为什么分配给空列表(例如 [] = “")不是错误? [英] Why isn't assigning to an empty list (e.g. [] = "") an error?
问题描述
在 python 3.4 中，我正在输入
In python 3.4, I am typing
[] = ""
并且它工作正常，没有引发异常.虽然当然 [] 不等于 "" 之后.
and it works fine, no Exception is raised. Though of course [] is not equal to "" afterwards.
[] = ()
也可以正常工作.
"" = []
虽然按预期引发异常，
() = ""
虽然按预期引发异常.发生什么了?
raises an exception as expected though. So, what's going on?
推荐答案
你不是为了平等而比较.您正在分配.
You are not comparing for equality. You are assigning.
Python 允许您分配给多个目标:
Python allows you to assign to multiple targets:
foo, bar = 1, 2
将这两个值分别分配给 foo 和 bar.您所需要的只是右侧的 sequence 或 iterable，以及左侧的名称列表或元组.
assigns the two values to foo and bar, respectively. All you need is a sequence or iterable on the right-hand side, and a list or tuple of names on the left.
当你这样做时:
[] = ""
您将一个 空 序列(空字符串仍然是序列)分配给一个空名称列表.
you assigned an empty sequence (empty strings are sequences still) to an empty list of names.
这和做的事情本质上是一样的:
It is essentially the same thing as doing:
[foo, bar, baz] = "abc"
你最终得到 foo = "a"、bar = "b" 和 baz = "c"，但更少字符.
where you end up with foo = "a", bar = "b" and baz = "c", but with fewer characters.
然而，您不能为字符串赋值，因此赋值左侧的 "" 永远不起作用，并且总是语法错误.
You cannot, however, assign to a string, so "" on the left-hand side of an assignment never works and is always a syntax error.
请参阅赋值语句文档一个>:
See the Assignment statements documentation:
赋值语句评估表达式列表(记住这可以是单个表达式或逗号分隔的列表，后者产生一个元组)并将单个结果对象从左到右分配给每个目标列表.
An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right.
和
将对象分配给目标列表，可选地括在圆括号或方括号中，递归定义如下.
Assignment of an object to a target list, optionally enclosed in parentheses or square brackets, is recursively defined as follows.
强调我的.
那个 Python 不会为空列表抛出语法错误实际上是一个错误！官方记录的语法不允许空的目标列表，对于空的 () 你确实会得到一个错误.请参阅错误 23275；它被认为是一个无害的错误:
That Python doesn't throw a syntax error for the empty list is actually a bit of a bug! The officially documented grammar doesn't allow for an empty target list, and for the empty () you do get an error. See bug 23275; it is considered a harmless bug:
首先要认识到这已经存在很长时间并且是无害的.
The starting point is recognizing that this has been around for very long time and is harmless.
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