButton(
            'View the most recently added book', self)
        self.view_button.clicked.connect(self.view)
        self.l.addWidget(self.view_button)

        self.update_metadata_button = QPushButton(
            'Update metadata in a book\'s files', self)
        self.update_metadata_button.clicked.connect(self.update_metadata)
        self.l.addWidget(self.update_metadata_button)

        self.conf_button = QPushButton(
                'Configure this plugin', self)
        self.conf_button.clicked.connect(self.config)
        self.l.addWidget(self.conf_button)

        self.resize(self.sizeHint())

    def about(self):
        # Get the about text from a file inside the plugin zip file
        # The get_resources function is a builtin function defined for all your
        # plugin code. It loads files from the plugin zip file. It returns
        # the bytes from the specified file.
        #
        # Note that if you are loading more than one file, for performance, you
        # should pass a list of names to get_resources. In this case,
        # get_resources will return a dictionary mapping names to bytes. Names that
        # are not found in the zip file will not be in the returned dictionary.
        text = get_resources('about.txt')
        QMessageBox.about(self, 'About the Interface Plugin Demo',
                text.decode('utf-8'))

    def marked(self):
        ''' Show books with only one format '''
        db = self.db.new_api
        matched_ids = {book_id for book_id in db.all_book_ids() if len(db.formats(book_id)) == 1}
        # Mark the records with the matching ids
        # new_api does not know anything about marked books, so we use the full
        # db object
        self.db.set_marked_ids(matched_ids)

        # Tell the GUI to search for all marked records
        self.gui.search.setEditText('marked:true')
        self.gui.search.do_search()

    def view(self):
        ''' View the most recently added book '''
        most_recent = most_recent_id = None
        db = self.db.new_api
        for book_id, timestamp in db.all_field_for('timestamp', db.all_book_ids()).items():
            if most_recent is None or timestamp > most_recent:
                most_recent = timestamp
                most_recent_id = book_id

        if most_recent_id is not None:
            # Get a reference to the View plugin
            view_plugin = self.gui.iactions['View']
            # Ask the view plugin to launch the viewer for row_number
            view_plugin._view_calibre_books([most_recent_id])

    def update_metadata(self):
        '''
        Set the metadata in the files in the selected book's record to
        match the current metadata in the database.
        '''
        from calibre.ebooks.metadata.meta import set_metadata
        from calibre.gui2 import error_dialog, info_dialog

        # Get currently selected books
        rows = self.gui.library_view.selectionModel().selectedRows()
        if not rows or len(rows) == 0:
            return error_dialog(self.gui, 'Cannot update metadata',
                             'No books selected', show=True)
        # Map the rows to book ids
        ids = list(map(self.gui.library_view.model().id, rows))
        db = self.db.new_api
        for book_id in ids:
            # Get the current metadata for this book from the db
            mi = db.get_metadata(book_id, get_cover=True, cover_as_data=True)
            fmts = db.formats(book_id)
            if not fmts:
                continue
            for fmt in fmts:
                fmt = fmt.lower()
                # Get a python file object for the format. This will be either
                # an in memory file or a temporary on disk file
                ffile = db.format(book_id, fmt, as_file=True)
                ffile.seek(0)
                # Set metadata in the format
                set_metadata(ffile, mi, fmt)
                ffile.seek(0)
                # Now replace the file in the calibre library with the updated
                # file. We dont use add_format_with_hooks as the hooks were
                # already run when the file was first added to calibre.
                db.add_format(book_id, fmt, ffile, run_hooks=False)

        info_dialog(self, 'Updated files',
                'Updated the metadata in the files of %d book(s)'%len(ids),
                show=True)

    def config(self):
        self.do_user_config(parent=self)
        # Apply the changes
        self.label.setText(prefs['hello_world_msg'])

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 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:

    def is_customizable(self):
        '''
        This method must return True to enable customization via
        Preferences->Plugins
        '''
        return True

    def config_widget(self):
        '''
        Implement this method and :meth:`save_settings` in your plugin to
        use a custom configuration dialog.

