`TypeError` in `ChordReducer.run()` on multi-part `stream.Score` without instrument `partId`s

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崩溃位于 music21/tree/timespanTree.py（约第 743 行）的 TimespanTree.allParts()，这里按 getInstrument().partId 对声部排序，而该值可能为 None。请从 music21/analysis/reduceChords.py 中的 ChordReducer.run() 开始，运行 issue 中的复现脚本。完成的标准是脚本运行时不再出现 TypeError，且声部保持合理的顺序，例如它们在乐谱中出现的顺序。issue 提出了两种排序方案，因此需要与维护者确认使用哪一种。

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描述

music21 version

11.0.0b11

Problem summary

ChordReducer.run() was found to raise a TypeError when given a multi-part stream.Score whose instruments were not assigned partIds.

Steps to reproduce

from music21 import chord, instrument, note, stream
from music21.analysis.reduceChords import ChordReducer

upperInstrument = instrument.Instrument()  
# upperInstrument.partId = 'P1'
lowerInstrument = instrument.Instrument()    
# lowerInstrument.partId = 'P2'

upper = stream.Part([upperInstrument, stream.Measure([chord.Chord('C4 E4', type='whole')])])                                                                                               
lower = stream.Part([lowerInstrument, stream.Measure([note.Note('G4', type='whole')])])                                                                                                    

score = stream.Score([upper, lower])                                                                                                                            
ChordReducer().run(score)

TypeError: '<' not supported between instances of 'NoneType' and 'NoneType'

This error was discovered when calling ChordReducer.run() on scores loaded from MIDI.

Expected vs. actual behavior
Expected ChordReducer.run(score) to complete without error; instead, raises TypeError at music21/tree/timespanTree.py, line 743, in TimespanTree.allParts().

More information
ChordReducer.run() calls TimespanTree.allParts() internally, which returns a list of the tree's parts. This list is ordered by the partId of each part's first instrument, using the following sort:

parts = sorted(parts, key=lambda x: x.getInstrument().partId)

x.getInstrument() returns the first available instrument in the part, or a new default instrument. In either case, the partId could be None.

One possible fix for the NoneType comparison: having allParts() utilize the input source's ordering to sort the parts. The ordering is available for at least this use case, since ChordReducer.run() already validates that its input is a stream.Score, and the TimespanTree that it uses already keeps track of the source in its source attribute. For just the stream.Score case, allParts() could then return the parts in order of their appearance in the score. Or, more generally, for a solution that works with other stream types that TimespanTree can process, allParts() could always return the list of parts ordered using the source's ordering.

Applying chordify() to the input score beforehand (stream.Score([score.chordify()])), avoids the TypeError being raised, although the chordified score contains just one part.

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