Raw string of r'\' incorrectly being flagged as SyntaxError: unterminated string literal

[Loki-waterAIC]

Bug report

Bug description:

# Add a code block here, if required
Python 3.13.1 (tags/v3.13.1:0671451, Dec  3 2024, 19:06:28) [MSC v.1942 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
r'\'
SyntaxError: unterminated string literal (detected at line 1); perhaps you escaped the end quote?

The raw string of the single character '' incorrectly gets flagged as an unterminated string when both opening and closing quotations are present.

This also occurs in 3.12, 3.13; both on windows and linux.

CPython versions tested on:

3.13

Operating systems tested on:

Windows

[JeffersGlass]

I believe this is behaving as expected, as described in the docs (Section 2.4.1.1 - Escape Sequences):

Even in a raw literal, quotes can be escaped with a backslash, but the backslash remains in the result; for example, r""" is a valid string literal consisting of two characters: a backslash and a double quote; r"" is not a valid string literal (even a raw string cannot end in an odd number of backslashes). Specifically, a raw literal cannot end in a single backslash (since the backslash would escape the following quote character). Note also that a single backslash followed by a newline is interpreted as those two characters as part of the literal, not as a line continuation.

[Loki-waterAIC]

Unless an 'r' or 'R' prefix is present

If you look at the code above, the string is a raw string.

So, r'\' or R'\' should not have escape sequences considered during lexical analysis.
Which currently it is counting the \' as an escape sequence when it should not be considering the raw string indicator in-front of the string.

[Loki-waterAIC]

Note this error also occurs with 'r"", r''''''andr""""""` as well as their R counterparts, when it should not.

[JeffersGlass]

Yes, note the paragraph that I quoted that starts "even in a raw literal...". Note in particular that a raw literal cannot end in a single backslash, as noted in that section. This is a noted exception to the "raw strings do not have escape sequences" - quotes are special-cased to allow raw strings to contain quotes.

[Loki-waterAIC]

I see, I was confused due to preview formatting on GitHub.

This single exception in behavior is quite unexpected.

Do you know if there are any plans on changing this in the future to make raw strings consistent?

[JeffersGlass]

There's some context for it here: Design History and FAQ: Why can’t raw strings (r-strings) end with a backslash?.

[Loki-waterAIC]

Interesting, okay, I am satisfied with that answer. Thank you for explaining it to me.