import traceback

class DBCError(Exception): pass
class CheckError(DBCError): pass
class PreError(DBCError): pass
class PostError(DBCError): pass
class SentinelError(DBCError): pass

def check(test, msg, cls=CheckError):
    print(">>> ", msg)
    traceback.print_stack(limit=-1)
    if not test:
        raise cls(msg)

def pre(test, msg):
    check(test, msg, PreError)

def post(test, msg):
    check(test, msg, PostError)

def sentinel():
    traceback.print_stack(limit=-1)
    raise SentinelError()