Given an absolute chord (such as 'Ab') and an absolute key (such as 'C') return the chord's scale degree ('bVI').

absolute_chord2rn.py

from ms3 import fifths2rn, name2fifths, rel2abs_key

def absolute_chord2rn(abs_chord, abs_key):
    """Given an absolute chord (such as 'Ab') and an absolute key (such as 'C') return the chord's scale degree ('bVI')."""
    is_minor_chord, is_minor_key = abs_chord.islower(), abs_key.islower()
    chord_fifths, key_fifths = name2fifths(abs_chord), name2fifths(abs_key)
    interval = chord_fifths - key_fifths
    result = fifths2rn(interval, is_minor_key)
    if is_minor_chord:
        result = result.lower()
    return result
  
tests = {
    ('G', 'G'): 'I',
    ('g', 'G'): 'i',
    ('G', 'g'): 'I',
    ('g', 'g'): 'i',
    ('Bb', 'G'): 'bIII',
    ('b', 'G'): 'iii',
    ('Bb', 'g'): 'III',
    ('b', 'g'): '#iii',
    ('d', 'G'): 'v',
    ('D', 'g'): 'V',
    ('f', 'A'): 'bvi',
    ('F#', 'A'): 'VI',
    ('F', 'a'): 'VI',
    ('f#', 'a'): '#vi',
}

for params, true_result in tests.items():
    result = absolute_chord2rn(*params)
    test_result = 'OK' if result == true_result else f"FAIL (expected '{true_result}')"
    print(f"{test_result} {params} => {result}")

### OUTPUT:
#
# OK ('G', 'G') => I
# OK ('g', 'G') => i
# OK ('G', 'g') => I
# OK ('g', 'g') => i
# OK ('Bb', 'G') => bIII
# OK ('b', 'G') => iii
# OK ('Bb', 'g') => III
# OK ('b', 'g') => #iii
# OK ('d', 'G') => v
# OK ('D', 'g') => V
# OK ('f', 'A') => bvi
# OK ('F#', 'A') => VI
# OK ('F', 'a') => VI
# OK ('f#', 'a') => #vi