prize17-1, Exercise 2: A twelve-bar blues in G

The I, IV and V chords in G are G, C and D, so the form becomes:

form = ["G7", "G7", "G7", "G7",
        "C7", "C7", "G7", "G7",
        "D7", "C7", "G7", "D7"]
assert len(form) == 12
for i in range(0, 12, 4):
    print(" | ".join(form[i:i+4]))

Expected output:

G7 | G7 | G7 | G7
C7 | C7 | G7 | G7
D7 | C7 | G7 | D7

Why it works: the pattern depends only on the chord's position (I, IV, V), so moving to a new key means swapping the three chord names. The assert line stops the program if you accidentally drop or add a bar.

Care: the last bar (D7 here) is a turnaround; many players use G7 instead.