class CONST(object):
__slots__ = ()
FOO = 1234
CONST = CONST()
print(CONST.FOO) # 1234
CONST.FOO = 4321 # AttributeError: 'CONST' object attribute 'FOO' is read-only
CONST.BAR = 5678 # AttributeError: 'CONST' object has no attribute 'BAR'