为什么不能在f字符串中使用反斜杠? [英] Why isn't it possible to use backslashes in f-strings?

问题描述

在Python> = 3.6中，f字符串可用作str.format方法的替代.举一个简单的例子，它们是等效的:

In Python >=3.6, f-strings can be used as a replacement for the str.format method. As a simple example, these are equivalent:

'{} {}'.format(2+2, "hey")
f'{2+2} {"hey"}'

不管格式说明符，我基本上可以将f字符串中大括号内的str.format位置参数.特别要注意的是，尽管看起来有点笨拙，但我只允许在其中输入str文字.

Disregarding format specifiers, I can basically move the positional arguments of str.format inside braces in an f-string. Note specifically that I am allowed to just put str literals in here, although it may seem a bit unwieldy.

但是有一些限制.具体来说，任何形状或形式的反斜杠都不允许在其中f弦的花括号:

There are however some limitations. Specifically, backslashes in any shape or form are disallowed inside the braces of an f-string:

'{}'.format("new\nline")  # legal
f'{"new\nline"}'          # illegal
f'{"\\"}'                 # illegal

如果在括号内，我什至不能使用\来分隔长行；

I cannot even use \ to split up a long line if it's inside the braces;

f'{2+\
2}'     # illegal

即使在普通str的内部完全允许使用\，

even though this usage of \ is perfectly allowed inside normal str's;

'{\
}'.format(2+2)  # legal

在我看来，如果解析器在f字符串大括号内的所有位置都看到\字符，则将硬停止编码到解析器中.为什么要实施此限制?尽管 docs 指定了此行为，但这并不能说明为什么

It seems to me that a hard stop is coded into the parser if it sees the \ character at all inside the braces of an f-string. Why is this limitation implemented? Though the docs specify this behavior, it does not justify why.

推荐答案

您似乎期望

'{}'.format("new\nline")

和

f'{"new\nline"}'

等同.这不是我所期望的，也不是f字符串中的反斜杠在Python 3.6的预发行版本中如何起作用的，其中允许在括号之间使用反斜杠.那时，您会收到一个错误消息，因为

to be equivalent. That's not what I would expect, and it's not how backslashes in f-strings worked back in the pre-release versions of Python 3.6 where backslashes between the braces were allowed. Back then, you'd get an error because

"new
line"

不是有效的Python表达式.

is not a valid Python expression.

如前所述，花括号中的反斜杠令人困惑且不明确，它们为已禁止以免引起混淆:

As just demonstrated, backslashes in the braces are confusing and ambiguous, and they were banned to avoid confusion:

这样做的目的是禁止像这样的复杂代码:

The point of this is to disallow convoluted code like:

>>> d = {'a': 4}
>>> f'{d[\'a\']}'
'4'

此外，我禁止将转义用于方括号，如:

In addition, I'll disallow escapes to be used for brackets, as in:

>>> f'\x7bd["a"]}'
'4'

(其中chr(0x7b)=="{").

(where chr(0x7b) == "{").

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