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

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PYTHON-LY MUSICXML CHORD STAFF + BAR REFERENCE BUG FIX
========================================================

Date: 2026-02-18
Affected package: python-ly (frescobaldi/python-ly)
Related to: python-ly-musicxml-chord-fix.txt (bar duration fix)
Branch name: fix-musicxml-chord-staff-and-bar

PREREQUISITE
------------
This fix builds on top of the chord bar duration fix (see python-ly-musicxml-chord-fix.txt).
That fix added increase_bar_dura() calls to new_chordbase() and copy_prev_chord().
However, it introduced two new issues described below.

THE BUGS
--------
When exporting LilyPond files with chords in multi-staff parts (e.g. PianoStaff)
to MusicXML, MuseScore reports "corrupted" files with "Incomplete measure" errors.

Two bugs in ly/musicxml/ly2xml_mediator.py:

BUG 1: Missing <staff> element on chord notes
  new_chordnote() never calls set_staff() on chord notes. In a PianoStaff, each
  note needs a <staff> element (1 or 2) so MuseScore knows which staff the note
  belongs to. Regular notes get this via check_current_note() (line 505-506), but
  chord notes bypass that method.

  Symptom: MuseScore imports the file but reports corruption. Notes may appear on
  the wrong staff.

BUG 2: Chord notes added to wrong measure after bar boundary
  When increase_bar_dura() (added by the previous chord fix) triggers new_bar()
  at a bar boundary, self.bar advances to the NEXT measure. Any subsequent
  new_chordnote() calls (for additional notes in the same chord) use self.bar.add()
  which adds them to the NEW measure — with a <chord/> flag but no preceding
  non-chord note. This creates an orphaned chord note as the first element of a
  measure, which is invalid MusicXML.

  Symptom: MuseScore reports "Incomplete measure" errors. Measures have wrong
  note counts.

  Example: In 3/4 time, <c'' e''>2. spans the full measure. new_chordbase()
  processes c'', calls increase_bar_dura() which fills the bar and triggers
  new_bar(). Then new_chordnote() for e'' adds it to the NEW bar instead of
  the bar where c'' is.

ROOT CAUSE
----------
In ly/musicxml/ly2xml_mediator.py:

1. new_chordnote() (line ~569-585) never calls set_staff() — unlike
   check_current_note() which does it for regular notes at line 505-506.

2. new_chordbase() calls increase_bar_dura() which can trigger new_bar(),
   advancing self.bar. Then new_chordnote() uses self.bar.add() which
   references the wrong (new) bar.

THE FIX
-------

FIX 0: Initialize _chord_bar in __init__()

Add after self.prev_tremolo = 8:
    self._chord_bar = None

FIX 1: new_chordbase() — save bar reference before increase_bar_dura()

Original (after previous chord-duration fix):
    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)

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._chord_bar = self.bar          # Save bar ref before possible new_bar()
        self.increase_bar_dura(duration)

FIX 2: new_chordnote() — set staff + use saved bar reference

Original:
    def new_chordnote(self, note, rel):
        chord_note = self.create_barnote_from_note(note)
        ...
        chord_note.chord = True
        self.bar.add(chord_note)
        return chord_note

Fixed:
    def new_chordnote(self, note, rel):
        chord_note = self.create_barnote_from_note(note)
        ...
        chord_note.chord = True
        if self.staff:
            chord_note.set_staff(self.staff)
        self._chord_bar.add(chord_note)     # Use saved bar ref, not self.bar
        return chord_note

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

TESTING
-------
Test script: tests/chord-staff-bug/test_chords.py
Test LilyPond: tests/chord-staff-bug/test-chords.ly

Tests verify:
- All chord notes have <staff> elements (BUG 1 fix)
- No orphaned chord notes at measure start (BUG 2 fix)
- Correct measure count preserved

Additional verification:
- Full Cello+Piano composition (Für Julia, 37 bars with many chords) exports
  to MusicXML without MuseScore corruption warnings
- MuseScore CLI successfully converts the MusicXML to MP3

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 (or use existing fork)
2. Clone fork locally
3. Create branch: git checkout -b fix-musicxml-chord-staff-and-bar
4. Apply all 3 changes to ly/musicxml/ly2xml_mediator.py:
   - Add self._chord_bar = None in __init__()
   - Add self._chord_bar = self.bar in new_chordbase() before increase_bar_dura()
   - In new_chordnote(): add staff assignment + change self.bar.add() to self._chord_bar.add()
5. Commit with descriptive message
6. Push and create PR

Note: This fix depends on the previous chord bar duration fix (increase_bar_dura
calls in new_chordbase and copy_prev_chord). Both fixes should be in the same PR
or the bar duration fix should be merged first.