1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
|
#!/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[k] = 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()
|
