backend/api/midi_utils.py

import mido
from io import BytesIO

def process_midi(midi_bytes: bytes) -> BytesIO:
    """
    Processes a MIDI file by transposing all notes up by one semitone.
    
    Args:
        midi_bytes: The raw bytes of the input MIDI file.
        
    Returns:
        A BytesIO buffer containing the processed MIDI data.
    """
    # Create an in-memory file-like object from the input bytes
    with BytesIO(midi_bytes) as input_buffer:
        mid = mido.MidiFile(file=input_buffer)
        
        # Iterate over all tracks in the MIDI file
        for i, track in enumerate(mid.tracks):
            # Iterate over all messages in the track
            for msg in track:
                # Check if the message is a note_on or note_off event
                if msg.type in ['note_on', 'note_off']:
                    # Transpose the note by +1 semitone
                    # We add a check to ensure the note doesn't go above 127
                    if msg.note < 127:
                        msg.note += 1
        
        # Save the processed MIDI data to an in-memory output buffer
        output_buffer = BytesIO()
        mid.save(file=output_buffer)
        # Reset the buffer's position to the beginning
        output_buffer.seek(0)
        
        return output_buffer