Writing Plugins

Plugins modify how Leo works. With plugins you can give Leo new commands, modify how existing commands work, or change any other aspect of Leo’s look and feel.

leoPlugins.leo contains all of Leo’s official plugins. Studying this file is a good way to learn how to write plugins.

Writing plugins is like writing any other Leo script. See Scripting Leo with Python. In particular:

  1. Plugins can use any of Leo’s source code simply by importing any module defined in leoPy.leo.
  2. Plugins can register event handlers just like any other Leo script. For full details, see the section called Handling Events later in this chapter.

The rest of this chapters discusses topics related specifically to plugins.

Writing Plugins

A plugin is a Python file in Leo’s plugins folder.

Every plugin should have a top-level init function that returns True if the plugin has been initialized properly. The init function typically:

  1. Registers an onCreate event handler, called when Leo creates a new window.
  2. Calls g.plugin_signon(__name__)

For example:

def init():
    if << all imports successful >>:
        return True
        return False

Plugins do not have automatic access to c, g and p.

Plugins define g by importing it:

import leo.core.leoGlobals as g

Plugins gain access to c using event handlers:

controllers = {}

def init():
    return True

def onCreate (tag, keys):
    global controllers
    c = keys.get('c')
    if c:
        hash = c.hash()
        if hash not in controllers.keys():
            controllers(hash) = PluginController(c)

def eventHander(tag,keys):
    global controllers
    c = keys.get('c')
    if c:
        controller = controllers.get(c.hash())

Some plugins inject ivars into the Commands class rather than using a global controllers dict:

def onCreate (tag, keys):
    c = keys.get('c')
    if c:
        c.my_plugin_controller = ControllerClass(c)

def eventHander(tag,keys):
    c = keys.get('c')
    if c:

Once c is determined, the presently selected position is simply c.p.

Important security warnings

Naively using plugins can expose you and your .leo files to malicious attacks. The fundamental principles are:

Scripts and plugins must never blindly execute code from untrusted sources.


.leo files obtained from other people may potentially contain hostile code.

Stephen Schaefer summarizes the danger this way:

I foresee a future in which the majority of leo projects come from
marginally trusted sources...a world of leo documents sent hither
and yon - resumes, project proposals, textbooks, magazines,
contracts - and as a race of Pandora's, we cannot resist wanting
to see "What's in the box?" And are we going to fire up a text
editor to make a detailed examination of the ASCII XML? Never!
We're going to double click on the cute leo file icon, and leo
will fire up in all its raging glory. Just like Word (and its
macros) or Excel (and its macros).

In other words:

When we share "our" .leo files we can NOT assume that we know what
is in our "own" documents!

Not all environments are untrustworthy. Code in a commercial cvs repository is probably trustworthy: employees might be terminated for posting malicious code. Still, the potential for abuse exists anywhere.

In Python it is very easy to write a script that will blindly execute other scripts:

# Warning: extremely dangerous code

# Execute the body text of all nodes that start with `@script`.
def onLoadFile():
    for p in c.all_positions():
        h = p.h.lower()
        if g.match_word(h,0,"@script"):
            s = p.b
            if s and len(s) > 0:
                try: # SECURITY BREACH: s may be malicious!
                    exec(s + '\n')

Executing this kind of code is typically an intolerable security risk. Important: rexec provides no protection whatever. Leo is a repository of source code, so any text operation is potentially malicious. For example, consider the following script, which is valid in rexec mode:

badNode = c.p
for p in c.all_positions():
    << change `rexec` to `exec` in p's body >>
<< delete badNode >>
<< clear the undo stack >>

This script will introduce a security hole the .leo file without doing anything prohibited by rexec, and without leaving any traces of the perpetrating script behind. The damage will become permanent outside this script when the user saves the .leo file.

Documenting plugins

Documenting new plugins is important for users to be able understand and use the features they add. To that effect, there are a few documentation steps that should not be overlooked.

