`converter` parsed midi data stream `getElementsByClass` throws `ValueError`

[khteh]

    midi_data = converter.parse(data_fn)
    # Get melody part, compress into single voice.
    melody_stream = midi_data[5]     # For Metheny piece, Melody is Part #5.
    melody1, melody2 = melody_stream.getElementsByClass(stream.Voice)

Error:

---> 29 melody1, melody2 = melody_stream.getElementsByClass(stream.Voice)
     30 for j in melody2:
     31     melody1.insert(j.offset, j)

ValueError: not enough values to unpack (expected 2, got 0)

[mscuthbert]

Please read through the user's guide more on expected hierarchies. Part-Measure-Voice. You're skipping measures and you're getting parts in a very unsafe way.

[mscuthbert]

Closing -- continue discussion on the same place in StackOverflow.

[khteh]

Spent a couple of hours and this is what I have done:

    midi_data = converter.parse(data_fn)
    assert len(midi_data.getElementsByClass(stream.Part)) == len(midi_data.parts)
    measure_part5_len = len(midi_data.getElementsByClass(stream.Part)[5].getElementsByClass(stream.Measure))
    measure_part5 = midi_data.parts[5].measures(1, measure_part5_len)
    measure_stream = midi_data.parts[5].getElementsByClass(stream.Measure)
    print(f"\nMeasure (Part[5]): {len(measure_part5)} {measure_part5}")
    #melody_stream = midi_data[5]     # For Metheny piece, Melody is Part #5.
    print(f"\nMelody Voice: {len(measure_part5.recurse().getElementsByClass(stream.Voice))}")
    melody_voice = []
    for el in measure_part5.recurse().getElementsByClass(stream.Voice):
        #print(el.offset, el, el.activeSite)
        print(f"el offset: {el.offset}, {el}")
        melody_voice.insert(int(el.offset), el)
    for i in melody_voice:
        if i.quarterLength == 0.0:
            i.quarterLength = 0.25
    # Change key signature to adhere to comp_stream (1 sharp, mode = major).
    # Also add Electric Guitar. 
    melody_voice.insert(0, instrument.ElectricGuitar())
    melody_voice.insert(0, key.KeySignature(sharps=1))

    # The accompaniment parts. Take only the best subset of parts from
    # the original data. Maybe add more parts, hand-add valid instruments.
    # Should add least add a string part (for sparse solos).
    # Verified are good parts: 0, 1, 6, 7 '''
    partIndices = [0, 1, 6, 7]
    comp_stream = stream.Voice()
    #comp_stream = measure_stream.voices.stream()
    comp_stream.append([j for i, j in enumerate(midi_data.flatten()) 
        if i in partIndices and not isinstance(j, metadata.Metadata)])

    print(f"\ncomp_stream: {comp_stream}")
    comp_stream.show("text")

    # Full stream containing both the melody and the accompaniment. 
    # All parts are flattened. 
    full_stream = stream.Voice()
    print(f"\nfull_stream: {full_stream}")
    full_stream.show("text")

    #full_stream = measure_stream.voices.stream()
    for i in range(len(comp_stream)):
        full_stream.append(comp_stream[i])
    full_stream.append(melody_voice)

    # Extract solo stream, assuming you know the positions ..ByOffset(i, j).
    # Note that for different instruments (with stream.flat), you NEED to use
    # stream.Part(), not stream.Voice().
    # Accompanied solo is in range [478, 548)
    solo_stream = stream.Voice()
    #solo_stream = measure_stream.voices.stream()
    print(f"\ninstrument.Instrument:")
    for part in full_stream.getElementsByClass(instrument.Instrument):
        print(f"part: {part}")
        part.show("text")
        curr_part = stream.Part()
        curr_part.append(part)
        cp = curr_part.flatten()
        solo_stream.insert(cp)
    print(f"\ntempo.MetronomeMark:")
    for part in full_stream.getElementsByClass(tempo.MetronomeMark):
        print(f"part: {part}")
        part.show("text")
        curr_part = stream.Part()
        curr_part.append(part)
        cp = curr_part.flatten()
        solo_stream.insert(cp)
    print(f"\nkey.KeySignature:")
    for part in full_stream.getElementsByClass(key.KeySignature):
        print(f"part: {part}")
        part.show("text")
        curr_part = stream.Part()
        curr_part.append(part)
        cp = curr_part.flatten()
        solo_stream.insert(cp)
    print(f"\nmeter.TimeSignature:")
    for part in full_stream.getElementsByClass(meter.TimeSignature):
        print(f"part: {part}")
        part.show("text")
        curr_part = stream.Part()
        curr_part.append(part)
        cp = curr_part.flatten()
        solo_stream.insert(cp)
    print(f"\nmeter.getElementsByOffset:")
    for part in full_stream.getElementsByOffset(476, 548, includeEndBoundary=True):
        print(f"part: {part}")
        part.show("text")
        curr_part = stream.Part()
        curr_part.append(part)
        cp = curr_part.flatten()
        solo_stream.insert(cp)

