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`music21.midi.MidiEvent.getBytes()` confuses `SEQUENCER_SPECIFIC_META_EVENT` (0x7F) with `POLY_MODE_ON` (also 0x7F)

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新手友好度
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Issue 类型
缺陷
描述清晰度
描述清楚
活跃度
停滞
技术栈
python
领域
audio-video-rtc

调研方向

The bug is in getBytes() in music21/midi/init.py, which dispatches on the raw enum value so SEQUENCER_SPECIFIC_META_EVENT (0x7F) matches ChannelModeMessages.POLY_MODE_ON and crashes when channel is None. Reproduce with the snippet in the issue, then make the branch checks also verify the enum class of self.type. Done when the snippet no longer raises and a MidiFile read-then-write round trip succeeds.

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描述

music21 version
9.5.0

Operating System(s) checked
OS agnostic
(I am on Ubuntu 24.04, though, but that should be irrelevant.)

Problem summary
music21.midi.MidiEvent.getBytes() confuses SEQUENCER_SPECIFIC_META_EVENT (0x7F) with POLY_MODE_ON (also 0x7F) and will therefore cause a TypeError if the former does not care about channels (i.e self.channel == None).

Steps to reproduce

from music21 import midi

ev = midi.MidiEvent(type=midi.MetaEvents.SEQUENCER_SPECIFIC_META_EVENT, channel=None)
print(ev)
b = ev.getBytes()

FYI: Actually I encountered this issue not by instantiating a MidiEvent manually but by writing a MidiFile which comes with some SEQUENCER_SPECIFIC_META_EVENT's.

mf = midi.MidiFile()
mf.open(inputfile)
mf.read()
mf.close()

# write the contents back immediately without touching anything

mf.open(outputfile, 'wb')
mf.write()
mf.close()

Expected vs. actual behavior
EXPECTED BEHAVIOUR:
music21.midi.MidiEvent.getBytes() should process all MIDI events correctly without triggering any exception.

ACTUAL BEHAVIOUR:

File "/foo/bar/lib/python3.12/site-packages/music21/midi/__init__.py", line 982, in getBytes
channelMode = bytes([0xB0 + self.channel - 1])
                        ~~~~~^~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'

More information
The current code of music21.midi.MidiEvent.getBytes() checks the event type essentially by looking at its value only:

def getBytes(self):
    if self.type in ChannelVoiceMessages:
        ...
    elif self.type in ChannelModeMessages:
        channelMode = bytes([0xB0 + self.channel - 1])
        ...
    elif self.type in SysExEvents:
        ...
    elif self.type in MetaEvents:
        ...
    else:
        ...

However:

class ChannelModeMessages(_ContainsEnum):
    ...
    POLY_MODE_ON = 0x7F
    ...

class MetaEvents(_ContainsEnum):
    ...
    SEQUENCER_SPECIFIC_META_EVENT = 0x7F
    ...

Apparently, getBytes() will mistake a SEQUENCER_SPECIFIC_META_EVENT as a POLY_MODE_ON, and will then fail immediately if self.channel is null.

Therefore, it is probably more appropriate to also check the event class, in addition to the event type value:

if isinstance(self.type, ChannelVoiceMessages) and self.type in ChannelVoiceMessages:
    ...
elif isinstance(self.type, ChannelModeMessages) and self.type in ChannelModeMessages:
    ...
elif isinstance(self.type, SysExEvents) and self.type in SysExEvents:
    ...
elif isinstance(self.type, MetaEvents) and self.type in MetaEvents:
    ...
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