Georg Brandl | 856898b | 2010-12-30 22:11:50 +0000 | [diff] [blame] | 1 | #!/usr/bin/env python3 |
| 2 | |
| 3 | """ |
| 4 | Support Eiffel-style preconditions and postconditions for functions. |
| 5 | |
| 6 | An example for Python metaclasses. |
| 7 | """ |
| 8 | |
| 9 | import unittest |
| 10 | from types import FunctionType as function |
| 11 | |
| 12 | class EiffelBaseMetaClass(type): |
| 13 | |
| 14 | def __new__(meta, name, bases, dict): |
| 15 | meta.convert_methods(dict) |
| 16 | return super(EiffelBaseMetaClass, meta).__new__( |
| 17 | meta, name, bases, dict) |
| 18 | |
| 19 | @classmethod |
| 20 | def convert_methods(cls, dict): |
| 21 | """Replace functions in dict with EiffelMethod wrappers. |
| 22 | |
| 23 | The dict is modified in place. |
| 24 | |
| 25 | If a method ends in _pre or _post, it is removed from the dict |
| 26 | regardless of whether there is a corresponding method. |
| 27 | """ |
| 28 | # find methods with pre or post conditions |
| 29 | methods = [] |
| 30 | for k, v in dict.items(): |
| 31 | if k.endswith('_pre') or k.endswith('_post'): |
| 32 | assert isinstance(v, function) |
| 33 | elif isinstance(v, function): |
| 34 | methods.append(k) |
| 35 | for m in methods: |
| 36 | pre = dict.get("%s_pre" % m) |
| 37 | post = dict.get("%s_post" % m) |
| 38 | if pre or post: |
Serhiy Storchaka | 6a98fe9 | 2013-07-16 22:12:03 +0300 | [diff] [blame^] | 39 | dict[m] = cls.make_eiffel_method(dict[m], pre, post) |
Georg Brandl | 856898b | 2010-12-30 22:11:50 +0000 | [diff] [blame] | 40 | |
| 41 | |
| 42 | class EiffelMetaClass1(EiffelBaseMetaClass): |
| 43 | # an implementation of the "eiffel" meta class that uses nested functions |
| 44 | |
| 45 | @staticmethod |
| 46 | def make_eiffel_method(func, pre, post): |
| 47 | def method(self, *args, **kwargs): |
| 48 | if pre: |
| 49 | pre(self, *args, **kwargs) |
| 50 | rv = func(self, *args, **kwargs) |
| 51 | if post: |
| 52 | post(self, rv, *args, **kwargs) |
| 53 | return rv |
| 54 | |
| 55 | if func.__doc__: |
| 56 | method.__doc__ = func.__doc__ |
| 57 | |
| 58 | return method |
| 59 | |
| 60 | |
| 61 | class EiffelMethodWrapper: |
| 62 | |
| 63 | def __init__(self, inst, descr): |
| 64 | self._inst = inst |
| 65 | self._descr = descr |
| 66 | |
| 67 | def __call__(self, *args, **kwargs): |
| 68 | return self._descr.callmethod(self._inst, args, kwargs) |
| 69 | |
| 70 | |
| 71 | class EiffelDescriptor: |
| 72 | |
| 73 | def __init__(self, func, pre, post): |
| 74 | self._func = func |
| 75 | self._pre = pre |
| 76 | self._post = post |
| 77 | |
| 78 | self.__name__ = func.__name__ |
| 79 | self.__doc__ = func.__doc__ |
| 80 | |
| 81 | def __get__(self, obj, cls): |
| 82 | return EiffelMethodWrapper(obj, self) |
| 83 | |
| 84 | def callmethod(self, inst, args, kwargs): |
| 85 | if self._pre: |
| 86 | self._pre(inst, *args, **kwargs) |
| 87 | x = self._func(inst, *args, **kwargs) |
| 88 | if self._post: |
| 89 | self._post(inst, x, *args, **kwargs) |
| 90 | return x |
| 91 | |
| 92 | |
| 93 | class EiffelMetaClass2(EiffelBaseMetaClass): |
| 94 | # an implementation of the "eiffel" meta class that uses descriptors |
| 95 | |
| 96 | make_eiffel_method = EiffelDescriptor |
| 97 | |
| 98 | |
| 99 | class Tests(unittest.TestCase): |
| 100 | |
| 101 | def testEiffelMetaClass1(self): |
| 102 | self._test(EiffelMetaClass1) |
| 103 | |
| 104 | def testEiffelMetaClass2(self): |
| 105 | self._test(EiffelMetaClass2) |
| 106 | |
| 107 | def _test(self, metaclass): |
| 108 | class Eiffel(metaclass=metaclass): |
| 109 | pass |
| 110 | |
| 111 | class Test(Eiffel): |
| 112 | def m(self, arg): |
| 113 | """Make it a little larger""" |
| 114 | return arg + 1 |
| 115 | |
| 116 | def m2(self, arg): |
| 117 | """Make it a little larger""" |
| 118 | return arg + 1 |
| 119 | |
| 120 | def m2_pre(self, arg): |
| 121 | assert arg > 0 |
| 122 | |
| 123 | def m2_post(self, result, arg): |
| 124 | assert result > arg |
| 125 | |
| 126 | class Sub(Test): |
| 127 | def m2(self, arg): |
| 128 | return arg**2 |
| 129 | |
| 130 | def m2_post(self, Result, arg): |
| 131 | super(Sub, self).m2_post(Result, arg) |
| 132 | assert Result < 100 |
| 133 | |
| 134 | t = Test() |
| 135 | self.assertEqual(t.m(1), 2) |
| 136 | self.assertEqual(t.m2(1), 2) |
| 137 | self.assertRaises(AssertionError, t.m2, 0) |
| 138 | |
| 139 | s = Sub() |
| 140 | self.assertRaises(AssertionError, s.m2, 1) |
| 141 | self.assertRaises(AssertionError, s.m2, 10) |
| 142 | self.assertEqual(s.m2(5), 25) |
| 143 | |
| 144 | |
| 145 | if __name__ == "__main__": |
| 146 | unittest.main() |