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266 lines
3.9 KiB
Markdown
266 lines
3.9 KiB
Markdown
# 属性
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## 只读属性
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只读属性，顾名思义，指的是只可读不可写的属性，之前我们定义的属性都是可读可写的，对于只读属性，我们需要使用 `@property` 修饰符来得到：
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In [1]:
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```py
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class Leaf(object):
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def __init__(self, mass_mg):
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self.mass_mg = mass_mg
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# 这样 mass_oz 就变成属性了
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@property
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def mass_oz(self):
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return self.mass_mg * 3.53e-5
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```
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这里 `mass_oz` 就是一个只读不写的属性（注意是属性不是方法），而 `mass_mg` 是可读写的属性：
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In [2]:
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```py
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leaf = Leaf(200)
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print leaf.mass_oz
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```
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```py
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0.00706
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```
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可以修改 `mass_mg` 属性来改变 `mass_oz`：
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In [3]:
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```py
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leaf.mass_mg = 150
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print leaf.mass_oz
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```
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```py
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0.005295
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```
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是属性不是方法：
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In [4]:
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```py
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leaf.mass_oz()
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```
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```py
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---------------------------------------------------------------------------
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TypeError Traceback (most recent call last)
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<ipython-input-4-aac6717ebc82> in <module>()
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----> 1 leaf.mass_oz()
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TypeError: 'float' object is not callable
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```
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而且是只读属性，不可写：
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In [5]:
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```py
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leaf.mass_oz = 0.001
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```
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```py
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---------------------------------------------------------------------------
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AttributeError Traceback (most recent call last)
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<ipython-input-5-d232052cd2dc> in <module>()
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----> 1 leaf.mass_oz = 0.001
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AttributeError: can't set attribute
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```
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回到 `forest` 的例子，我们希望加入几个只读属性：
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In [6]:
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```py
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import numpy as np
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class Forest(object):
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""" Forest can grow trees which eventually die."""
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def __init__(self, size=(150,150)):
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self.size = size
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self.trees = np.zeros(self.size, dtype=bool)
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self.fires = np.zeros((self.size), dtype=bool)
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def __repr__(self):
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my_repr = "{}(size={})".format(self.__class__.__name__, self.size)
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return my_repr
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def __str__(self):
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return self.__class__.__name__
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@property
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def num_cells(self):
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"""Number of cells available for growing trees"""
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return np.prod(self.size)
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@property
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def tree_fraction(self):
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"""
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Fraction of trees
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"""
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num_trees = self.trees.sum()
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return float(num_trees) / self.num_cells
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@property
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def fire_fraction(self):
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"""
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Fraction of fires
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"""
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num_fires = self.fires.sum()
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return float(num_fires) / self.num_cells
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```
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查看属性：
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In [7]:
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```py
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forest = Forest()
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forest.num_cells
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```
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Out[7]:
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```py
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22500
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```
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生成一个较小的森林：
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In [8]:
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```py
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small_forest = Forest((10, 10))
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small_forest.num_cells
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```
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Out[8]:
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```py
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100
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```
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初始状态下，树和火灾的比例都是 0：
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In [9]:
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```py
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small_forest.tree_fraction
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```
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Out[9]:
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```py
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0.0
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```
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In [10]:
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```py
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small_forest.fire_fraction
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```
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Out[10]:
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```py
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0.0
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```
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## 可读写的属性
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对于 `@property` 生成的只读属性，我们可以使用相应的 `@attr.setter` 修饰符来使得这个属性变成可写的：
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In [11]:
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```py
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class Leaf(object):
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def __init__(self, mass_mg):
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self.mass_mg = mass_mg
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# 这样 mass_oz 就变成属性了
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@property
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def mass_oz(self):
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return self.mass_mg * 3.53e-5
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# 使用 mass_oz.setter 修饰符
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@mass_oz.setter
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def mass_oz(self, m_oz):
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self.mass_mg = m_oz / 3.53e-5
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```
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测试：
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In [12]:
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```py
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leaf = Leaf(200)
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print leaf.mass_oz
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leaf.mass_mg = 150
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print leaf.mass_oz
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```
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```py
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0.00706
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0.005295
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```
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修改 `mass_oz` 属性：
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In [13]:
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```py
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leaf.mass_oz = 0.01
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print leaf.mass_mg
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```
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```py
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283.28611898
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```
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一个等价的替代如下：
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```py
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class Leaf(object):
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def __init__(self, mass_mg):
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self.mass_mg = mass_mg
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def get_mass_oz(self):
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return self.mass_mg * 3.53e-5
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def set_mass_oz(self, m_oz):
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self.mass_mg = m_oz / 3.53e-5
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mass_oz = property(get_mass_oz, set_mass_oz)
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``` |