mandolin_chords.py

"""Lists ways to play a certain chord on mandolin.

Example invocation:
$ python mandolin_chords.py Abm

Example output: 
The chord Abm has notes Ab, B, Eb

(4, 6, 6, 4) plays B, Ab, Eb, Ab
(8, 6, 6, 7) plays Eb, Ab, Eb, B
(1, 1, 2, 4) plays Ab, Eb, B, Ab
(4, 1, 2, 4) plays B, Eb, B, Ab
(4, 6, 6, 7) plays B, Ab, Eb, B

"""

import itertools
import sys

NOTES = ["A", "Bb", "B", "C", "Db", "D", "Eb", "E", "F", "Gb", "G", "Ab"]

def note_up(note, dist):
  """Returns the note $dist semitones above base note $note."""
  return NOTES[(NOTES.index(note) + dist) % len(NOTES)]

def notes_in_chord(chord):
  """Returns the three notes that make $chord. $chord examples: 'A', 'Cbm'."""
  targets = []
  if chord[-1] == "m":
    assert chord[:-1] in NOTES, "%s isn't a valid note." % chord[:-1]
    targets.append(chord[:-1])
    targets.append(note_up(targets[0], 3))
  else:
    assert chord in NOTES, "%s isn't a valid note." % chord
    targets.append(chord)
    targets.append(note_up(targets[0], 4))
  targets.append(note_up(targets[0], 7))
  return targets

def fingers_for_chord(chord):
  targets = set(notes_in_chord(chord))
  options = [] # List of 4 ints, representing which fret to play on each string.
  print "The chord %s has notes %s" % (chord, ", ".join(notes_in_chord(chord)))
  print
  # Loop over every possible combination of frets on each of the four strings
  for g, d, a, e in itertools.product(range(16), range(16), range(16), range(16)):
    positions = {"G": g, "D": d, "A": a, "E": e}
    notes = [note_up(note, i) for note, i in positions.items()]
    # If this combination of frets plays the chord, add it to the options list.
    if targets == set(notes):
      options.append((g, d, a, e))
  # Sort the list by the amount your fingers have to stretch.
  options.sort(key=lambda option: max(option) - min(option))
  for option in options[:5]:
    ma, mi = max(option), min(option)
    print option, "plays", ", ".join([note_up(n,i) for n,i in zip(["G", "D", "A", "E"], option)])

if __name__ == "__main__":
  fingers_for_chord(sys.argv[1])