赋值语句值 [英] Assignment statement value

问题描述

每个人都知道在 Python 中赋值不返回值，大概是为了避免在通常只是比较的情况下对 if 语句进行赋值:

<预><代码>>>>如果 a = b:文件<stdin>"，第 1 行如果 a = b:^语法错误:无效语法>>>如果 a == b:... 经过...

出于同样的原因，人们可能会怀疑同一语句上的多个赋值也是语法错误.

事实上，a = (b = 2) 不是一个有效的表达式:

<预><代码>>>>a = (b = 2)文件<stdin>"，第 1 行a = (b = 2)^语法错误:无效语法

所以，我的问题是:为什么 a = b = 2 在 Python 中可以像在其他赋值语句具有值的语言(如 C)中一样工作?

<预><代码>>>>a = b = c = 2>>>甲、乙、丙(2, 2, 2)

这种行为有记录吗?我在赋值语句文档中找不到任何关于此的内容:http://docs.python.org/reference/simple_stmts.html#assignment-statements

解决方案

就在语法中:

assignment_stmt ::= (target_list "=")+ (expression_list | yield_expression)

(target_list "=")+ 末尾的小+ 表示一个或多个".因此，a = b = c = 2 行不包含 3 个赋值语句，而是包含 一个带有 3 个目标列表的赋值语句.

每个目标列表依次只包含一个目标(在本例中为标识符).

它也在文本中(强调我的):

<块引用>

赋值语句 [...] 将单个结果对象从左到右分配给每个目标列表.

这可能会导致有趣的结果:

<预><代码>>>>(a,b) = c = (1,2)>>>(a, b, c)(1, 2, (1, 2))

Everybody knows that in Python assignments do not return a value, presumably to avoid assignments on if statements when usually just a comparison is intended:

>>> if a = b:
  File "<stdin>", line 1
    if a = b:
         ^
SyntaxError: invalid syntax

>>> if a == b:
...     pass
...

For the same reason, one could suspect that multiple assignments on the same statement were also syntax errors.

In fact, a = (b = 2) is not a valid expression:

>>> a = (b = 2)
  File "<stdin>", line 1
    a = (b = 2)
           ^
SyntaxError: invalid syntax

So, my question is: why a = b = 2 works in Python as it works in other languages where assignment statements have a value, like C?

>>> a = b = c = 2
>>> a, b, c
(2, 2, 2)

Is this behavior documented? I could not found anything about this in the assignment statement documentation: http://docs.python.org/reference/simple_stmts.html#assignment-statements

解决方案

It's right there in the syntax:

assignment_stmt ::=  (target_list "=")+ (expression_list | yield_expression)

The tiny + at the end of (target_list "=")+ means "one or more". So the line a = b = c = 2 does not consist of 3 assignment statements, but of a single assignment statement with 3 target lists.

Each target list in turn consist only of a single target (an identifier in this case).

It's also in the text (emphasis mine):

An assignment statement [...] assigns the single resulting object to each of the target lists, from left to right.

This can lead to interesting results:

>>> (a,b) = c = (1,2)
>>> (a, b, c)
(1, 2, (1, 2))

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