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README for developers
The Frescobaldi LilyPond sheet music editor is written in Python and uses
Qt for its user interface, via the PyQt bindings (Python3 with Qt5 for the
current development from version 3 onwards, Python2/3 with Qt4 for the previous
stable release from version 2. Any future development will be done with
Python3 and Qt5.)
All code and application data is in the frescobaldi_app/ directory. The
frescobaldi script just imports frescobaldi_app.main. You can simply run
./frescobaldi from the command line after unpacking Frescobaldi.
You can also install the 'frescobaldi_app' package in the system- or user-wide
python directories and the frescobaldi script in your PATH. The distutils-based
installation procedure can do this.
'frescobaldi_app' is not a real package: on startup, the absolute
'frescobaldi_app' directory is added to sys.path and its own __path__ is cleared
so all modules and packages inside frescobaldi_app are available as toplevel
modules and packages.
Running Frescobaldi in an interactive shell
To test features or to experiment, you can run Frescobaldi in an interactive
Python shell. It is recommended to open a shell (e.g. by simply running python
without arguments, or by using Dreampie or IPython) and then enter:
from frescobaldi_app.debug import *
This ensures the application starts up and uses the correct SIP API (2) for
QString and QVariant. This will also install some handlers that print
debugging information on certain events. And it imports the most used modules.
Currently Python 3.2 and newer is supported.
How Frescobaldi is organized
There can be one or more mainwindow.MainWindow instances and one or more
document.Document instances (when the last Document is closed, another one is
always constructed). The app module keeps references to those and contains the
Signals emitted when something changes.
Many parts inside Frescobaldi need to store or cache additional information or
add features to Documents, MainWindows etc. Instead of clobbering those basic
classes with an ever-growing number of unrelated groups of methods, a different
approach is chosen: the plugin module.
This keeps all classes small and only have methods that directly apply to
themselves and not to other parts of Frescobaldi (separation of concerns).
So e.g. the resultfiles, highlighter or documentinfo modules contain classes
for objects that coexist with a Document, and providing their own relevant
methods, while keeping a weak reference to the Document.
Exchange of messages is done as much as possible using signals (PyQt4 signals
or from the signals module), or event filters, so adding new features changes as
less existing code as possible.
Some important modules:
main: Entry point
info: Information about the application, such as the version
toplevel: Adds the path of frescobaldi_app to sys.path, clears __path__
so all modules inside frescobaldi_app can be imported as
toplevel modules and packages
app: Central hub with global signals, also keeping references to
mainwindow and document instances
mainwindow: MainWindow (QMainWindow)
document: Document (QTextDocument)
view: View (QPlainTextEdit)
menu: Here the menubar is constructed (by importing all the relevant
modules and adding the actions they define)
plugin: A simple way to extend objects without them knowing it
metainfo: Stores (optionally) meta information about the document, such
as last cursor position, whether to enable auto indent, etc
panel: The base class of all dock widgets
panelmanager: Add new dock widget tools here
symbols: Provides icons of LilyPond-generated SVG files that draw
themselves in the default text color.
Some completely generic modules (don't have anything to do with Frescobaldi or
qpopplerview: PDF viewer widget using the popplerqt4 binding to Poppler-Qt4
signals: An alternative to Qt signals that allows for connections to have
priorities, and objects don't have to be Qt objects
cachedproperty: Caches properties that can be asynchronously computed
slexer: A Stateful Lexer, used to build regular expression-based parsers
hyphenator: Hyphenate text using hyphenation dictionaries
node: A list-like type to build tree structures with
cursortools: Some useful functions manipulating QTextCursor instances
portmidi: Access the PortMidi library in different ways
midifile: Load and play MIDI files
Contributing, Coding Style
A clean pythonic coding style is preferred, with short methods and method names,
and small modules and classes that encapsulate functionality in a sensible way.
See also PEP8 (
Indent: 4 spaces indent with no tabs
Case: ClassName, module_name, methodname() (or methodName() for classes that
inherit from Qt classes). Instance attributes that are not meant to be
used from outside an instance are prefixed with an underscore, e.g.
There is a habit, inspired by Qt, to use setters and getters for such
attributes: obj.setDocument(document) and obj.document().
