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Fix syntax error false positives for escapes and quotes in f-strings (#20867)

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Summary
--

Fixes #20844 by refining the unsupported syntax error check for [PEP
701]
f-strings before Python 3.12 to allow backslash escapes and escaped
outer quotes
in the format spec part of f-strings. These are only disallowed within
the
f-string expression part on earlier versions. Using the examples from
the PR:

```pycon
>>> f"{1:\x64}"
'1'
>>> f"{1:\"d\"}"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier '"d"' for object of type 'int'
```

Note that the second case is a runtime error, but this is actually
avoidable if
you override `__format__`, so despite being pretty weird, this could
actually be
a valid use case.

```pycon
>>> class C:
...     def __format__(*args, **kwargs): return "<C>"
...
>>> f"{C():\"d\"}"
'<C>'
```

At first I thought narrowing the range we check to exclude the format
spec would
only work for escapes, but it turns out that cases like `f"{1:""}"` are
already
covered by an existing `ParseError`, so we can just narrow the range of
both our
escape and quote checks.

Our comment check also seems to be working correctly because it's based
on the
actual tokens. A case like
[this](https://play.ruff.rs/9f1c2ff2-cd8e-4ad7-9f40-56c0a524209f):

```python
f"""{1:# }"""
```

doesn't include a comment token, instead the `#` is part of an
`InterpolatedStringLiteralElement`.

Test Plan
--

New inline parser tests

[PEP 701]: https://peps.python.org/pep-0701/
This commit is contained in:
Brent Westbrook 2025-10-15 09:23:16 -04:00 • committed by GitHub
parent 8817ea5c84
commit 8b9ab48ac6
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9 changed files with 317 additions and 62 deletions
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54
crates/ruff_python_formatter/tests/snapshots/format@expression__fstring.py.snap
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@ -712,8 +712,6 @@ f'{1:hy "user"}'
f'{1: abcd "{1}" }'
f'{1: abcd "{'aa'}" }'
f'{1=: "abcd {'aa'}}'
# FIXME(brent) This should not be a syntax error on output. The escaped quotes are in the format
# spec, which is valid even before 3.12.
f'{x:a{z:hy "user"}} \'\'\''
# Changing the outer quotes is fine because the format-spec is in a nested expression.
@ -1536,8 +1534,6 @@ f'{1:hy "user"}'
f'{1: abcd "{1}" }'
f'{1: abcd "{"aa"}" }'
f'{1=: "abcd {'aa'}}'
# FIXME(brent) This should not be a syntax error on output. The escaped quotes are in the format
# spec, which is valid even before 3.12.
f"{x:a{z:hy \"user\"}} '''"
# Changing the outer quotes is fine because the format-spec is in a nested expression.
@ -2365,8 +2361,6 @@ f'{1:hy "user"}'
f'{1: abcd "{1}" }'
f'{1: abcd "{"aa"}" }'
f'{1=: "abcd {'aa'}}'
# FIXME(brent) This should not be a syntax error on output. The escaped quotes are in the format
# spec, which is valid even before 3.12.
f"{x:a{z:hy \"user\"}} '''"
# Changing the outer quotes is fine because the format-spec is in a nested expression.
@ -2418,30 +2412,6 @@ print(f"{ {}, 1 }")
### Unsupported Syntax Errors
error[invalid-syntax]: Cannot use an escape sequence (backslash) in f-strings on Python 3.10 (syntax was added in Python 3.12)
--> fstring.py:764:19
|
762 | # FIXME(brent) This should not be a syntax error on output. The escaped quotes are in the format
763 | # spec, which is valid even before 3.12.
764 | f"{x:a{z:hy \"user\"}} '''"
| ^
765 |
766 | # Changing the outer quotes is fine because the format-spec is in a nested expression.
|
warning: Only accept new syntax errors if they are also present in the input. The formatter should not introduce syntax errors.
error[invalid-syntax]: Cannot use an escape sequence (backslash) in f-strings on Python 3.10 (syntax was added in Python 3.12)
--> fstring.py:764:13
|
762 | # FIXME(brent) This should not be a syntax error on output. The escaped quotes are in the format
763 | # spec, which is valid even before 3.12.
764 | f"{x:a{z:hy \"user\"}} '''"
| ^
765 |
766 | # Changing the outer quotes is fine because the format-spec is in a nested expression.
|
warning: Only accept new syntax errors if they are also present in the input. The formatter should not introduce syntax errors.
error[invalid-syntax]: Cannot reuse outer quote character in f-strings on Python 3.10 (syntax was added in Python 3.12)
--> fstring.py:178:8
|
@ -2452,27 +2422,3 @@ error[invalid-syntax]: Cannot reuse outer quote character in f-strings on Python
179 | f"foo {'"bar"'}"
|
warning: Only accept new syntax errors if they are also present in the input. The formatter should not introduce syntax errors.
error[invalid-syntax]: Cannot reuse outer quote character in f-strings on Python 3.10 (syntax was added in Python 3.12)
--> fstring.py:773:14
|
771 | f'{1=: "abcd \'\'}' # Don't change the outer quotes, or it results in a syntax error
772 | f"{1=: abcd \'\'}" # Changing the quotes here is fine because the inner quotes aren't the opposite quotes
773 | f"{1=: abcd \"\"}" # Changing the quotes here is fine because the inner quotes are escaped
| ^
774 | # Don't change the quotes in the following cases:
775 | f'{x=:hy "user"} \'\'\''
|
warning: Only accept new syntax errors if they are also present in the input. The formatter should not introduce syntax errors.
error[invalid-syntax]: Cannot reuse outer quote character in f-strings on Python 3.10 (syntax was added in Python 3.12)
--> fstring.py:764:14
|
762 | # FIXME(brent) This should not be a syntax error on output. The escaped quotes are in the format
763 | # spec, which is valid even before 3.12.
764 | f"{x:a{z:hy \"user\"}} '''"
| ^
765 |
766 | # Changing the outer quotes is fine because the format-spec is in a nested expression.
|
warning: Only accept new syntax errors if they are also present in the input. The formatter should not introduce syntax errors.