Python 运算符重载（十六）

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已于 2023-03-27 09:35:52 修改

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分类专栏： Python 笔记文章标签： python

于 2021-11-01 11:14:30 首次发布

本文链接：https://blog.youkuaiyun.com/chennai1101/article/details/121076347

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首先申明下，本文为笔者学习《Python学习手册》的笔记，并加入笔者自己的理解和归纳总结。

1. 索引getitem和setitem

>>> class Sample:
    def __init__(self, len):
        self.L = [x * 2 for x in range(len)]
    def __getitem__(self, index):
        return self.L[index]
    def __setitem__(self, index, value):
        self.L[index] = value

>>> s = Sample(5)
>>> s[2]                          # 索引操作
4

>>> s[0:3]                        # 切片操作
[0, 2, 4]

>>> s[2] = 1234                   # 索引设置
>>> s[2:]
[1234, 6, 8]

>>> for val in s:                 # for函数调用
    print val, 
0 2 1234 6 8

2. 迭代器iter和next

__iter__方法用来返回一个迭代器，next方法返回下一个值。

>>> class Sample:
    def __init__(self, len):
        self.L = [x * 2 for x in range(len)]
    def __iter__(self):           # 返回一个迭代器
        print "iter",
        self.index = -1
        return self
    def next(self):               # next方法，返回当前值
        print "next",
        if self.index >= (len(self.L) - 1):
            raise StopIteration
        self.index += 1
        return self.L[self.index]

>>> s = Sample(5)
>>> S = iter(s)
iter

>>> next(S)
next
0

>>> next(S)
next
2

3. 包含关系contains

__contains__必须配合迭代器使用，用来查询是否包含。

没有contains方法

>>> s = Sample(5)
>>> 3 in s                       # 默认从迭代器中取值比较
iter next next next next next next
False

>>> 2 in s
iter next next
True

增加contains方法。

>>> class Sample:
    def __init__(self, len):
        self.L = [x * 2 for x in range(len)]
    def __iter__(self):
        print "iter",
        self.index = -1
        return self
    def next(self):
        print "next",
        if self.index >= (len(self.L) - 1):
            raise StopIteration
        self.index += 1
        return self.L[self.index]
    def __contains__(self, value):
        return value in (10, 20, 30)

>>> s = Sample(5)
>>> 2 in s
False

>>> 10 in s
True

4. 属性引用getattr和setattr

自定义类，只包含name属性。

>>> class Sample:
    def __getattr__(self, name):
        if (name == "name"):
            return "Mike"
        else:
            raise AttributeError
    def __setattr__(self, name, value):
        if (name == "name"):
            print value
        else:
            raise AttributeError

>>> s = Sample()
>>> s.name                       # 访问name属性
'Mike'

>>> s.name = "Jack"              # 设置name属性
Jack

>>> s.age                        # 访问age属性，发生异常
Traceback (most recent call last):
  File "<pyshell#61>", line 1, in <module>
    s.age
  File "<pyshell#57>", line 6, in __getattr__
    raise AttributeError
AttributeError

>>> s.age = 14                   # 设置age属性，发生异常
Traceback (most recent call last):
  File "<pyshell#62>", line 1, in <module>
    s.age = 14
  File "<pyshell#57>", line 11, in __setattr__
    raise AttributeError
AttributeError

5. 加法add、radd和iadd

add方法，实现加法操作

>>> class Sample:
    def __init__(self, val):
        self.val = val
    def __add__(self, val):
        print "add", val 
        return Sample(self.val + val)
    def __repr__(self):
        return str(self.val)

>>> s = Sample(4)
>>> s + 4                        # 加法在实例右边，调用add方法
add 4
8

>>> s                            # 实例值不改变
4

>>> s += 5                       # 自加操作，同样调用add方法
add 5

>>> s                            # 实例值被修改
9

>>> 4 + s                        # 不支持加号在实例左边
Traceback (most recent call last):
  File "<pyshell#87>", line 1, in <module>
    4 + s
TypeError: unsupported operand type(s) for +: 'int' and 'instance'

radd方法支持加号在实例右边

>>> class Sample:
    def __init__(self, val):
        self.val = val
    def __add__(self, val):
        print "add", val 
        return Sample(self.val + val)
    def __radd__(self, val):
        print "radd", val 
        return Sample(self.val + val)
    def __repr__(self):
        return str(self.val)

>>> s = Sample(4)
>>> 3 + s                        # 加号在实例左边，使用radd方法
radd 3
7

>>> s
4

iadd方法实现自加。

>>> class Sample:
    def __init__(self, val):
        self.val = val
    def __add__(self, val):
        print "add", val 
        return Sample(self.val + val)
    def __iadd__(self, val):
        print "iadd", val 
        return Sample(self.val + val)
    def __repr__(self):
        return str(self.val)

>>> s = Sample(4)                # 自加使用优先使用iadd方法
>>> s += 6
iadd 6

>>> s
10

6. call方法

call方法使类成为一个可调用函数。

>>> class Sample:
    def __call__(self, *pargs, **kargs):
        print "call", pargs, kargs

>>> S = Sample()
>>> S(1, 3, 5, name="Mike", age=23)
call (1, 3, 5) {'age': 23, 'name': 'Mike'}

7. 比较表达式lt、gt、eq和ne

>>> class Sample:
    def __init__(self, val):
        self.val = val
    def __gt__(self, val):
        print "gt"
    	return self.val > val
    def __lt__(self, val):
        print "lt"
        return self.val < val
    def __eq__(self, val):
        print "eq"
        return self.val == val
    def __ne__(self, val):
        print "ne"
        return self.val != val

>>> s = Sample(5)
>>> s > 4                        # > 调用gt方法
gt
True

>>> s < 6                        # < 调用lt方法
lt
True

>>> 4 == s                       # == 调用eq方法
eq
False

>>> 4 != s                       # != 调用ne方法
ne
True

8. 内置函数bool和len

>>> class Sample:
    def __init__(self, val):
        self.val = val
    def __nonzero__(self):
        return self.val > 5
    def __len__(self):
        return self.val

>>> bool(Sample(4)), bool(Sample(6))
(False, True)

>>> len(Sample(4)), len(Sample(6))
(4, 6)