MidiGen

A Python library for creating and manipulating MIDI files with a music-theory-aware API. Build anything from simple melodies to multi-track compositions with chord progressions, scales, and rhythmic patterns.

Built on mido with a custom Roman numeral analysis engine for chord progression parsing. No heavy dependencies.

The package is published on PyPI as midigen-lib but you import it as midigen.

Installation

Requires Python 3.10+:

pip install midigen-lib

For development:

git clone https://github.com/cainky/midigen.git
cd midigen
uv sync

Quick Start

from midigen import Song, Section, Key, MidiCompiler

song = Song(key=Key("C", "major"), tempo=120)
song.add_section(Section("Verse", 8, "I-V-vi-IV"))
song.add_section(Section("Chorus", 4, "IV-I-V-vi"))
song.add_instrument("Acoustic Grand Piano")

MidiCompiler(song).compile().save("my_song.mid")

Song holds your musical intent. MidiCompiler handles MIDI protocol details (channels, tracks, timing).

Features

• Song structure - Compose with Song and Section objects using familiar concepts like verses and choruses
• Chord progressions - Roman numeral notation (e.g., "I-IV-V-I") with automatic voice leading
• 14 scale types - Major, minor (natural/harmonic/melodic), pentatonic, blues, whole tone, chromatic, and all seven modes
• 22 chord types - Triads, suspended, seventh, ninth, eleventh, thirteenth, augmented, and diminished voicings
• Melody generation - From note names, scale degrees, or random walk algorithm
• Arpeggio patterns - Ascending, descending, and alternating
• Drum programming - DrumKit class with standard General MIDI drum names
• Multi-track support - Layer instruments across separate MIDI tracks with General MIDI instrument names
• Time utilities - Convert between musical time (measures, beats) and MIDI ticks

Examples

Melodies

from midigen import Melody, Scale

# From note names
melody = Melody.from_note_names("C4 E4 G4 E4 C4", durations=480)

# From scale degrees with rhythm
scale = Scale.major(60)  # C major
melody = Melody.from_degrees(
    scale,
    degrees=[1, 3, 5, 8, 5, 3, 1],
    rhythms="quarter quarter quarter half quarter quarter half"
)

# Random walk (reproducible with seed)
melody = Melody.random_walk(
    start_pitch=60, length=16, scale=scale, max_interval=3, seed=42
)

# Transform
transposed = melody.transpose(5)
retrograde = melody.reverse()

Scales and Modes

from midigen import Scale

# Basic
c_major = Scale.major(60)
a_minor = Scale.minor(57)
a_harmonic_minor = Scale.harmonic_minor(57)

# Pentatonic and blues
c_pent = Scale.major_pentatonic(60)
c_blues = Scale.blues(60)

# Modes
d_dorian = Scale.dorian(62)
e_phrygian = Scale.phrygian(64)
f_lydian = Scale.lydian(65)
g_mixolydian = Scale.mixolydian(67)
b_locrian = Scale.locrian(71)

# Other
c_whole_tone = Scale.whole_tone(60)
c_chromatic = Scale.chromatic(60)

Multi-Track Compositions

from midigen import Song, Section, Key, MidiCompiler

song = Song(key=Key("Am", "minor"), tempo=90)

song.add_section(Section(name="Intro", length=4, chord_progression="i-VI-III-VII"))
song.add_section(Section(name="Verse", length=8, chord_progression="i-VI-III-VII-i-VI-iv-V"))

song.add_instrument("Synth Bass 1")
song.add_instrument("String Ensemble 1")
song.add_instrument("Lead 1 (square)")

compiler = MidiCompiler(song)
compiler.compile_instrument("Synth Bass 1", octave=3)
compiler.compile_instrument("String Ensemble 1", octave=4)
compiler.compile_instrument("Lead 1 (square)", octave=5)
compiler.save("multi_track.mid", output_dir="./output")

Drum Patterns

from midigen import MidiGen, DrumKit, Key

midi = MidiGen(key=Key("C"))
drum_kit = DrumKit()

# Simple 4/4 rock beat
for i in range(4):
    t = i * 480
    drum_kit.add_drum("Bass Drum 1", time=t)
    drum_kit.add_drum("Acoustic Snare", time=t + 240)
    drum_kit.add_drum("Closed Hi Hat", time=t)
    drum_kit.add_drum("Closed Hi Hat", time=t + 120)
    drum_kit.add_drum("Closed Hi Hat", time=t + 240)
    drum_kit.add_drum("Closed Hi Hat", time=t + 360)

track = midi.get_active_track()
track.add_drum_kit(drum_kit)
midi.save("drum_beat.mid")

Arpeggios

from midigen import MidiGen, Note, Arpeggio, ArpeggioPattern, Key, KEY_MAP

midi = MidiGen(tempo=140, key=Key("C"))
track = midi.get_active_track()

notes = [
    Note(pitch=KEY_MAP["C4"], velocity=70, duration=120, time=0),
    Note(pitch=KEY_MAP["E4"], velocity=70, duration=120, time=0),
    Note(pitch=KEY_MAP["G4"], velocity=70, duration=120, time=0),
    Note(pitch=KEY_MAP["B4"], velocity=70, duration=120, time=0),
]

arpeggio = Arpeggio(
    notes=notes,
    pattern=ArpeggioPattern.ASCENDING,  # or DESCENDING, ALTERNATING
    delay=120,
    loops=4,
)

track.add_arpeggio(arpeggio)
midi.save("arpeggio.mid")

Time Conversions

from midigen import TimeConverter

tc = TimeConverter(ticks_per_quarter_note=480)

one_measure = tc.measures_to_ticks(1)         # 1920 ticks in 4/4
half_note = tc.note_duration("half")           # 960 ticks
dotted_quarter = tc.note_duration("dotted_quarter")  # 720
triplet_eighth = tc.note_duration("triplet_eighth")  # 160

# Different time signatures
waltz_measure = tc.measures_to_ticks(
    1, time_signature_numerator=3, time_signature_denominator=4
)

Low-Level API

For full control, bypass Song/MidiCompiler and work directly with tracks and notes:

from midigen import MidiGen, Note, Chord, Key, KEY_MAP

midi = MidiGen(tempo=120, time_signature=(4, 4), key_signature=Key("C"))
track = midi.get_active_track()

track.add_note(Note(pitch=KEY_MAP["C4"], velocity=64, duration=480, time=0))

chord = Chord([
    Note(pitch=KEY_MAP["E4"], velocity=64, duration=480, time=480),
    Note(pitch=KEY_MAP["G4"], velocity=64, duration=480, time=480),
])
track.add_chord(chord)

midi.save("low_level.mid")

Architecture

The library is organized into four layers:

• midigen.theory — Pure music theory (Note, Key, Scale, roman numeral parsing, time conversion)
• midigen.composition — Musical structures (Chord, ChordProgression, Arpeggio, Melody, DrumKit, Section)
• midigen.protocol — MIDI protocol (Track, ChannelPool, instruments)
• midigen.api — High-level API (Song, MidiGen, MidiCompiler)

All public names are available via from midigen import ....

Contributing

Contributions are welcome. See CONTRIBUTING.md for details.

License

GPL-3.0. See LICENSE.