Source

cpython_sandbox / Lib / test / test_descrtut.py

Full commit
  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
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
# This contains most of the executable examples from Guido's descr
# tutorial, once at
#
#     http://www.python.org/2.2/descrintro.html
#
# A few examples left implicit in the writeup were fleshed out, a few were
# skipped due to lack of interest (e.g., faking super() by hand isn't
# of much interest anymore), and a few were fiddled to make the output
# deterministic.

from test.support import sortdict
import pprint

class defaultdict(dict):
    def __init__(self, default=None):
        dict.__init__(self)
        self.default = default

    def __getitem__(self, key):
        try:
            return dict.__getitem__(self, key)
        except KeyError:
            return self.default

    def get(self, key, *args):
        if not args:
            args = (self.default,)
        return dict.get(self, key, *args)

    def merge(self, other):
        for key in other:
            if key not in self:
                self[key] = other[key]

test_1 = """

Here's the new type at work:

    >>> print(defaultdict)              # show our type
    <class 'test.test_descrtut.defaultdict'>
    >>> print(type(defaultdict))        # its metatype
    <class 'type'>
    >>> a = defaultdict(default=0.0)    # create an instance
    >>> print(a)                        # show the instance
    {}
    >>> print(type(a))                  # show its type
    <class 'test.test_descrtut.defaultdict'>
    >>> print(a.__class__)              # show its class
    <class 'test.test_descrtut.defaultdict'>
    >>> print(type(a) is a.__class__)   # its type is its class
    True
    >>> a[1] = 3.25                     # modify the instance
    >>> print(a)                        # show the new value
    {1: 3.25}
    >>> print(a[1])                     # show the new item
    3.25
    >>> print(a[0])                     # a non-existent item
    0.0
    >>> a.merge({1:100, 2:200})         # use a dict method
    >>> print(sortdict(a))              # show the result
    {1: 3.25, 2: 200}
    >>>

We can also use the new type in contexts where classic only allows "real"
dictionaries, such as the locals/globals dictionaries for the exec
statement or the built-in function eval():

    >>> print(sorted(a.keys()))
    [1, 2]
    >>> a['print'] = print              # need the print function here
    >>> exec("x = 3; print(x)", a)
    3
    >>> print(sorted(a.keys(), key=lambda x: (str(type(x)), x)))
    [1, 2, '__builtins__', 'print', 'x']
    >>> print(a['x'])
    3
    >>>

Now I'll show that defaultdict instances have dynamic instance variables,
just like classic classes:

    >>> a.default = -1
    >>> print(a["noway"])
    -1
    >>> a.default = -1000
    >>> print(a["noway"])
    -1000
    >>> 'default' in dir(a)
    True
    >>> a.x1 = 100
    >>> a.x2 = 200
    >>> print(a.x1)
    100
    >>> d = dir(a)
    >>> 'default' in d and 'x1' in d and 'x2' in d
    True
    >>> print(sortdict(a.__dict__))
    {'default': -1000, 'x1': 100, 'x2': 200}
    >>>
"""

class defaultdict2(dict):
    __slots__ = ['default']

    def __init__(self, default=None):
        dict.__init__(self)
        self.default = default

    def __getitem__(self, key):
        try:
            return dict.__getitem__(self, key)
        except KeyError:
            return self.default

    def get(self, key, *args):
        if not args:
            args = (self.default,)
        return dict.get(self, key, *args)

    def merge(self, other):
        for key in other:
            if key not in self:
                self[key] = other[key]

test_2 = """

The __slots__ declaration takes a list of instance variables, and reserves
space for exactly these in the instance. When __slots__ is used, other
instance variables cannot be assigned to:

    >>> a = defaultdict2(default=0.0)
    >>> a[1]
    0.0
    >>> a.default = -1
    >>> a[1]
    -1
    >>> a.x1 = 1
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    AttributeError: 'defaultdict2' object has no attribute 'x1'
    >>>

"""

test_3 = """

