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147 lines
3.8 KiB
147 lines
3.8 KiB
#!/usr/bin/env python3
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"""
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Support Eiffel-style preconditions and postconditions for functions.
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An example for Python metaclasses.
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"""
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import unittest
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from types import FunctionType as function
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class EiffelBaseMetaClass(type):
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def __new__(meta, name, bases, dict):
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meta.convert_methods(dict)
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return super(EiffelBaseMetaClass, meta).__new__(
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meta, name, bases, dict)
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@classmethod
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def convert_methods(cls, dict):
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"""Replace functions in dict with EiffelMethod wrappers.
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The dict is modified in place.
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If a method ends in _pre or _post, it is removed from the dict
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regardless of whether there is a corresponding method.
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"""
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# find methods with pre or post conditions
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methods = []
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for k, v in dict.items():
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if k.endswith('_pre') or k.endswith('_post'):
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assert isinstance(v, function)
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elif isinstance(v, function):
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methods.append(k)
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for m in methods:
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pre = dict.get("%s_pre" % m)
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post = dict.get("%s_post" % m)
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if pre or post:
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dict[m] = cls.make_eiffel_method(dict[m], pre, post)
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class EiffelMetaClass1(EiffelBaseMetaClass):
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# an implementation of the "eiffel" meta class that uses nested functions
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@staticmethod
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def make_eiffel_method(func, pre, post):
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def method(self, *args, **kwargs):
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if pre:
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pre(self, *args, **kwargs)
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rv = func(self, *args, **kwargs)
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if post:
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post(self, rv, *args, **kwargs)
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return rv
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if func.__doc__:
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method.__doc__ = func.__doc__
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return method
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class EiffelMethodWrapper:
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def __init__(self, inst, descr):
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self._inst = inst
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self._descr = descr
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def __call__(self, *args, **kwargs):
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return self._descr.callmethod(self._inst, args, kwargs)
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class EiffelDescriptor:
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def __init__(self, func, pre, post):
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self._func = func
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self._pre = pre
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self._post = post
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self.__name__ = func.__name__
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self.__doc__ = func.__doc__
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def __get__(self, obj, cls):
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return EiffelMethodWrapper(obj, self)
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def callmethod(self, inst, args, kwargs):
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if self._pre:
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self._pre(inst, *args, **kwargs)
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x = self._func(inst, *args, **kwargs)
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if self._post:
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self._post(inst, x, *args, **kwargs)
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return x
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class EiffelMetaClass2(EiffelBaseMetaClass):
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# an implementation of the "eiffel" meta class that uses descriptors
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make_eiffel_method = EiffelDescriptor
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class Tests(unittest.TestCase):
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def testEiffelMetaClass1(self):
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self._test(EiffelMetaClass1)
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def testEiffelMetaClass2(self):
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self._test(EiffelMetaClass2)
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def _test(self, metaclass):
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class Eiffel(metaclass=metaclass):
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pass
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class Test(Eiffel):
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def m(self, arg):
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"""Make it a little larger"""
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return arg + 1
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def m2(self, arg):
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"""Make it a little larger"""
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return arg + 1
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def m2_pre(self, arg):
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assert arg > 0
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def m2_post(self, result, arg):
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assert result > arg
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class Sub(Test):
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def m2(self, arg):
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return arg**2
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def m2_post(self, Result, arg):
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super(Sub, self).m2_post(Result, arg)
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assert Result < 100
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t = Test()
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self.assertEqual(t.m(1), 2)
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self.assertEqual(t.m2(1), 2)
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self.assertRaises(AssertionError, t.m2, 0)
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s = Sub()
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self.assertRaises(AssertionError, s.m2, 1)
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self.assertRaises(AssertionError, s.m2, 10)
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self.assertEqual(s.m2(5), 25)
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if __name__ == "__main__":
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unittest.main()
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