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#!/usr/bin/env python3

"""

Support Eiffel-style preconditions and postconditions for functions.

An example for Python metaclasses.

"""

import unittest

from types import FunctionType as function

class EiffelBaseMetaClass(type):

def __new__(meta, name, bases, dict):

meta.convert_methods(dict)

return super(EiffelBaseMetaClass, meta).__new__(

meta, name, bases, dict)

@classmethod

def convert_methods(cls, dict):

"""Replace functions in dict with EiffelMethod wrappers.

The dict is modified in place.

If a method ends in _pre or _post, it is removed from the dict

regardless of whether there is a corresponding method.

"""

# find methods with pre or post conditions

methods = []

for k, v in dict.items():

if k.endswith('_pre') or k.endswith('_post'):

assert isinstance(v, function)

elif isinstance(v, function):

methods.append(k)

for m in methods:

pre = dict.get("%s_pre" % m)

post = dict.get("%s_post" % m)

if pre or post:

dict[m] = cls.make_eiffel_method(dict[m], pre, post)

class EiffelMetaClass1(EiffelBaseMetaClass):

# an implementation of the "eiffel" meta class that uses nested functions

@staticmethod

def make_eiffel_method(func, pre, post):

def method(self, *args, **kwargs):

if pre:

pre(self, *args, **kwargs)

rv = func(self, *args, **kwargs)

if post:

post(self, rv, *args, **kwargs)

return rv

if func.__doc__:

method.__doc__ = func.__doc__

return method

class EiffelMethodWrapper:

def __init__(self, inst, descr):

self._inst = inst

self._descr = descr

def __call__(self, *args, **kwargs):

return self._descr.callmethod(self._inst, args, kwargs)

class EiffelDescriptor:

def __init__(self, func, pre, post):

self._func = func

self._pre = pre

self._post = post

self.__name__ = func.__name__

self.__doc__ = func.__doc__

def __get__(self, obj, cls):

return EiffelMethodWrapper(obj, self)

def callmethod(self, inst, args, kwargs):

if self._pre:

self._pre(inst, *args, **kwargs)

x = self._func(inst, *args, **kwargs)

if self._post:

self._post(inst, x, *args, **kwargs)

return x

class EiffelMetaClass2(EiffelBaseMetaClass):

# an implementation of the "eiffel" meta class that uses descriptors

make_eiffel_method = EiffelDescriptor

class Tests(unittest.TestCase):

def testEiffelMetaClass1(self):

self._test(EiffelMetaClass1)

def testEiffelMetaClass2(self):

self._test(EiffelMetaClass2)

def _test(self, metaclass):

class Eiffel(metaclass=metaclass):

pass

class Test(Eiffel):

def m(self, arg):

"""Make it a little larger"""

return arg + 1

def m2(self, arg):

"""Make it a little larger"""

return arg + 1

def m2_pre(self, arg):

assert arg > 0

def m2_post(self, result, arg):

assert result > arg

class Sub(Test):

def m2(self, arg):

return arg**2

def m2_post(self, Result, arg):

super(Sub, self).m2_post(Result, arg)

assert Result < 100

t = Test()

self.assertEqual(t.m(1), 2)

self.assertEqual(t.m2(1), 2)

self.assertRaises(AssertionError, t.m2, 0)

s = Sub()

self.assertRaises(AssertionError, s.m2, 1)

self.assertRaises(AssertionError, s.m2, 10)

self.assertEqual(s.m2(5), 25)

if __name__ == "__main__":

unittest.main()

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