Introspecting instances of built-in types

For instance of built-in types, x.__class__ is now the same as type(x):

    >>> type([])
    <class 'list'>
    >>> [].__class__
    <class 'list'>
    >>> list
    <class 'list'>
    >>> isinstance([], list)
    True
    >>> isinstance([], dict)
    False
    >>> isinstance([], object)
    True
    >>>

You can get the information from the list type:

    >>> pprint.pprint(dir(list))    # like list.__dict__.keys(), but sorted
    ['__add__',
     '__class__',
     '__contains__',
     '__delattr__',
     '__delitem__',
     '__dir__',
     '__doc__',
     '__eq__',
     '__format__',
     '__ge__',
     '__getattribute__',
     '__getitem__',
     '__gt__',
     '__hash__',
     '__iadd__',
     '__imul__',
     '__init__',
     '__iter__',
     '__le__',
     '__len__',
     '__lt__',
     '__mul__',
     '__ne__',
     '__new__',
     '__reduce__',
     '__reduce_ex__',
     '__repr__',
     '__reversed__',
     '__rmul__',
     '__setattr__',
     '__setitem__',
     '__sizeof__',
     '__str__',
     '__subclasshook__',
     'append',
     'clear',
     'copy',
     'count',
     'extend',
     'index',
     'insert',
     'pop',
     'remove',
     'reverse',
     'sort']

The new introspection API gives more information than the old one:  in
addition to the regular methods, it also shows the methods that are
normally invoked through special notations, e.g. __iadd__ (+=), __len__
(len), __ne__ (!=). You can invoke any method from this list directly:

    >>> a = ['tic', 'tac']
    >>> list.__len__(a)          # same as len(a)
    2
    >>> a.__len__()              # ditto
    2
    >>> list.append(a, 'toe')    # same as a.append('toe')
    >>> a
    ['tic', 'tac', 'toe']
    >>>

This is just like it is for user-defined classes.
"""

test_4 = """

Static methods and class methods

The new introspection API makes it possible to add static methods and class
methods. Static methods are easy to describe: they behave pretty much like
static methods in C++ or Java. Here's an example:

    >>> class C:
    ...
    ...     @staticmethod
    ...     def foo(x, y):
    ...         print("staticmethod", x, y)

    >>> C.foo(1, 2)
    staticmethod 1 2
    >>> c = C()
    >>> c.foo(1, 2)
    staticmethod 1 2

Class methods use a similar pattern to declare methods that receive an
implicit first argument that is the *class* for which they are invoked.

    >>> class C:
    ...     @classmethod
    ...     def foo(cls, y):
    ...         print("classmethod", cls, y)

    >>> C.foo(1)
    classmethod <class 'test.test_descrtut.C'> 1
    >>> c = C()
    >>> c.foo(1)
    classmethod <class 'test.test_descrtut.C'> 1

    >>> class D(C):
    ...     pass

    >>> D.foo(1)
    classmethod <class 'test.test_descrtut.D'> 1
    >>> d = D()
    >>> d.foo(1)
    classmethod <class 'test.test_descrtut.D'> 1

This prints "classmethod __main__.D 1" both times; in other words, the
class passed as the first argument of foo() is the class involved in the
call, not the class involved in the definition of foo().

But notice this:

    >>> class E(C):
    ...     @classmethod
    ...     def foo(cls, y): # override C.foo
    ...         print("E.foo() called")
    ...         C.foo(y)

    >>> E.foo(1)
    E.foo() called
    classmethod <class 'test.test_descrtut.C'> 1
    >>> e = E()
    >>> e.foo(1)
    E.foo() called
    classmethod <class 'test.test_descrtut.C'> 1

In this example, the call to C.foo() from E.foo() will see class C as its
first argument, not class E. This is to be expected, since the call
specifies the class C. But it stresses the difference between these class
methods and methods defined in metaclasses (where an upcall to a metamethod
would pass the target class as an explicit first argument).
"""

test_5 = """