Console output:

Measure (Part[5]): 217 <music21.stream.Part 0x79ec52af8050>

Melody Voice: 84

comp_stream: <music21.stream.Voice 0x7487a8b0c0a0>
{0.0} <music21.instrument.Piano 'Piano: Piano'>
{0.0} <music21.instrument.StringInstrument 'String: StringInstrument'>
{0.0} <music21.instrument.Piano 'Piano Elec.: Piano Elec.'>

full_stream: <music21.stream.Voice 0x7487af60b1b0>


instrument.Instrument:
part: Piano: Piano
<music21.instrument.Piano 'Piano: Piano'>
part: String: StringInstrument
<music21.instrument.StringInstrument 'String: StringInstrument'>
part: Piano Elec.: Piano Elec.
<music21.instrument.Piano 'Piano Elec.: Piano Elec.'>
part: Electric Guitar
<music21.instrument.ElectricGuitar 'Electric Guitar'>

tempo.MetronomeMark:

key.KeySignature:
part: <music21.key.KeySignature of 1 sharp>
<music21.key.KeySignature of 1 sharp>

meter.TimeSignature:

meter.getElementsByOffset:

solo_stream: <music21.stream.Voice 0x7487adc16a30>
{0.0} <music21.stream.Part 0x7487aec79e50>
    {0.0} <music21.instrument.Piano 'Piano: Piano'>
{0.0} <music21.stream.Part 0x7487acd75130>
    {0.0} <music21.instrument.StringInstrument 'String: StringInstrument'>
{0.0} <music21.stream.Part 0x7487acb4fd90>
    {0.0} <music21.instrument.Piano 'Piano Elec.: Piano Elec.'>
{0.0} <music21.stream.Part 0x7487a8af9310>
    {0.0} <music21.instrument.ElectricGuitar 'Electric Guitar'>
{0.0} <music21.stream.Part 0x7487a8af9630>
    {0.0} <music21.key.KeySignature of 1 sharp>

In the end, the following code only manage to gather 1 chord and 1 measure:

    # Group by measure so you can classify. 
    # Note that measure 0 is for the time signature, metronome, etc. which have
    # an offset of 0.0.
    melody_stream = solo_stream[-1]
    measures = OrderedDict()
    offsetTuples = [(int(n.offset / 4), n) for n in melody_stream]
    measureNum = 0 # for now, don't use real m. nums (119, 120)
    for key_x, group in groupby(offsetTuples, lambda x: x[0]):
        measures[measureNum] = [n[1] for n in group]
        measureNum += 1

    # Get the stream of chords.
    # offsetTuples_chords: group chords by measure number.
    chordStream = solo_stream[0]
    chordStream.removeByClass(note.Rest)
    chordStream.removeByClass(note.Note)
    offsetTuples_chords = [(int(n.offset / 4), n) for n in chordStream]

    # Generate the chord structure. Use just track 1 (piano) since it is
    # the only instrument that has chords. 
    # Group into 4s, just like before. 
    chords = OrderedDict()
    measureNum = 0
    for key_x, group in groupby(offsetTuples_chords, lambda x: x[0]):
        chords[measureNum] = [n[1] for n in group]
        measureNum += 1

    # Fix for the below problem.
    #   1) Find out why len(measures) != len(chords).
    #   ANSWER: resolves at end but melody ends 1/16 before last measure so doesn't
    #           actually show up, while the accompaniment's beat 1 right after does.
    #           Actually on second thought: melody/comp start on Ab, and resolve to
    #           the same key (Ab) so could actually just cut out last measure to loop.
    #           Decided: just cut out the last measure. 
    #del chords[len(chords) - 1]
    assert len(chords) == len(measures), f"{len(chords)} chords, {len(measures)} measures"
    print(f"{len(chords)} measures and chords")

This fails the application which expects 19 chords....So, obviously I still don't know what I am doing....:(

[mscuthbert]

Music21 makes no guarantee that the sixth element of a stream [5] will be of a particular class. Why are you assuming that this is true?

[khteh]

Legacy code