在 Python 中， a += b 并不总是等价于 a = a + b

[itskingname]

大家经常在一些博客中看到这样的说法：

a += 1

等价于

a = a + 1

这种说法实际上并不准确。

我们来看一个例子：

>>> a = [1, 2, 3]
>>> a += (4,)
>>> a
[1, 2, 3, 4]

>>> a = [1, 2, 3]
>>> a = a + (4,)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "tuple") to list

这里报错了，说明a += b和a = a + b并不是完全等价的。

请点击查看完整内容：在 Python 中，a += b 并不总是等价于 a = a + b

[misaka19000]

不错，学习了

[OysterQAQ]

学习了，查了下，在 java 中从本质讲这两者确实是两码事

[sdijeenx]

是这样的。

These methods are called to implement the augmented arithmetic assignments (+=, -=, *=, @=, /=, //=, %=, **=, <<=, >>=, &=, ^=, |=). These methods should attempt to do the operation in-place (modifying self) and return the result (which could be, but does not have to be, self). If a specific method is not defined, the augmented assignment falls back to the normal methods. For instance, if x is an instance of a class with an __iadd__() method, x += y is equivalent to x = x.__iadd__(y) . Otherwise, x.__add__(y) and y.__radd__(x) are considered, as with the evaluation of x + y. In certain situations, augmented assignment can result in unexpected errors (see Why does a_tuple[i] += [‘ item ’] raise an exception when the addition works?), but this behavior is in fact part of the data model.
https://docs.python.org/3/reference/datamodel.html


Why does a_tuple[i] += [‘ item ’] raise an exception when the addition works?
This is because of a combination of the fact that augmented assignment operators are assignment operators, and the difference between mutable and immutable objects in Python.

This discussion applies in general when augmented assignment operators are applied to elements of a tuple that point to mutable objects, but we ’ ll use a list and += as our exemplar.
https://docs.python.org/3/faq/programming.html#faq-augmented-assignment-tuple-error

[rekulas]

这个，我觉得别人说的是数值吧，你这个改成了 dict 相加不符合别人的原意了?（非 python 研究者，这似乎是 dict 吧）

[bumz]

本来就不是一回事

只不过对于值语义的类型，这二者的结果期望是等价的

list 是引用语义的类型，不等价

[whwq2012]

偷换概念。。一般说的+=是数字运算，又不是其他对象。。

[helloworld000]

现在从 python 开始学编程的人都已经不知道 数据类型这么个东西了吗。。。。

[mario85]

推公众号的吧。
@Livid

[itskingname]

@mario85 已 block

[pkookp8]

这算是运算符重载吧
你想写成什么样都可以

[lance6716]

@itskingname 竟然还反向 b，牛逼牛逼

[junmoxiao]

，，，，，这，，，水平有点低呀

[ETiV]

大家还是拒绝服务攻击吧

[leido]

这是运算符重载，楼主偷换概念

[Trumeet]

数据类型 数据类型

[master13]

那按照楼主这逻辑，a+b=b+a 也不准确，当 a 和 b 都是 str 的时候……

玩这个有意思吗

[Qzier]

废话，+ 对应的是 __add__ 方法，+= 对应的是 __iadd__ 或 __iconcat__ 方法，当然不同，完全取决于你的实现！

[Qzier]

补充下 + 对应的方法还有 __concat__，是针对序列类型的

[xiri]

这是 python 里面特殊的运算符重载吧，一般说的+=是针对数值运算的。按楼主这样描述，我自己重定义一下，还可以实现其他功能呢

[fan123199]

介绍知识点不错，但不要搞个大新闻（常指标题浮夸），因为这个知识点很可能是常识。