Attributes defined by get/set methods


    >>> class property(object):
    ...
    ...     def __init__(self, get, set=None):
    ...         self.__get = get
    ...         self.__set = set
    ...
    ...     def __get__(self, inst, type=None):
    ...         return self.__get(inst)
    ...
    ...     def __set__(self, inst, value):
    ...         if self.__set is None:
    ...             raise AttributeError("this attribute is read-only")
    ...         return self.__set(inst, value)

Now let's define a class with an attribute x defined by a pair of methods,
getx() and setx():

    >>> class C(object):
    ...
    ...     def __init__(self):
    ...         self.__x = 0
    ...
    ...     def getx(self):
    ...         return self.__x
    ...
    ...     def setx(self, x):
    ...         if x < 0: x = 0
    ...         self.__x = x
    ...
    ...     x = property(getx, setx)

Here's a small demonstration:

    >>> a = C()
    >>> a.x = 10
    >>> print(a.x)
    10
    >>> a.x = -10
    >>> print(a.x)
    0
    >>>

Hmm -- property is builtin now, so let's try it that way too.

    >>> del property  # unmask the builtin
    >>> property
    <class 'property'>

    >>> class C(object):
    ...     def __init__(self):
    ...         self.__x = 0
    ...     def getx(self):
    ...         return self.__x
    ...     def setx(self, x):
    ...         if x < 0: x = 0
    ...         self.__x = x
    ...     x = property(getx, setx)


    >>> a = C()
    >>> a.x = 10
    >>> print(a.x)
    10
    >>> a.x = -10
    >>> print(a.x)
    0
    >>>
"""

test_6 = """

Method resolution order

This example is implicit in the writeup.

>>> class A:    # implicit new-style class
...     def save(self):
...         print("called A.save()")
>>> class B(A):
...     pass
>>> class C(A):
...     def save(self):
...         print("called C.save()")
>>> class D(B, C):
...     pass

>>> D().save()
called C.save()

>>> class A(object):  # explicit new-style class
...     def save(self):
...         print("called A.save()")
>>> class B(A):
...     pass
>>> class C(A):
...     def save(self):
...         print("called C.save()")
>>> class D(B, C):
...     pass

>>> D().save()
called C.save()
"""

class A(object):
    def m(self):
        return "A"

class B(A):
    def m(self):
        return "B" + super(B, self).m()

class C(A):
    def m(self):
        return "C" + super(C, self).m()

class D(C, B):
    def m(self):
        return "D" + super(D, self).m()


test_7 = """

Cooperative methods and "super"

>>> print(D().m()) # "DCBA"
DCBA
"""

test_8 = """

Backwards incompatibilities

>>> class A:
...     def foo(self):
...         print("called A.foo()")

>>> class B(A):
...     pass

>>> class C(A):
...     def foo(self):
...         B.foo(self)

>>> C().foo()
called A.foo()

>>> class C(A):
...     def foo(self):
...         A.foo(self)
>>> C().foo()
called A.foo()
"""

__test__ = {"tut1": test_1,
            "tut2": test_2,
            "tut3": test_3,
            "tut4": test_4,
            "tut5": test_5,
            "tut6": test_6,
            "tut7": test_7,
            "tut8": test_8}

# Magic test name that regrtest.py invokes *after* importing this module.
# This worms around a bootstrap problem.
# Note that doctest and regrtest both look in sys.argv for a "-v" argument,
# so this works as expected in both ways of running regrtest.
def test_main(verbose=None):
    # Obscure:  import this module as test.test_descrtut instead of as
    # plain test_descrtut because the name of this module works its way
    # into the doctest examples, and unless the full test.test_descrtut
    # business is used the name can change depending on how the test is
    # invoked.
    from test import support, test_descrtut
    support.run_doctest(test_descrtut, verbose)

# This part isn't needed for regrtest, but for running the test directly.
if __name__ == "__main__":
    test_main(1)