python-ly/python-ly-musicxml-chord-fix.txt at master · hexbold/python-ly · GitHub

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PYTHON-LY MUSICXML CHORD BUG FIX
=================================

Date: 2026-01-17 (updated 2026-01-26)
Affected package: python-ly (frescobaldi/python-ly)
GitHub issue: https://github.com/frescobaldi/python-ly/issues/171
Branch name: fix-musicxml-chord-bar-duration

THE BUG
-------
When exporting LilyPond files to MusicXML, chord notation causes measure
boundaries to be lost. A 3-bar piece becomes 2 bars.

Two variants affected:

1. Explicit chords:
   c''1 | <d'' f'' a''>1 | e''1    <- Should be 3 measures, was 2

2. Duration shorthand after chords (from issue #171):
   \time 3/4
   <e f>4 4 4 | 4 4 8 r | r4 a g   <- Should be 3 measures, was 2

ROOT CAUSE
----------
In ly/musicxml/ly2xml_mediator.py, two functions set chord durations but
never increment the bar duration counter:

1. new_chordbase() - handles explicit chord notation <c e g>
2. copy_prev_chord() - handles duration shorthand after chords (4 4)

Bar boundaries are calculated by accumulating note durations. When the
total reaches the time signature value (e.g., 4 beats in 4/4), a new
measure is created. Chords were not adding their duration to this counter.

THE FIX
-------

FIX 1: new_chordbase() - line 562-567

Original:
    def new_chordbase(self, note, duration, rel=False):
        self.current_note = self.create_barnote_from_note(note)
        self.current_note.set_duration(duration)
        self.current_lynote = note
        self.check_current_note(rel)

Fixed:
    def new_chordbase(self, note, duration, rel=False):
        self.current_note = self.create_barnote_from_note(note)
        self.current_note.set_duration(duration)
        self.current_lynote = note
        self.check_current_note(rel)
        self.increase_bar_dura(duration)  # FIX: count chord duration

FIX 2: copy_prev_chord() - line 587-604

Original:
    def copy_prev_chord(self, duration):
        ...
        self.tied = False

Fixed:
    def copy_prev_chord(self, duration):
        ...
        self.tied = False
        self.increase_bar_dura(duration)  # FIX: count repeated chord duration

LOCAL INSTALLATION
------------------
The fix is applied locally in this project:
  /Users/lukasbold/Documents/Programming/Arpegiora/ly/

When running python3 -m ly from this directory, Python uses the local
fixed copy instead of the system package.

To use from other directories:
  PYTHONPATH="/Users/lukasbold/Documents/Programming/Arpegiora:$PYTHONPATH" python3 -m ly musicxml file.ly

TESTING
-------
Test files:
- test-issue-171.ly   (shorthand notation)
- test-issue-171b.ly  (explicit chords)
- test-original.ly    (original example)

All tests: 3 measures expected, 3 measures output

Chord types tested:
- 2-note chords: FIXED
- 3-note chords: FIXED
- Chords with tremolo: FIXED
- Duration shorthand after chord: FIXED

SUBMITTING UPSTREAM
-------------------
To fix this for everyone, submit a PR to:
https://github.com/frescobaldi/python-ly

Steps:
1. Fork https://github.com/frescobaldi/python-ly
2. Clone fork locally
3. Create branch: git checkout -b fix-musicxml-chord-bar-duration
4. Apply both fixes to ly/musicxml/ly2xml_mediator.py
5. Commit with message referencing "Fixes #171"
6. Push and create PR