(AI) Preserve microtones through Interval.transpose (fix accidental corruption / AccidentalException)

[gaoflow]

The bug

Transposing a Pitch (or Note) that carries a microtone corrupts its spelling, and for larger microtones raises an exception — even when the transposition is a no-op:

from music21 import pitch, interval

p = pitch.Pitch('C4'); p.microtone = 30
p.transpose(interval.Interval('P1'))     # C~4(-20c)   -- expected C4(+30c)

p = pitch.Pitch('G#5'); p.microtone = 50
p.transpose(interval.Interval('A1'))     # AccidentalException: 2.5 is not a supported accidental type

Transposing by a perfect unison must be the identity, so C4(+30c) → P1 → C4(+30c) is unambiguous. The corruption kicks in above 25¢ (25¢ survives, 26¢ flips to a half-sharp) and affects ordinary intervals too — Note('C4', microtone=30).transpose('P5') gives G~4(-20c) instead of G4(+30c). It's also internally inconsistent: a 25¢ microtone is preserved by P5, and a 30¢ microtone is preserved by intervals that resolve to a sharp/flat — so identical inputs behave differently depending on the interval.

Root cause

music21/interval.py, Interval._diatonicTransposePitch:

pitch2.microtone = None
interval2 = Interval(pitch1, pitch2)                       # pitch1 still carries its microtone
halfStepsToFix = self.chromatic.semitones - interval2.chromatic.semitones
...
pitch2.accidental = halfStepsToFix                         # fractional alter -> corrupt accidental

interval2 is measured from the microtonal pitch1, so the source cents (0.30 semitone) leak into halfStepsToFix. That fractional value is then assigned to the accidental, which snaps to the nearest quarter-tone (half-sharp = 0.5) and dumps the −0.20 remainder into a spurious microtone — or, when the alter lands on an unsupported value like 2.5, raises AccidentalException.

The method even contains a dead comment, # centsOrigin = pitch1.microtone.cents # unused!!, marking the intended-but-never-wired-up fix.

The fix

Resurrect centsOrigin, peel it off halfStepsToFix before the accidental is set, and restore it as a microtone afterward. The whole thing is guarded by if centsOrigin: so the common, non-microtonal (performance-critical) path is byte-identical:

centsOrigin = pitch1.microtone.cents
...
halfStepsToFix = self.chromatic.semitones - interval2.chromatic.semitones
if centsOrigin:
    halfStepsToFix = round(halfStepsToFix - centsOrigin / 100.0, 9)
...
if centsOrigin:
    pitch2.microtone = pitch2.microtone.cents + centsOrigin

Now C4(+30c) → P1 → C4(+30c), G#5(+50c) → A1 → G##5(+50c) (no crash), and a combined quarter-tone-plus-microtone pitch like C~4(+30c) round-trips through a unison unchanged.

Tests

testTransposeMicrotonePreserved covers the unison identity, the formerly-crashing A1 case, a combined accidental+microtone pitch, and a sweep over eight intervals × four microtone magnitudes asserting the cents are preserved, the sounding pitch (ps) moves by exactly the interval, and no quarter-tone accidental is introduced on an integer-semitone interval. It fails on master and passes with the fix. The existing interval, pitch, and note test/doctest suites stay green; ruff, pylint -j0, and mypy are clean.

---

Disclosure (per CONTRIBUTING.md): prepared with AI assistance under my direction. I reproduced the bug, wrote and reviewed the fix and tests, and verified it locally — full interval/pitch/note suites, ruff/pylint/mypy, and an ~1,200-case oracle (intervals × microtones × accidentals) with zero microtone/ps failures and zero crashes.

[mscuthbert]

Great PR -- a few smaller changes requested, with guidance to you and agent for future PRs.

Btw -- version 11 or a future version might deprecate the ~ and ` representations of half-flats and sharps, so if you want to make future microtonal contributions please use Accidental('half-flat') instead of ``` etc. Thanks!

[mscuthbert]

this should gate on if not pitch1.isTwelveTone(): ... else: centsOrigin = 0.0

[mscuthbert]

I have a patch that should make isTwelveTone run faster -- will push in next PR. isTwelveTone guards against creating Microtone objects unnecessarily.

[mscuthbert]

See #1963

[mscuthbert]

pitch1 isn't necessarily microtonal. This comments should be trimmed to essentials and moved into the if centsOrigin: clause -- in general agents write too much about their particular task and leave too much baggage for people interested in other tasks. Think about 10-15 words

[mscuthbert]

this does not restore the source microtone, it restores the microtone's .cents. And again comment inside the if.

[mscuthbert]

pitch2.microtone.cents = ...

[mscuthbert]

good test