代码之家 › 专栏 › 技术社区 › Samuel Muldoon

如果“\uuuu add\uuuuuu”引发“NotImplementedError”,是否调用“\uuuu radd”?

operator-overloading python-3.x python

Samuel Muldoon · 技术社区 · 6 年前

假设我们写一个小类:

class K:
    pass
obj = K()

下面是代码。。。

total = 4 + obj

... 基本上与以下内容相同?

import io
try:
    total = 4.__add__(obj)
except NotImplementedError:
    try:
        total = obj.__radd__(4)
    except AttributeError:
        # type(obj) does not have an `__radd__` method
        with io.StringIO() as string_stream:
            print(
                "unsupported operand type(s) for +:",
                repr(type(4).__name__),
                "and",
                repr(type(obj).__name__),
                file=string_stream
            ) # `repr` puts quotes around the type names
            msg = string_stream.getvalue()
        raise TypeError(msg) from None

1 回复 | 直到 6 年前

Green Cloak Guy 6 年前

触发的行为 __radd__() 事实上,它不是一个 NotImplementedError 而是一个叫做 NotImplemented :

>>> help(NotImplemented)
Help on NotImplementedType object:

class NotImplementedType(object)
 |  Methods defined here:
 |  
 |  __reduce__(...)
 |      Helper for pickle.
 |  
 |  __repr__(self, /)
 |      Return repr(self).
 |  
 |  ----------------------------------------------------------------------
 |  Static methods defined here:
 |  
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.

A. 未实现错误 仍将作为错误传播。然而,返回 未实施 对象(而不是引发错误)将允许 __radd___;() 触发:

>>> class A:
...     def __add__(self, other):
...         raise NotImplementedError()
... 
>>> class B:
...     def __add__(self, other):
...         print("__add__ was called")
...     def __radd__(self, other):
...         print("__radd__ was called")
... 
>>> class C:
...     def __add__(self, other):
...         return NotImplemented
... 
>>> a, b, c = A(), B(), C()
>>> b + a
__add__ was called
>>> a + b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __add__
NotImplementedError
>>> b + c
__add__ was called
>>> c + b
__radd__ was called

推荐文章

Google User · Django管理员在`list_display中未显示`creation_date`字段`

1 年前

user29747013 · 如何创建一个新的数据框架,其中包含原始数据框架中列的聚合列?

1 年前

ÎÎÎ½Î· ÎÎ®Î¹Î½Î¿Ï · Python lxml.html语法错误:使用lxml find时XPATH的谓词无效

1 年前

user29715306 · from_users=和chats=电视节目中的差异

1 年前

Redshoe · 当执行numpy.genfromtxt()时,python是否会读取文件的所有行?

1 年前

RASEL MAHMUD · 为什么以及如何在is_even()函数内的IF条件中递归X变量在满足0后递增?[副本]

1 年前

prayner · 更新嵌套字典包含列表中的项

1 年前

Bringo Jr · 我可以在O(n)中解决这个问题吗?

1 年前

Dave · 如何在for循环中修改列表值

1 年前

Shukurullox Komiljonov · 从记录中获得相互和解。使用SQL

1 年前