  • Document the plugin thoroughly in the plugin’s docstring. This allows the documentation to be accessed from the Plugins menu.

  • Document any new commands with a proper docstring. This allows the minibuffer command help-for-command to provide help for the command.

  • In leo/doc/sphinx-docs/sphinxDocs.leo, to the node @file leo.plugins.rst, add the following snippet (preferably in alphabetical order), with the name of the plugin modified to the name of your plugin (here ipython). This allows the API docs to be automatically updated:

    :mod:`ipython` Module
    .. automodule:: leo.plugins.ipython

c ivars & properties

For any commander c:

Property Value
c.p the presently selected position
Ivar Value
c.frame the leoFrame representing the main window.
c.frame.body the leoBody representing the body pane.
c.frame.body.wrapper a leoQTextEditWidget.
c.frame.body.wrapper.widget a LeoQTextBrowser (a QTextBrowser)
c.frame.tree a leoQtTree, representing the tree pane
c.frame.tree.treeWidget a LeoQTreeWidget (a QTreeWidget)
c.user_dict a Python dictionary for use by scripts and plugins. Does not persist when Leo exists.

Handling events

Plugins and other scripts can register event handlers (also known as hooks):

leoPlugins.registerHandler("idle", on_idle)
leoPlugins.registerHandler(("start2","open2","command2"), create_open_with_menu)

As shown above, a plugin may register one or more event handlers with a single call to leoPlugins.registerHandler. Once a hook is registered, Leo will call the registered function’ at the named hook time. For example:

leoPlugins.registerHandler("idle", on_idle)

causes Leo to call on_idle at “idle” time.

Event handlers must have the following signature:

def myHook (tag, keywords):
  • tag is the name of the hook (a string).
  • keywords is a Python dictionary containing additional information. The following section describes the contents of the keywords dictionary in detail.

Important: hooks should get the proper commander this way:

c = keywords.get('c')

Summary of event handlers

The following table tells about each event handler: its name, when it is called, and the additional arguments passed to the hook in the keywords dictionary. For some kind of hooks, Leo will skip its own normal processing if the hook returns anything other than None. The table indicates such hooks with ‘yes’ in the ‘Stop?’ column.

Important: Ever since Leo 4.2, the v, old_v and new_v keys in the keyword dictionary contain positions, not vnodes. These keys are deprecated. The new_c key is also deprecated. Plugins should use the c key instead.

