Python全栈之路系列之面向对象运算符重载

运算符重载的概念如下:

  1. 运算符重载让类拦截常规的Python运算;
  2. 类可重载所有Python表达式运算符;
  3. 类也可重载打印、函数调用、属性点号运算等内置运算;
  4. 重载是类实例的行为想内置类型;
  5. 重载是通过提供特殊名称的类方法来实现的;

常见的运算符重载方法

方法 重载 调用
__init__ 构造函数 对象建立:X = Class(args)
__del__ 解析函数 X对象收回
__add__ 运算符+ 如果没有__iadd__,X+Y,X+=Y
__or__ 运算符或 如果没有__ior__
__repr__,__str__ 打印、转换 print(X)、repr(X)、str(X)
__call__ 函数调用 X(*args, **kwargs)
__getattr__ 点号运算 X.undefined
__setattr__ 属性赋值语句 X.any = value
__delattr__ 属性删除 del X.any
__getattribute__ 属性获取 X.any
__getitem__ 索引运算 X[key],X[i:j],没__iter__时的for循环和其他迭代器
__setitem__ 索引赋值语句 X[key]=value,X[i:k]=sequence
__delitem__ 索引和分片删除 del X[key], del X[i:j]
__len__ 长度 len(X),如果没有__bool__,真值测试
__bool__ 布尔测试 bool(X),真测试
__lt____gt____le____ge____eq____ne__ 特定的比较 X<Y,X>Y…
__radd__ 右侧加法 Other + X
__iadd__ 增强的加法 X += Y
__iter____next__ 迭代环境 I=iter(X),next(I)
__contains__ 成员关系测试 item in X(任何可迭代对象)
__index__ 整数值 hex(X),bin(X),oct(X),o[X],O[X:]
__enter__,__exit__ 环境管理器 with obj as var:
__get__,__set__,__delete__ 描述符属性 X.attr,X.attr=Value,del X.attr
__new__ 创建 __init__之前创建对象

所有重载方法的名称前后都有两个下划线字符,以便把同类中定义的变量名区别开来。

构造函数和表达式:__init____sub__

>>> class Number:
... def __init__(self, start):
... self.data = start
... def __sub__(self, other):
... return Number(self.data - other)
...
>>> X = Number(5)
>>> Y = X - 2
>>> Y
<__main__.Number object at 0x10224d550>
>>> Y.data
3

索引和分片: __getitem____setitem__

基本索引

>>> class Index:
... def __getitem__(self, item):
... return item ** 2
...
>>>
>>> for i in range(5):
... I = Index()
... print(I[i], end=' ')
...
0 1 4 9 16

切片索引

>>> class Index:
... data = [5, 6, 7, 8, 9]
... def __getitem__(self, item):
... print('getitem: ', item)
... return self.data[item]
... def __setitem__(self, key, value):
... self.data[key] = value
...
>>> X = Index()
>>> print(X[1:4])
getitem: slice(1, 4, None)
[6, 7, 8]
>>> X[1:4] = (1, 1, 1)
>>> print(X[1:4])
getitem: slice(1, 4, None)
[1, 1, 1]

索引迭代:__getitem__

如果重载了这个方法,for循环每次循环时都会调用类的getitem方法;

>>> class stepper:
... def __getitem__(self, item):
... return self.data[item].upper()
...
>>>
>>> X = stepper()
>>> X.data = 'ansheng'
>>> for item in X:
... print(item)
...
A
N
S
H
E
N
G

迭代器对象:__iter____next__

>>> class Squares:
... def __init__(self, start, stop):
... self.value = start - 1
... self.stop = stop
... def __iter__(self):
... return self
... def __next__(self):
... if self.value == self.stop:
... raise StopIteration
... self.value += 1
... return self.value ** 2
...
>>> for i in Squares(1, 5):
... print(i)
...
1
4
9
16
25

成员关系:__contains____iter____getitem__

class Iters:
def __init__(self, value):
self.data = value

def __getitem__(self, item):
print('get[%s]' % item, end='')
return self.data[item]

def __iter__(self):
print('iter>==', end='')
self.ix = 0
return self

def __next__(self):
print('next:', end='')
if self.ix == len(self.data): raise StopIteration
item = self.data[self.ix]
self.ix += 1
return item

def __contains__(self, item):
print('contains: ', end=' ')
return item in self.data


