特殊方法

Python 使用 __ 开头的名字来定义特殊的方法和属性，它们有：

• init()
• repr()
• str()
• call()
• iter()
• add()
• sub()
• mul()
• rmul()
• class
• name

构造方法 init()

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：

In [1]:

class Leaf(object):
    """
    A leaf falling in the woods.
    """
    def __init__(self, color='green'):
        self.color = color

默认属性值：

In [2]:

leaf1 = Leaf()
 
print leaf1.color

green

传入有参数的值：

In [3]:

leaf2 = Leaf('orange')
 
print leaf2.color

orange

回到森林的例子：

In [4]:

import numpy as np
 
class Forest(object):
    """ Forest can grow trees which eventually die."""
    def __init__(self):
        self.trees = np.zeros((150,150), dtype=bool)
        self.fires = np.zeros((150,150), dtype=bool)

我们在构造方法中定义了两个属性 trees 和 fires：

In [5]:

forest = Forest()
 
forest.trees

Out[5]:

array([[False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       ..., 
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False]], dtype=bool)

In [6]:

forest.fires

Out[6]:

array([[False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       ..., 
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False]], dtype=bool)

修改属性的值：

In [7]:

forest.trees[0,0]=True
forest.trees

Out[7]:

array([[ True, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       ..., 
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False]], dtype=bool)

改变它的属性值不会影响其他对象的属性值：

In [8]:

forest2 = Forest()
 
forest2.trees

Out[8]:

array([[False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       ..., 
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False],
       [False, False, False, ..., False, False, False]], dtype=bool)

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：

leaf = Leaf()

相当于

my_new_leaf = Leaf.__new__(Leaf)
Leaf.__init__(my_new_leaf)
leaf = my_new_leaf

表示方法 repr() 和 str()

In [9]:

class Leaf(object):
    """
    A leaf falling in the woods.
    """
    def __init__(self, color='green'):
        self.color = color
    def __str__(self):
        "This is the string that is printed."
        return "A {} leaf".format(self.color)
    def __repr__(self):
        "This string recreates the object."
        return "{}(color='{}')".format(self.__class__.__name__, self.color)

str() 是使用 print 函数显示的结果：

In [10]:

leaf = Leaf()
 
print leaf

A green leaf

repr() 返回的是不使用 print 方法的结果：

In [11]:

leaf

Out[11]:

Leaf(color='green')

回到森林的例子：

In [12]:

import numpy as np
 
class Forest(object):
    """ Forest can grow trees which eventually die."""
    def __init__(self, size=(150,150)):
        self.size = size
        self.trees = np.zeros(self.size, dtype=bool)
        self.fires = np.zeros((self.size), dtype=bool)
 
    def __repr__(self):
        my_repr = "{}(size={})".format(self.__class__.__name__, self.size)
        return my_repr
 
    def __str__(self):
        return self.__class__.__name__

In [13]:

forest = Forest()

str() 方法：

In [14]:

print forest

Forest

repr() 方法：

In [15]:

forest

Out[15]:

Forest(size=(150, 150))

name 和 class 为特殊的属性：

In [16]:

forest.__class__

Out[16]:

__main__.Forest

In [17]:

forest.__class__.__name__

Out[17]:

'Forest'

原文: https://nbviewer.jupyter.org/github/lijin-THU/notes-python/blob/master/08-object-oriented-programming/08.05-special-method.ipynb