08.11 接口
接口
在 Python 中，鸭子类型（duck typing）是一种动态类型的风格。所谓鸭子类型，来自于 James Whitcomb Riley 的“鸭子测试”：
当看到一只鸟走起来像鸭子、游泳起来像鸭子、叫起来也像鸭子，那么这只鸟就可以被称为鸭子。
假设我们需要定义一个函数，这个函数使用一个类型为鸭子的参数，并调用它的走和叫方法。
在鸭子类型的语言中，这样的函数可以接受任何类型的对象，只要这个对象实现了走和叫的方法，否则就引发一个运行时错误。换句话说，任何拥有走和叫方法的参数都是合法的。
先看一个例子，父类：
1class Leaf(object):
2 def __init__(self, color="green"):
3 self.color = color
4 def fall(self):
5 print "Splat!"
子类：
1class MapleLeaf(Leaf):
2 def fall(self):
3 self.color = 'brown'
4 super(MapleLeaf, self).fall()
新的类：
1class Acorn(object):
2 def fall(self):
3 print "Plunk!"
这三个类都实现了 fall() 方法，因此可以这样使用：
1objects = [Leaf(), MapleLeaf(), Acorn()]
2
3for obj in objects:
4 obj.fall()
Splat!
Splat!
Plunk!
这里 fall() 方法就一种鸭子类型的体现。
不仅方法可以用鸭子类型，属性也可以：
1import numpy as np
2from scipy.ndimage.measurements import label
3
4class Forest(object):
5 """ Forest can grow trees which eventually die."""
6 def __init__(self, size=(150,150), p_sapling=0.0025):
7 self.size = size
8 self.trees = np.zeros(self.size, dtype=bool)
9 self.p_sapling = p_sapling
10
11 def __repr__(self):
12 my_repr = "{}(size={})".format(self.__class__.__name__, self.size)
13 return my_repr
14
15 def __str__(self):
16 return self.__class__.__name__
17
18 @property
19 def num_cells(self):
20 """Number of cells available for growing trees"""
21 return np.prod(self.size)
22
23 @property
24 def losses(self):
25 return np.zeros(self.size)
26
27 @property
28 def tree_fraction(self):
29 """
30 Fraction of trees
31 """
32 num_trees = self.trees.sum()
33 return float(num_trees) / self.num_cells
34
35 def _rand_bool(self, p):
36 """
37 Random boolean distributed according to p, less than p will be True
38 """
39 return np.random.uniform(size=self.trees.shape) < p
40
41 def grow_trees(self):
42 """
43 Growing trees.
44 """
45 growth_sites = self._rand_bool(self.p_sapling)
46 self.trees[growth_sites] = True
47
48 def advance_one_step(self):
49 """
50 Advance one step
51 """
52 self.grow_trees()
53
54class BurnableForest(Forest):
55 """
56 Burnable forest support fires
57 """
58 def __init__(self, p_lightning=5.0e-6, **kwargs):
59 super(BurnableForest, self).__init__(**kwargs)
60 self.p_lightning = p_lightning
61 self.fires = np.zeros((self.size), dtype=bool)
62
63 def advance_one_step(self):
64 """
65 Advance one step
66 """
67 super(BurnableForest, self).advance_one_step()
68 self.start_fires()
69 self.burn_trees()
70
71 @property
72 def losses(self):
73 return self.fires
74
75 @property
76 def fire_fraction(self):
77 """
78 Fraction of fires
79 """
80 num_fires = self.fires.sum()
81 return float(num_fires) / self.num_cells
82
83 def start_fires(self):
84 """
85 Start of fire.
86 """
87 lightning_strikes = (self._rand_bool(self.p_lightning) &
88 self.trees)
89 self.fires[lightning_strikes] = True
90
91 def burn_trees(self):
92 pass
93
94class SlowBurnForest(BurnableForest):
95 def burn_trees(self):
96 """
97 Burn trees.
98 """
99 fires = np.zeros((self.size[0] + 2, self.size[1] + 2), dtype=bool)
100 fires[1:-1, 1:-1] = self.fires
101 north = fires[:-2, 1:-1]
102 south = fires[2:, 1:-1]
103 east = fires[1:-1, :-2]
104 west = fires[1:-1, 2:]
105 new_fires = (north | south | east | west) & self.trees
106 self.trees[self.fires] = False
107 self.fires = new_fires
108
109class InstantBurnForest(BurnableForest):
110 def burn_trees(self):
111 # 起火点
112 strikes = self.fires
113 # 找到连通区域
114 groves, num_groves = label(self.trees)
115 fires = set(groves[strikes])
116 self.fires.fill(False)
117 # 将与着火点相连的区域都烧掉
118 for fire in fires:
119 self.fires[groves == fire] = True
120 self.trees[self.fires] = False
121 self.fires.fill(False)
测试：
1forest = Forest()
2b_forest = BurnableForest()
3sb_forest = SlowBurnForest()
4ib_forest = InstantBurnForest()
5
6forests = [forest, b_forest, sb_forest, ib_forest]
7
8losses_history = []
9
10for i in xrange(1500):
11 for fst in forests:
12 fst.advance_one_step()
13 losses_history.append(tuple(fst.losses.sum() for fst in forests))
显示结果：
1import matplotlib.pyplot as plt
2%matplotlib inline
3
4plt.figure(figsize=(10,6))
5
6plt.plot(losses_history)
7plt.legend([f.__str__() for f in forests])
8
9plt.show()
