keys-and-harmony-with-pytheory

Keys & Harmony

Working with keys and progressions — the harmonic level above a single chord.

Keys and their diatonic content

from pytheory import Key

k = Key("C", "major")
k.chords            # ['C major', 'D minor', 'E minor', 'F major', 'G major', 'A minor', 'B diminished']
k.seventh_chords    # ['C major 7th', 'D minor 7th', …, 'G dominant 7th', …]
k.note_names        # ['C', 'D', 'E', 'F', 'G', 'A', 'B', 'C']
k.signature         # {'sharps': 0, 'flats': 0, 'accidentals': []}
k.relative          # <Key A minor>

Detect the key

Key.detect("C", "E", "G", "B", "D")     # -> <Key C major>

Progressions

k = Key("G", "major")
[c.symbol for c in k.progression("I", "V", "vi", "IV")]   # ['G', 'D', 'Em', 'C']
[c.symbol for c in k.nashville(1, 5, 6, 4)]               # ['G', 'D', 'Em', 'C']
k.random_progression(4)                                    # a diatonic [Chord, …]

Roman-numeral analysis of an existing progression:

from pytheory import analyze_progression, Chord
chords = [Chord.from_symbol(s) for s in ["C", "G", "Am", "F"]]
analyze_progression(chords, key="C", mode="major")        # ['I', 'V', 'vi', 'IV']

Secondary dominants — applied dominants that tonicise a non-tonic degree. detect_secondary_dominant identifies one chord (the analytical inverse of Key.secondary_dominant(degree), which builds one); pass secondary_dominants=True to analyze_progression to label them in context (instead of the bare degree):

from pytheory import detect_secondary_dominant, analyze_progression, Chord
detect_secondary_dominant(Chord.from_symbol("D7"), "C")   # 'V7/V'  (D7 -> G)
detect_secondary_dominant(Chord.from_symbol("E7"), "C")   # 'V7/vi' (E7 -> Am)
prog = [Chord.from_symbol(s) for s in ("C", "D7", "G7", "C")]
analyze_progression(prog, "C", secondary_dominants=True)  # ['I', 'V7/V', 'V7', 'I']

From the terminal, pytheory analyze C D7 G7 C prints the whole picture — detected key, Roman numerals (with secondary dominants), and cadences (add --key/--mode to fix the key).