Event name (tag argument) Stop? When called Keys in keywords dict
‘after-auto’   after each @auto file loaded c,p (note 13)
‘after-create-leo-frame’   after creating any frame c
‘after-redraw-outline’   end of tree.redraw c (note 6)
‘before-create-leo-frame’   before frame.finishCreate c
‘bodyclick1’ yes before normal click in body c,p,v,event
‘bodyclick2’   after normal click in body c,p,v,event
‘bodydclick1’ yes before double click in body c,p,v,event
‘bodydclick2’   after double click in body c,p,v,event
‘bodykey1’ yes before body keystrokes c,p,v,ch,oldSel,undoType
‘bodykey2’   after body keystrokes c,p,v,ch,oldSel,undoType
‘bodyrclick1’ yes before right click in body c,p,v,event
‘bodyrclick2’   after right click in body c,p,v,event
‘boxclick1’ yes before click in +- box c,p,v,event
‘boxclick2’   after click in +- box c,p,v,event
‘clear-all-marks’   after clear-all-marks command c,p,v
‘clear-mark’   when mark is set c,p,v
‘close-frame’   in app.closeLeoWindow c
‘color-optional-markup’ yes * (note 7) colorer,p,v,s,i,j,colortag (note 7)
‘command1’ yes before each command c,p,v,label (note 2)
‘command2’   after each command c,p,v,label (note 2)
‘create-optional-menus’   (note 8) c (note 8)
‘create-popup-menu-items’   in tree.OnPopup c,p,v,event (new)
‘draw-outline-box’ yes when drawing +- box tree,p,v,x,y
‘draw-outline-icon’ yes when drawing icon tree,p,v,x,y
‘draw-outline-node’ yes when drawing node tree,p,v,x,y
‘draw-outline-text-box’ yes when drawing headline tree,p,v,x,y
‘drag1’ yes before start of drag c,p,v,event
‘drag2’   after start of drag c,p,v,event
‘dragging1’ yes before continuing to drag c,p,v,event
‘dragging2’   after continuing to drag c,p,v,event
‘enable-popup-menu-items’   in tree.OnPopup c,p,v,event
‘end1’   start of app.quit() None
‘enddrag1’ yes before end of drag c,p,v,event
‘enddrag2’   after end of drag c,p,v,event
‘headclick1’ yes before normal click in headline c,p,v,event
‘headclick2’   after normal click in headline c,p,v,event
‘headrclick1’ yes before right click in headline c,p,v,event
‘headrclick2’   after right click in headline c,p,v,event
‘headkey1’ yes before headline keystrokes c,p,v,ch (note 12)
‘headkey2’   after headline keystrokes c,p,v,ch (note 12)
‘hoist-changed’   whenever the hoist stack changes c
‘hypercclick1’ yes before control click in hyperlink c,p,v,event
‘hypercclick2’   after control click in hyperlink c,p,v,event
‘hyperenter1’ yes before entering hyperlink c,p,v,event
‘hyperenter2’   after entering hyperlink c,p,v,event
‘hyperleave1’ yes before leaving hyperlink c,p,v,event
‘hyperleave2’   after leaving hyperlink c,p,v,event
‘iconclick1’ yes before single click in icon box c,p,v,event (note 15)
‘iconclick2’   after single click in icon box c,p,v,event (note 15)
‘iconrclick1’ yes before right click in icon box c,p,v,event (note 15)
‘iconrclick2’   after right click in icon box c,p,v,event (note 15)
‘icondclick1’ yes before double click in icon box c,p,v,event (note 15)
‘icondclick2’   after double click in icon box c,p,v,event (note 15)
‘idle’   periodically (at idle time) c
‘init-color-markup’   (note 7) colorer,p,v (note 7)
‘menu1’ yes before creating menus c,p,v (note 3)
‘menu2’ yes during creating menus c,p,v (note 3)
‘menu-update’ yes before updating menus c,p,v
‘new’   start of New command c,old_c,new_c (note 9)
‘open1’ yes before opening any file c,old_c,new_c,fileName (note 4)
‘open2’   after opening any file c,old_c,new_c,fileName (note 4)
‘openwith1’ yes before Open With command c,p,v,d (note 14)
‘openwith2’   after Open With command c,p,v,(note 14)
‘recentfiles1’ yes before Recent Files command c,p,v,fileName,closeFlag
‘recentfiles2’   after Recent Files command c,p,v,fileName,closeFlag
‘redraw-entire-outline’ yes start of tree.redraw c (note 6)
‘save1’ yes before any Save command c,p,v,fileName
‘save2’   after any Save command c,p,v,fileName
‘scan-directives’   in scanDirectives c,p,v,s,old_dict,dict,pluginsList (note 10)
‘select1’ yes before selecting a position c,new_p,old_p,new_v,old_v
‘select2’   after selecting a position c,new_p,old_p,new_v,old_v
‘select3’   after selecting a position c,new_p,old_p,new_v,old_v
‘set-mark’   when a mark is set c,p,v
‘show-popup-menu’   in tree.OnPopup c,p,v,event
‘start1’   after app.finishCreate() None
‘start2’   after opening first Leo window c,p,v,fileName
‘unselect1’ yes before unselecting a vnode c,new_p,old_p,new_v,old_v
‘unselect2’   after unselecting a vnode c,new_p,old_p,old_v,old_v
‘@url1’ yes before double-click @url node c,p,v,url (note 5)
‘@url2’   after double-click @url node c,p,v(note 5)


  1. ‘activate’ and ‘deactivate’ hooks have been removed because they do not work as expected.