        This method, if implemented, must return a QWidget. The widget can have
        an optional method validate() that takes no arguments and is called
        immediately after the user clicks OK. Changes are applied if and only
        if the method returns True.

        If for some reason you cannot perform the configuration at this time,
        return a tuple of two strings (message, details), these will be
        displayed as a warning dialog to the user and the process will be
        aborted.

        The base class implementation of this method raises NotImplementedError
        so by default no user configuration is possible.
        '''
        # It is important to put this import statement here rather than at the
        # top of the module as importing the config class will also cause the
        # GUI libraries to be loaded, which we do not want when using calibre
        # from the command line
        from calibre_plugins.interface_demo.config import ConfigWidget
        return ConfigWidget()

    def save_settings(self, config_widget):
        '''
        Save the settings specified by the user with config_widget.

        :param config_widget: The widget returned by :meth:`config_widget`.
        '''
        config_widget.save_settings()

        # Apply the changes
        ac = self.actual_plugin_
        if ac is not None:
            ac.apply_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:

from calibre.utils.config import JSONConfig

# This is where all preferences for this plugin will be stored
# Remember that this name (i.e. plugins/interface_demo) is also
# in a global namespace, so make it as unique as possible.
# You should always prefix your config file name with plugins/,
# so as to ensure you dont accidentally clobber a calibre config file
prefs = JSONConfig('plugins/interface_demo')

# Set defaults
prefs.defaults['hello_world_msg'] = 'Hello, World!'


class ConfigWidget(QWidget):

    def __init__(self):
        QWidget.__init__(self)
        self.l = QHBoxLayout()
        self.setLayout(self.l)

        self.label = QLabel('Hello world &message:')
        self.l.addWidget(self.label)

        self.msg = QLineEdit(self)
        self.msg.setText(prefs['hello_world_msg'])
        self.l.addWidget(self.msg)
        self.label.setBuddy(self.msg)

    def save_settings(self):
        prefs['hello_world_msg'] = self.msg.text()

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:

    def config(self):
        self.do_user_config(parent=self)
        # Apply the changes
        self.label.setText(prefs['hello_world_msg'])

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: editor_demo_plugin.zip.

The first step, as for all plugins is to create the import name empty txt file, as described above. 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.

from calibre.customize import EditBookToolPlugin


class DemoPlugin(EditBookToolPlugin):

    name = 'Edit Book plugin demo'
    version = (1, 0, 0)
    author = 'Kovid Goyal'
    supported_platforms = ['windows', 'osx', 'linux']
    description = 'A demonstration of the plugin interface for the ebook editor'
    minimum_calibre_version = (1, 46, 0)

A single editor plugin can provide multiple tools each tool corresponds to a single button in the toolbar and entry in the 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 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 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.

import re

from css_parser.css import CSSRule
from qt.core import QAction, QInputDialog

from calibre import force_unicode
from calibre.ebooks.oeb.polish.container import OEB_DOCS, OEB_STYLES, serialize
from calibre.gui2 import error_dialog

# The base class that all tools must inherit from
from calibre.gui2.tweak_book.plugin import Tool


class DemoTool(Tool):

    #: Set this to a unique name it will be used as a key
    name = 'demo-tool'

    #: If True the user can choose to place this tool in the plugins toolbar
    allowed_in_toolbar = True

