knights_and_knaves.py

# knights and knaves

class Meta(type):
    def __repr__(cls):
        return f'<{cls.__name__}>'

class Knight(metaclass = Meta):
    pass

class Knave(metaclass = Meta):
    pass

# uppercase letters: declare unsolved identities as globals
A, B, C, D = None, None, None, None

# lowercase letters: statements
a = lambda: C is Knave
b = lambda: A is Knight and C is Knave
c = lambda: B is not C
d = lambda: A is Knave

# test until all 4 statements satisfied
def test():
    return [stmt() if person is Knight
            else not stmt() # invert logic if person is Knave
            for person, stmt
            in ( (A, a), (B, b), (C, c), (D, d) )
           ]

for A in (Knight, Knave):
    for B in (Knight, Knave):
        for C in (Knight, Knave):
            for D in (Knight, Knave):
                if all(test()):
                    print(A, B, C, D)
                    exit()