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[英]how to see the body of __add__() method in python. whenever we see + operator, __add__() is called
[英]How to get __add__ called
class C(object):
def __init__(self, value):
self.value = value
def __add__(self, other):
if isinstance(other, C):
return self.value + other.value
if isinstance(other, Number):
return self.value + other
raise Exception("error")
c = C(123)
print c + c
print c + 2
print 2 + c
显然,前两个print语句将起作用,而第三个print语句由于int而失败。 add ()无法处理C类实例。
246
125
print 2 + c
TypeError: unsupported operand type(s) for +: 'int' and 'C'
有没有办法解决这个问题,所以2 + c会导致C. add ()被调用?
您还需要添加__radd__
来处理相反的情况:
def __radd__(self, other):
if isinstance(other, C):
return other.value + self.value
if isinstance(other, Number):
return other + self.value
return NotImplemented
并注意您不应提出例外情况; 返回NotImplemented
单例。 这样, 另一个对象仍然可以尝试为您的对象支持__add__
或__radd__
,并且也将有机会实现加法。
当您尝试添加两个类型a
和b
,Python首先尝试调用a.__add__(b)
; 如果该调用返回NotImplemented
,则尝试尝试b.__radd__(a)
。
演示:
>>> from numbers import Number
>>> class C(object):
... def __init__(self, value):
... self.value = value
... def __add__(self, other):
... print '__add__ called'
... if isinstance(other, C):
... return self.value + other.value
... if isinstance(other, Number):
... return self.value + other
... return NotImplemented
... def __radd__(self, other):
... print '__radd__ called'
... if isinstance(other, C):
... return other.value + self.value
... if isinstance(other, Number):
... return other + self.value
... return NotImplemented
...
>>> c = C(123)
>>> c + c
__add__ called
246
>>> c + 2
__add__ called
125
>>> 2 .__add__(c)
NotImplemented
>>> 2 + c
__radd__ called
125
您需要在__radd__
上实现__radd__
。
def __radd__(self, other):
return self.value + other
这会自动调用,因为int类将引发NotImplemented
错误
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