    #: If True the user can choose to place this tool in the plugins menu
    allowed_in_menu = True

    def create_action(self, for_toolbar=True):
        # Create an action, this will be added to the plugins toolbar and
        # the plugins menu
        ac = QAction(get_icons('images/icon.png'), 'Magnify fonts', self.gui)  # noqa
        if not for_toolbar:
            # Register a keyboard shortcut for this toolbar action. We only
            # register it for the action created for the menu, not the toolbar,
            # to avoid a double trigger
            self.register_shortcut(ac, 'magnify-fonts-tool', default_keys=('Ctrl+Shift+Alt+D',))
        ac.triggered.connect(self.ask_user)
        return ac

    def ask_user(self):
        # Ask the user for a factor by which to multiply all font sizes
        factor, ok = QInputDialog.getDouble(
            self.gui, 'Enter a magnification factor', 'Allow font sizes in the book will be multiplied by the specified factor',
            value=2, min=0.1, max=4
        )
        if ok:
            # Ensure any in progress editing the user is doing is present in the container
            self.boss.commit_all_editors_to_container()
            try:
                self.magnify_fonts(factor)
            except Exception:
                # Something bad happened report the error to the user
                import traceback
                error_dialog(self.gui, _('Failed to magnify fonts'), _(
                    'Failed to magnify fonts, click "Show details" for more info'),
                    det_msg=traceback.format_exc(), show=True)
                # Revert to the saved restore point
                self.boss.revert_requested(self.boss.global_undo.previous_container)
            else:
                # Show the user what changes we have made, allowing her to
                # revert them if necessary
                self.boss.show_current_diff()
                # Update the editor UI to take into account all the changes we
                # have made
                self.boss.apply_container_update_to_gui()

    def magnify_fonts(self, factor):
        # Magnify all font sizes defined in the book by the specified factor
        # First we create a restore point so that the user can undo all changes
        # we make.
        self.boss.add_savepoint('Before: Magnify fonts')

        container = self.current_container  # The book being edited as a container object

        # Iterate over all style declarations in the book, this means css
        # stylesheets, <style> tags and style="" attributes
        for name, media_type in container.mime_map.items():
            if media_type in OEB_STYLES:
                # A stylesheet. Parsed stylesheets are css_parser CSSStylesheet
                # objects.
                self.magnify_stylesheet(container.parsed(name), factor)
                container.dirty(name)  # Tell the container that we have changed the stylesheet
            elif media_type in OEB_DOCS:
                # A HTML file. Parsed HTML files are lxml elements

                for style_tag in container.parsed(name).xpath('//*[local-name="style"]'):
                    if style_tag.text and style_tag.get('type', None) in {None, 'text/css'}:
                        # We have an inline CSS <style> tag, parse it into a
                        # stylesheet object
                        sheet = container.parse_css(style_tag.text)
                        self.magnify_stylesheet(sheet, factor)
                        style_tag.text = serialize(sheet, 'text/css', pretty_print=True)
                        container.dirty(name)  # Tell the container that we have changed the stylesheet
                for elem in container.parsed(name).xpath('//*[@style]'):
                    # Process inline style attributes
                    block = container.parse_css(elem.get('style'), is_declaration=True)
                    self.magnify_declaration(block, factor)
                    elem.set('style', force_unicode(block.getCssText(separator=' '), 'utf-8'))

    def magnify_stylesheet(self, sheet, factor):
        # Magnify all fonts in the specified stylesheet by the specified
        # factor.
        for rule in sheet.cssRules.rulesOfType(CSSRule.STYLE_RULE):
            self.magnify_declaration(rule.style, factor)

    def magnify_declaration(self, style, factor):
        # Magnify all fonts in the specified style declaration by the specified
        # factor
        val = style.getPropertyValue('font-size')
        if not val:
            return
        # see if the font-size contains a number
        num = re.search(r'[0-9.]+', val)
        if num is not None:
            num = num.group()
            val = val.replace(num, '%f' % (float(num) * factor))
            style.setProperty('font-size', val)
        # We should also be dealing with the font shorthand property and
        # font sizes specified as non numbers, but those are left as exercises
        # for the reader

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 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 calibre.gui2.tweak_book.boss.Boss class.

Finally, there is self.current_container which is a reference to the book being edited as a 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 API documentation for the e-book editing tools.

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 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 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 API documentation for 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 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 Preferences button or using the 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.