X = Iters([1, 2, 3, 4, 5])
print(3 in X)
for i in X:
print(i, end='|')

print([i ** 2 for i in X])
print(list(map(bin, X)))

I = iter(X)
while True:
try:
print(next(I), end=' @')
except StopIteration as e:
break

属性引用:__getattr____setattr__

当通过未定义的属性名称和实例通过点号进行访问时,就会用属性名称作为字符串调用这个方法,但如果类使用了继承,并且在超类中可以找到这个属性,那么就不会触发。

>>> class empty:
... def __getattr__(self, item):
... if item == 'age':
... return 40
... else:
... raise AttributeError(item)
...
>>>
>>> x = empty()
>>> print(x.age)
40
>>> print(x.name)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in __getattr__
AttributeError: name
>>> class accesscontrol:
... def __setattr__(self, key, value):
... if key == 'age':
... self.__dict__[key] = value
... else:
... raise AttributeError(key + ' not allowed')
...
>>>
>>> x = accesscontrol()
>>> x.age = 40
>>> print(x.age)
40
>>> x.name = 'Hello'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in __setattr__
AttributeError: name not allowed

__repr____str__会返回字符串表达式

__repr____str__都是为了更友好的显示,具体来说,如果在终端下print(Class)则会调用__repr__,非终端下会调用__str__方法,且这两个方法只能返回字符串;

class adder:
def __init__(self, value=0):
self.data = value

def __add__(self, other):
self.data += other

def __repr__(self):
return 'addrepr(%s)' % self.data

def __str__(self):
return 'N: %s' % self.data


x = adder(2)
x + 1
print(x)
print((str(x), repr(x)))

右侧加法和原处加法: __radd____iadd__

只有当+右侧的对象是类实例,而左边对象不是类实例的时候,Python才会调用__radd__

class Commuter:
def __init__(self, val):
self.val = val

def __add__(self, other):
print('add', self.val, other)
return self.val + other

def __radd__(self, other):
print('radd', self.val, other)
return other + self.val


x = Commuter(88)
y = Commuter(99)
print(x + 1)
print('')
print(1 + y)
print('')
print(x + y)

使用__iadd__进行原处加法

class Number:
def __init__(self, val):
self.val = val

def __iadd__(self, other):
self.val += other
return self


x = Number(5)
x += 1
x += 1
print(x.val)


class Number:
def __init__(self, val):
self.val = val

def __add__(self, other):
return Number(self.val + other)


x = Number(5)
x += 1
x += 1
print(x.val)

Call表达式:__call__

当调用类实例时执行__call__方法

class Callee:
def __call__(self, *args, **kwargs):
print('Callee:', args, kwargs)


C = Callee()
C(1, 2, 3)
C(1, 2, 3, x=1, y=2, z=3)


class Prod:
def __init__(self, value):
self.value = value

def __call__(self, other):
return self.value * other


x = Prod(3)
print(x(3))
print(x(4))

比较:__lt__,__gt__和其他方法

类可以定义方法来捕获所有的6种比较运算符:<、>、<=、>=、==和!=

class C:
data = 'spam'

def __gt__(self, other):
return self.data > other

def __lt__(self, other):
return self.data < other

x = C()
print(x > 'han')
print(x < 'han')

布尔值测试:boollen

class Truth:
def __bool__(self):
return True


X = Truth()
if X: print('yes')


class Truth:
def __bool__(self):
return False

X = Truth()
print(bool(X))

如果没有这个方法,Python退而求其次的求长度,因为一个非空对象看作是真:

>>> class Truth:
... def __len__(self): return 0
...
>>> X = Truth()
>>> if not X: print('no')
...
no

如果两个方法都有,__bool__会胜过__len__

>>> class Truth:
... def __bool__(self): return True
... def __len__(self): return 0
...
>>> X = Truth()
>>> bool(X)
True

如果两个方法都没有定义,对象毫无疑义的看作为真:

>>> class Truth: pass
...
>>> bool(Truth)
True

对象解析函数:__del__

每当实例产生时,就会调用init构造函数,每当实例空间被收回时,它的对立面__del__,也就是解析函数,就会自动执行;

class Life:
def __init__(self, name='unknown'):
print('Hello, ', name)
self.name = name

def __del__(self):
print('Goodbye', self.name)


brian = Life('Brian')
brian = 'loretta'

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