Help function for lilypond. get_interval will return an interval from a given note.

intervals.py

#! python3

CHR_SCALE_S = ('a', 'ais', 'b', 'c', 'cis', 'd', 'dis', 'e', 'f', 'fis', 'g', 'gis')
CHR_SCALE_F = ('a', 'bes', 'b', 'c', 'des', 'd', 'ees', 'e', 'f', 'ges', 'g', 'aes')
CHR_SCALES = (CHR_SCALE_S, CHR_SCALE_F)
SCALE = ('a', 'b', 'c', 'd', 'e', 'f', 'g')
INTERVAL = {'m2':1, 'M2':2, 'm3':3, 'M3':4, 'P4':5, '4+':6, '5-':6, 'P5': 7, 'm6':8, 'M6':9, 'm7':10, 'M7':11}

def get_interval(note, interval, up=True):
    '''get a note at interval from given note'''
    #get correct letter name, then modify sharp or flat
    half_steps = INTERVAL[interval]
    for scale in CHR_SCALES:
        if note in scale:
            if up:
                return scale[(scale.index(note) + half_steps) % len(scale)]
            else:
                return scale[(scale.index(note) - half_steps) % len(scale)]

if __name__ == "__main__":
    assert get_interval('aes', 'm2', up=True) == 'a'
    assert get_interval('a', 'M6', up=False) == 'c'
    assert get_interval('g', '4+', up=False) == 'cis'
    assert get_interval('fis', 'm3') == 'a'
    assert get_interval('f', 'm3') == 'aes'