1 class Descriptor:
 2     def __init__(self, name=None, **opts):
 3         self.name = name
 4         for key, value in opts.items():
 5             setattr(self, key, value)
 6 
 7     def __set__(self, instance, value):
 8         instance.__dict__[self.name] = value
 9 
10 
11 # Descriptor for enforcing types
12 class Typed(Descriptor):
13     expected_type = type(None)
14 
15     def __set__(self, instance, value):
16         if not isinstance(value, self.expected_type):
17             raise TypeError(‘expected ‘ + str(self.expected_type))
18         super().__set__(instance, value)
19 
20 
21 # Descriptor for enforcing values
22 class Unsigned(Descriptor):
23     def __set__(self, instance, value):
24         if value < 0:
25             raise ValueError(‘Expected >= 0‘)
26         super().__set__(instance, value)
27 
28 
29 class MaxSized(Descriptor):
30     def __init__(self, name=None, **opts):
31         if ‘size‘ not in opts:
32             raise TypeError(‘missing size option‘)
33         super().__init__(name, **opts)
34 
35     def __set__(self, instance, value):
36         if len(value) >= self.size:
37             raise ValueError(‘size must be < ‘ + str(self.size))
38         super().__set__(instance, value)
39 
40 class Integer(Typed):
41     expected_type = int   # 将上面的赋值覆盖了
42 
43 class UnsignedInteger(Integer, Unsigned):
44     pass
45 
46 class Float(Typed):
47     expected_type = float
48 
49 class UnsignedFloat(Float, Unsigned):
50     pass
51 
52 class String(Typed):
53     expected_type = str
54 
55 class SizedString(String, MaxSized):
56     pass
57 
58 
59 class Stock:
60     # Specify constraints
61     name = SizedString(‘name‘, size=8)
62     shares = UnsignedInteger(‘shares‘)
63     price = UnsignedFloat(‘price‘)
64 
65     def __init__(self, name, shares, price):
66         self.name = name
67         self.shares = shares
68         self.price = price
69 
70 s=Stock(‘sdf‘,23,4.3)
71 # s.shares = 2.3
72 s.shares=-10