Output chord progressions as Roman numerals for analysis

[BdR76]

First of all this is an excellent library. I'm using it at the moment to analyze pop song and their chord progression.

I have a question, is it possible to add support for Roman Numeral notations? I couldn't find it in documentation or code, but maybe I'm overlooking something.

So I would like to do something like this for example:

cp = pychord.ChordProgression(["Am", "Dm", "G", "C", "F", "Bdim", "E", "Am"])

cp = cp.as_roman_numeral(scale="Amin") # note: .as_roman_numeral() would be new function

>>> ["i", "iv", "VII", "III", "VI", "iidim", "V", "i"]

The way I do it now is using the code below, which I realise is not ideal and a bit hack-y

ROMAN_NUMERAL = {
   -1: "??",
    1: "I",
    2: "II",
    3: "III",
    4: "IV",
    5: "V",
    6: "VI",
    7: "VII",
    8: "VIII"
}

# create chord progression
cp = pychord.ChordProgression(["Am", "Dm", "G", "C", "F", "Bdim", "E", "Am"])
chords_roman = [];
scale = pychord.Chord("Amin")

# which scale
scale_root = scale.root
scale_type = ("min" if str(scale.quality) == 'min' else "maj")
scale_idx = NOTE_VAL_DICT[scale.root]

# iterate all chords
for c in cp:
    # next chord
    str_chord = c
    # determine root note, relative to scale, 
    note_idx = NOTE_VAL_DICT[c.root]
    relative_idx = (12 + note_idx - scale_idx) % 12

    # determine roman numeral
    roman_idx = (-1 if relative_idx not in RELATIVE_KEY_DICT[scale_type] else RELATIVE_KEY_DICT[scale_type].index(relative_idx) + 1)
    roman_num = ROMAN_NUMERAL[roman_idx]
    chord_qual = str(c.quality)
    roman_num = (roman_num.lower() if chord_qual in ['m', 'min'] else roman_num)
    roman_num = roman_num + (chord_qual if chord_qual in ['dim'] else "")

    chords_roman.append(roman_num)

# result
print(cp)
print(f'Roman numerals (scale is {scale_root}{scale_type})')
print(chords_roman)

>>> Am | Dm | G | C | F | Bdim | E | Am
>>> Roman numerals (scale is Amin)
>>> ['i', 'iv', 'VII', 'III', 'VI', 'IIdim', 'V', 'i']

[yuma-m]

Hello @BdR76, There's a Chord.note_from_index function to generate a Chord from a roman numeral expression, but there's no inversion function implemented. I appreciate it if you could create a pull request. I think Chord class should have as_roman_numeral method and ChordProgression class wraps it.