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
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
|
doctests = """
Basic class construction.
>>> class C:
... def meth(self): print("Hello")
...
>>> C.__class__ is type
True
>>> a = C()
>>> a.__class__ is C
True
>>> a.meth()
Hello
>>>
Use *args notation for the bases.
>>> class A: pass
>>> class B: pass
>>> bases = (A, B)
>>> class C(*bases): pass
>>> C.__bases__ == bases
True
>>>
Use a trivial metaclass.
>>> class M(type):
... pass
...
>>> class C(metaclass=M):
... def meth(self): print("Hello")
...
>>> C.__class__ is M
True
>>> a = C()
>>> a.__class__ is C
True
>>> a.meth()
Hello
>>>
Use **kwds notation for the metaclass keyword.
>>> kwds = {'metaclass': M}
>>> class C(**kwds): pass
...
>>> C.__class__ is M
True
>>> a = C()
>>> a.__class__ is C
True
>>>
Use a metaclass with a __prepare__ static method.
>>> class M(type):
... @staticmethod
... def __prepare__(*args, **kwds):
... print("Prepare called:", args, kwds)
... return dict()
... def __new__(cls, name, bases, namespace, **kwds):
... print("New called:", kwds)
... return type.__new__(cls, name, bases, namespace)
... def __init__(cls, *args, **kwds):
... pass
...
>>> class C(metaclass=M):
... def meth(self): print("Hello")
...
Prepare called: ('C', ()) {}
New called: {}
>>>
Also pass another keyword.
>>> class C(object, metaclass=M, other="haha"):
... pass
...
Prepare called: ('C', (<class 'object'>,)) {'other': 'haha'}
New called: {'other': 'haha'}
>>> C.__class__ is M
True
>>> C.__bases__ == (object,)
True
>>> a = C()
>>> a.__class__ is C
True
>>>
Check that build_class doesn't mutate the kwds dict.
>>> kwds = {'metaclass': type}
>>> class C(**kwds): pass
...
>>> kwds == {'metaclass': type}
True
>>>
Use various combinations of explicit keywords and **kwds.
>>> bases = (object,)
>>> kwds = {'metaclass': M, 'other': 'haha'}
>>> class C(*bases, **kwds): pass
...
Prepare called: ('C', (<class 'object'>,)) {'other': 'haha'}
New called: {'other': 'haha'}
>>> C.__class__ is M
True
>>> C.__bases__ == (object,)
True
>>> class B: pass
>>> kwds = {'other': 'haha'}
>>> class C(B, metaclass=M, *bases, **kwds): pass
...
Prepare called: ('C', (<class 'test.test_metaclass.B'>, <class 'object'>)) {'other': 'haha'}
New called: {'other': 'haha'}
>>> C.__class__ is M
True
>>> C.__bases__ == (B, object)
True
>>>
Check for duplicate keywords.
>>> class C(metaclass=type, metaclass=type): pass
...
Traceback (most recent call last):
[...]
TypeError: __build_class__() got multiple values for keyword argument 'metaclass'
>>>
Another way.
>>> kwds = {'metaclass': type}
>>> class C(metaclass=type, **kwds): pass
...
Traceback (most recent call last):
[...]
TypeError: __build_class__() got multiple values for keyword argument 'metaclass'
>>>
Use a __prepare__ method that returns an instrumented dict.
>>> class LoggingDict(dict):
... def __setitem__(self, key, value):
... print("d[%r] = %r" % (key, value))
... dict.__setitem__(self, key, value)
...
>>> class Meta(type):
... @staticmethod
... def __prepare__(name, bases):
... return LoggingDict()
...
>>> class C(metaclass=Meta):
... foo = 2+2
... foo = 42
... bar = 123
...
d['__module__'] = 'test.test_metaclass'
d['foo'] = 4
d['foo'] = 42
d['bar'] = 123
>>>
Use a metaclass that doesn't derive from type.
>>> def meta(name, bases, namespace, **kwds):
... print("meta:", name, bases)
... print("ns:", sorted(namespace.items()))
... print("kw:", sorted(kwds.items()))
... return namespace
...
>>> class C(metaclass=meta):
... a = 42
... b = 24
...
meta: C ()
ns: [('__module__', 'test.test_metaclass'), ('a', 42), ('b', 24)]
kw: []
>>> type(C) is dict
True
>>> print(sorted(C.items()))
[('__module__', 'test.test_metaclass'), ('a', 42), ('b', 24)]
>>>
And again, with a __prepare__ attribute.
>>> def prepare(name, bases, **kwds):
... print("prepare:", name, bases, sorted(kwds.items()))
... return LoggingDict()
...
>>> meta.__prepare__ = prepare
>>> class C(metaclass=meta, other="booh"):
... a = 1
... a = 2
... b = 3
...
prepare: C () [('other', 'booh')]
d['__module__'] = 'test.test_metaclass'
d['a'] = 1
d['a'] = 2
d['b'] = 3
meta: C ()
ns: [('__module__', 'test.test_metaclass'), ('a', 2), ('b', 3)]
kw: [('other', 'booh')]
>>>
The default metaclass must define a __prepare__() method.
>>> type.__prepare__()
{}
>>>
Make sure it works with subclassing.
>>> class M(type):
... @classmethod
... def __prepare__(cls, *args, **kwds):
... d = super().__prepare__(*args, **kwds)
... d["hello"] = 42
... return d
...
>>> class C(metaclass=M):
... print(hello)
...
42
>>> print(C.hello)
42
>>>
"""
__test__ = {'doctests' : doctests}
def test_main(verbose=False):
from test import support
from test import test_metaclass
support.run_doctest(test_metaclass, verbose)
if __name__ == "__main__":
test_main(verbose=True)
|
