python - 如何调用 __add__

Question

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 语句可以工作，第三个失败，因为 int.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？

Answer 1

您需要添加 __radd__以及处理相反的情况:

def __radd__(self, other):
    if isinstance(other, C):
        return other.value + self.value
    if isinstance(other, Number):
        return other + self.value
    return NotImplemented

并注意你不应该抛出异常；而是返回 NotImplemented 单例。这样，other 对象仍然可以尝试为您的对象支持 __add__ 或 __radd__，并且也有机会实现加法。

当您尝试添加两种类型a 和b 时，Python 首先尝试调用a.__add__(b)；如果该调用返回 NotImplemented，则会尝试 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