Function substitute

mingus/core/progressions.py:426–504

Give a list of possible substitutions for progression[substitute_index]. If depth > 0 the substitutions of each result will be recursively added as well. Example: >>> substitute(['I', 'IV', 'V', 'I'], 0) ['III', 'III7', 'VI', 'VI7', 'I7']

(progression, substitute_index, depth=0)

def substitute(progression, substitute_index, depth=0):
    """Give a list of possible substitutions for progression[substitute_index].

    If depth > 0 the substitutions of each result will be recursively added
    as well.

    Example:
    >>> substitute(['I', 'IV', 'V', 'I'], 0)
    ['III', 'III7', 'VI', 'VI7', 'I7']
    """
    res = []
    simple_substitutions = [
        ("I", "III"),
        ("I", "VI"),
        ("IV", "II"),
        ("IV", "VI"),
        ("V", "VII"),
        ("V", "VIIdim7"),
        ("V", "IIdim7"),
        ("V", "IVdim7"),
        ("V", "bVIIdim7"),
    ]
    p = progression[substitute_index]
    (roman, acc, suff) = parse_string(p)

    # Do the simple harmonic substitutions
    if suff == "" or suff == "7":
        for subs in simple_substitutions:
            r = None
            if roman == subs[0]:
                r = subs[1]
            elif roman == subs[1]:
                r = subs[0]
            if r != None:
                res.append(tuple_to_string((r, acc, "")))

                # Add seventh or triad depending on r
                if r[-1] != "7":
                    res.append(tuple_to_string((r, acc, "7")))
                else:
                    res.append(tuple_to_string((r[:-1], acc, "")))

    if suff == "" or suff == "M" or suff == "m":
        res.append(tuple_to_string((roman, acc, suff + "7")))

    if suff == "m" or suff == "m7":
        n = skip(roman, 2)
        a = interval_diff(roman, n, 3) + acc
        res.append(tuple_to_string((n, a, "M")))
        res.append(tuple_to_string((n, a, "M7")))

    # Major to minor substitution
    if suff == "M" or suff == "M7":
        n = skip(roman, 5)
        a = interval_diff(roman, n, 9) + acc
        res.append(tuple_to_string((n, a, "m")))
        res.append(tuple_to_string((n, a, "m7")))