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Writing an IDLE extension
=========================

An IDLE extension can define new key bindings and menu entries for IDLE
edit windows.  There is a simple mechanism to load extensions when IDLE
starts up and to attach them to each edit window. (It is also possible
to make other changes to IDLE, but this must be done by editing the IDLE
source code.)

The list of extensions loaded at startup time is configured by editing
the file config-extensions.def.  See below for details.

An IDLE extension is defined by a class.  Methods of the class define
actions that are invoked by event bindings or menu entries. Class (or
instance) variables define the bindings and menu additions; these are
automatically applied by IDLE when the extension is linked to an edit
window.

An IDLE extension class is instantiated with a single argument,
`editwin', an EditorWindow instance. The extension cannot assume much
about this argument, but it is guaranteed to have the following instance
variables:

    text	a Text instance (a widget)
    io		an IOBinding instance (more about this later)
    flist	the FileList instance (shared by all edit windows)

(There are a few more, but they are rarely useful.)

The extension class must not directly bind Window Manager (e.g. X) events.
Rather, it must define one or more virtual events, e.g. <<zoom-height>>, and
corresponding methods, e.g. zoom_height_event().  The virtual events will be
bound to the corresponding methods, and Window Manager events can then be bound
to the virtual events. (This indirection is done so that the key bindings can
easily be changed, and so that other sources of virtual events can exist, such
as menu entries.)

An extension can define menu entries.  This is done with a class or instance
variable named menudefs; it should be a list of pairs, where each pair is a
menu name (lowercase) and a list of menu entries. Each menu entry is either
None (to insert a separator entry) or a pair of strings (menu_label,
virtual_event).  Here, menu_label is the label of the menu entry, and
virtual_event is the virtual event to be generated when the entry is selected.
An underscore in the menu label is removed; the character following the
underscore is displayed underlined, to indicate the shortcut character (for
Windows).

At the moment, extensions cannot define whole new menus; they must define
entries in existing menus.  Some menus are not present on some windows; such
entry definitions are then ignored, but key bindings are still applied.  (This
should probably be refined in the future.)

Extensions are not required to define menu entries for all the events they
implement.  (They are also not required to create keybindings, but in that
case there must be empty bindings in cofig-extensions.def)

Here is a complete example:

class ZoomHeight:

    menudefs = [
        ('edit', [
            None, # Separator
            ('_Zoom Height', '<<zoom-height>>'),
         ])
    ]

    def __init__(self, editwin):
        self.editwin = editwin

    def zoom_height_event(self, event):
        "...Do what you want here..."

The final piece of the puzzle is the file "config-extensions.def", which is
used to configure the loading of extensions and to establish key (or, more
generally, event) bindings to the virtual events defined in the extensions.

See the comments at the top of config-extensions.def for information.  It's
currently necessary to manually modify that file to change IDLE's extension
loading or extension key bindings.

For further information on binding refer to the Tkinter Resources web page at
python.org and to the Tk Command "bind" man page.