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319 lines
4.8 KiB
Markdown
319 lines
4.8 KiB
Markdown
# 特殊方法
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**Python** 使用 `__` 开头的名字来定义特殊的方法和属性，它们有：
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* `__init__()`
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* `__repr__()`
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* `__str__()`
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* `__call__()`
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* `__iter__()`
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* `__add__()`
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* `__sub__()`
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* `__mul__()`
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* `__rmul__()`
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* `__class__`
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* `__name__`
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## 构造方法 `__init__()`
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之前说到，在产生对象之后，我们可以向对象中添加属性。事实上，还可以通过构造方法，在构造对象的时候直接添加属性：
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In [1]:
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```py
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class Leaf(object):
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"""
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A leaf falling in the woods.
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"""
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def __init__(self, color='green'):
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self.color = color
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```
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默认属性值：
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In [2]:
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```py
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leaf1 = Leaf()
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print leaf1.color
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```
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```py
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green
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```
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传入有参数的值：
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In [3]:
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```py
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leaf2 = Leaf('orange')
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print leaf2.color
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```
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```py
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orange
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```
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回到森林的例子：
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In [4]:
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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):
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self.trees = np.zeros((150,150), dtype=bool)
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self.fires = np.zeros((150,150), dtype=bool)
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```
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我们在构造方法中定义了两个属性 `trees` 和 `fires`：
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In [5]:
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```py
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forest = Forest()
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forest.trees
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```
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Out[5]:
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```py
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array([[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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...,
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False]], dtype=bool)
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```
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In [6]:
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```py
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forest.fires
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```
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Out[6]:
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```py
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array([[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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...,
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False]], dtype=bool)
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```
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修改属性的值：
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In [7]:
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```py
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forest.trees[0,0]=True
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forest.trees
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```
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Out[7]:
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```py
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array([[ True, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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...,
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False]], dtype=bool)
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```
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改变它的属性值不会影响其他对象的属性值：
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In [8]:
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```py
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forest2 = Forest()
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forest2.trees
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```
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Out[8]:
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```py
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array([[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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...,
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False],
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[False, False, False, ..., False, False, False]], dtype=bool)
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```
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事实上，`__new__()` 才是真正产生新对象的方法，`__init__()` 只是对对象进行了初始化，所以：
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```py
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leaf = Leaf()
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```
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相当于
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```py
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my_new_leaf = Leaf.__new__(Leaf)
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Leaf.__init__(my_new_leaf)
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leaf = my_new_leaf
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```
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## 表示方法 `__repr__()` 和 `__str__()`
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In [9]:
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```py
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class Leaf(object):
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"""
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A leaf falling in the woods.
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"""
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def __init__(self, color='green'):
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self.color = color
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def __str__(self):
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"This is the string that is printed."
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return "A {} leaf".format(self.color)
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def __repr__(self):
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"This string recreates the object."
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return "{}(color='{}')".format(self.__class__.__name__, self.color)
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```
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`__str__()` 是使用 `print` 函数显示的结果：
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In [10]:
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```py
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leaf = Leaf()
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print leaf
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```
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```py
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A green leaf
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```
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`__repr__()` 返回的是不使用 `print` 方法的结果：
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In [11]:
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```py
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leaf
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```
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Out[11]:
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```py
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Leaf(color='green')
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```
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回到森林的例子：
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In [12]:
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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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```
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In [13]:
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```py
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forest = Forest()
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```
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`__str__()` 方法：
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In [14]:
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```py
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print forest
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```
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```py
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Forest
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```
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`__repr__()` 方法：
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In [15]:
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```py
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forest
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```
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Out[15]:
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```py
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Forest(size=(150, 150))
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```
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`__name__` 和 `__class__` 为特殊的属性：
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In [16]:
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```py
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forest.__class__
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```
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Out[16]:
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```py
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__main__.Forest
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```
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In [17]:
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```py
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forest.__class__.__name__
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```
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Out[17]:
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```py
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'Forest'
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``` |