Names for actions (e.g. file_open) and QSettings keys should always be
lowercase and use underscores where needed.
Write code so that adding or extending functionality later only adds lines, not
changes or removes existing lines.
Module imports: one per line. First the Python standard library imports, then
PyQt4, then the generic Frescobaldi imports and then imports from the current
package. The different groups separated by a blank line:
import os # stdlib imports
import sys
from PyQt4.QtCore import ... # PyQt4 imports
import app # generic Frescobaldi imports
import icons
from . import widget # imports from current package
Avoid platform specific code, but when it is really needed, you can test
for Windows:
if sys.platform.startswith('win'):
if == "nt":
Testing for Mac OS X can be done using:
if sys.platform.startswith('darwin'):
Adding Functionality
Separate out core functionality from its GUI. Put core functionality in a basic
module. For the GUI preferably make a new package/, importing that
e.g. from while constructing the GUI. An example is the documentwatcher
base module, it automatically watches open documents on disk, simply by
listening to signals sent from app. It sends a documentChanged() signal when
something happens, nothing else. The externalchanges package that gets imported
from, starts up the documentwatcher if the user has it enabled.
So the documentwatcher watcher is a module just capable of performing a task,
without needing to have logic for accessing the preferences, constructing a
particular GUI, etc. It could, like app and many other modules, be used almost
unchanged in any other application.
New features that add menu actions can be imported in, while building
the menu. Features that should run in a global background thread can have their
toplevel file (<feature>/ imported from or,
loading as few as possible on first import.
Preparing patches or Pull Requests
When preparing a patch or pull request, please add new functionality first.
Old functionality that is superseded by the newer functions can be removed later
when the new functionality has been tested thoroughly. Change existing files
as less as possible. Preferably add new files and packages.
User Interface strings that are meant to be translated should be wrapped in
_("Message Text") constructs. See also README-translations for vital information
about how to use the translation mechanism and how to use variables inside
Objects that are longer-lived should be able to retranslate their GUIs. This can
be done by adding a method named translateUI(self), in which you set the texts
to the user interface objects, and calling app.translateUI() at the end of the
constructor. Frescobaldi will call your translateUI() method once and also take
care of calling it again when the user changes the UI language in the
preferences. You can also do this directly by connecting the method that sets
the texts to the app.languageChanged signal.
Guidelines for writing GUI texts
* Write short, directly and non-technical
* For help pages: each paragraph is one translatable string
* Avoid HTML in texts (but <code>\include</code> is OK)
* In help pages you can construct HTML using some helper functions
* Avoid hard line breaks (<br>) in any texts: use short paragraphs instead
* Use named format fields: not "page {0} of {1}" but "page {num} of {total}"
* Don't change existing strings just for cosmetics, it breaks translations
Testing and developing under Windows
Linux is the recommended developing platform, but it might be necessary to test
specific functionality or behaviour under Windows. It can be difficult to get
all the dependencies right, especially the right combination of poppler, pyqt4
and sip.
There is an easy way to get setup under Windows and be able to do quite a lot
of testing and development, you don't even need to install Python at all.
1. install a recent version of Frescobaldi into C:/Program Files/Frescobaldi,
using the Frescobaldi.Setup.exe installer
2. install Git Bash for Windows from
3. open up the Git Bash command prompt (now you're in a powerful UNIX shell!)
4. make a directory to do Frescobaldi development in:
mkdir dev
cd dev
5. download the frescobaldi source code from GitHub:
git clone
6. go to the frescobaldi directory and copy all the files from the installed
version, except for the frescobaldi_app directory:
cd frescobaldi
cp /c/Program\ Files/Frescobaldi/*.pyd .
cp /c/Program\ Files/Frescobaldi/frescobaldi.exe .
cp /c/Program\ Files/Frescobaldi/*.dll .
cp /c/Program\ Files/Frescobaldi/ .
cp -r /c/Program\ Files/Frescobaldi/iconengines .
cp -r /c/Program\ Files/Frescobaldi/imageformats .
7. Now you can modify files in frescobaldi_app and run and test Frescobaldi:
8. Make commits of your changes and either push them to a fork of yours of
frescobaldi on GitHub or send them via e-mail.