for habr

MessagedExpressions.py

import unittest
import logging


logging.basicConfig(level = logging.DEBUG)
log = logging.getLogger("messaged.expression")

class MessagedExpression(object):
    def __init__(self, val, msg):
        self.msg = msg
        self.val = val
        self.bool = None

    def __nonzero__(self):
        return self.__bool__()

    def __bool__(self):
        if self.bool is not None:
            return self.bool
        if callable(self.val):
            log.debug("%s evaluated" % self.val)
            self.val = self.val()
        return bool(self.val)

    def __and__(self, other):
        if bool(self) == False:
            return self
        self.bool = bool(other)
        self.msg = other.msg
        return self

    def __or__(self, other):
        if bool(self) == True:
            return self
        else:
            if not bool(other):
                if self.msg.__class__ is not list:
                    self.msg = [self.msg,]
                self.msg.append(other.msg)
        self.bool = bool(other)
        return self


_me = MessagedExpression


def calculate_me():
    log.debug("hardcore calculations")

class Test(unittest.TestCase):
    def test(self):
        result = _me("123","first") & _me(False, "second") &  _me(calculate_me,"calculated")
        self.assertEqual(result.msg,"second")


        result = _me("123","first") & _me(False, "second") |  _me(calculate_me,"calculated")
        self.assertEqual(result.msg,["second","calculated"])