  2. ‘commands’ hooks: The label entry in the keywords dict contains the ‘canonicalized’ form of the command, that is, the lowercase name of the command with all non-alphabetic characters removed. Commands hooks now set the label for undo and redo commands ‘undo’ and ‘redo’ rather than ‘cantundo’ and ‘cantredo’.

  3. ‘menu1’ hook: Setting g.app.realMenuNameDict in this hook is an easy way of translating menu names to other languages. Note: the ‘new’ names created this way affect only the actual spelling of the menu items, they do not affect how you specify shortcuts settings, nor do they affect the ‘official’ command names passed in g.app.commandName. For example:

    app().realMenuNameDict['Open...'] = 'Ouvre'.
  4. ‘open1’ and ‘open2’ hooks: These are called with a keywords dict containing the following entries:

    • c: The commander of the newly opened window.
    • old_c: The commander of the previously open window.
    • new_c: (deprecated: use ‘c’ instead) The commander of the newly opened window.
    • fileName: The name of the file being opened.

    You can use old_c.p and c.p to get the current position in the old and new windows. Leo calls the ‘open1’ and ‘open2’ hooks only if the file is not already open. Leo will also call the ‘open1’ and ‘open2’ hooks if: a) a file is opened using the Recent Files menu and b) the file is not already open.

  5. '@url1‘ and '@url2‘ hooks are only executed if the ‘icondclick1’ hook returns None.

  6. These hooks are useful for testing.

  7. These hooks allow plugins to parse and handle markup within doc parts, comments and Python ‘’’ strings. Note that these hooks are not called in Python ‘’’ strings. See the color_markup plugin for a complete example of how to use these hooks.

  8. Leo calls the ‘create-optional-menus’ hook when creating menus. This hook need only create new menus in the correct order, without worrying about the placement of the menus in the menu bar. See the plugins_menu and scripts_menu plugins for examples of how to use this hook.

  9. The New command calls ‘new’. The ‘new_c’ key is deprecated. Use the ‘c’ key instead.

  10. g.scanDirectives calls ‘scan-directives’ hook. g.scanDirectives returns a dictionary, say d. d.get(‘pluginsList’) is an a list of tuples (d,v,s,k) where:

    • d is the spelling of the @directive, without the leading @.
    • v is the vnode containing the directive, _not_ the original vnode.
    • s[k:] is a string containing whatever follows the @directive. k has already been moved past any whitespace that follows the @directive.

    See the add_directives plugins directive for a complete example of how to use the ‘scan-directives’ hook.

  11. g.app.closeLeoWindow calls the ‘close-frame’ hook just before removing the window from g.app.windowList. The hook code may remove the window from app.windowList to prevent g.app.closeLeoWindow from destroying the window.

  12. Leo calls the ‘headkey1’ and ‘headkey2’ when the headline might have changed.

  13. p is the new node (position) containing '@auto filename.ext’

  14. The d argument to the open-with event handlers is a python dictionary whose keys are all the tags specified by the user in the body of the @openwith node.

The following events can only be called be called by minibuffer commands:

Event name (tag argument) Stop? Keys in keywords dict
‘iconclick1’ yes c,p,v,event (note 15)
‘iconrclick1’ yes c,p,v,event (note 15)
‘iconrclick2’   c,p,v,event (note 15)
‘icondclick1’ yes c,p,v,event (note 15)
‘icondclick2’   c,p,v,event (note 15)
  1. The only way to trigger these event is with the following minibuffer commands:

    Ctrl+F3 double-click-icon-box

Support for unit testing

If a plugin has a function at the outer (module) level called unitTest, Leo’s unit tests will call that function.

So it would be good if writers of plugins would create such a unitTest function. To indicate a failure the unitTest can just throw an exception. Leo’s plugins test suite takes care of the rest.