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ruff/crates/ruff_python_parser
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Brent Westbrook 2fbc4d577e
[syntax-errors] Document behavior of global declarations in try nodes before 3.13 (#17285) 
Summary
--

This PR extends the documentation of the `LoadBeforeGlobalDeclaration`
check to specify the behavior on versions of Python before 3.13. Namely,
on Python 3.12, the `else` clause of a `try` statement is visited before
the `except` handlers:

```pycon
Python 3.12.9 (main, Feb 12 2025, 14:50:50) [Clang 19.1.6 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 10
>>> def g():
...     try:
...         1 / 0
...     except:
...         a = 1
...     else:
...         global a
...
>>> def f():
...     try:
...         pass
...     except:
...         global a
...     else:
...         print(a)
...
  File "<stdin>", line 5
SyntaxError: name 'a' is used prior to global declaration

```

The order is swapped on 3.13 (see
[CPython#111123](https://github.com/python/cpython/issues/111123)):

```pycon
Python 3.13.2 (main, Feb  5 2025, 08:05:21) [GCC 14.2.1 20250128] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 10
... def g():
...     try:
...         1 / 0
...     except:
...         a = 1
...     else:
...         global a
...
  File "<python-input-0>", line 8
    global a
    ^^^^^^^^
SyntaxError: name 'a' is assigned to before global declaration
>>> def f():
...     try:
...         pass
...     except:
...         global a
...     else:
...         print(a)
...
>>>
```

The current implementation of PLE0118 is correct for 3.13 but not 3.12:
[playground](https://play.ruff.rs/d7467ea6-f546-4a76-828f-8e6b800694c9)
(it flags the first case regardless of Python version).

We decided to maintain this incorrect diagnostic for Python versions
before 3.13 because the pre-3.13 behavior is very unintuitive and
confirmed to be a bug, although the bug fix was not backported to
earlier versions. This can lead to false positives and false negatives
for pre-3.13 code, but we also expect that to be very rare, as
demonstrated by the ecosystem check (before the version-dependent check
was reverted here).

Test Plan
--

N/a
2025-04-09 12:54:21 -04:00
..
resources [syntax-errors] Async comprehension in sync comprehension (#17177) 2025-04-08 12:50:52 -04:00
src [syntax-errors] Document behavior of global declarations in try nodes before 3.13 (#17285) 2025-04-09 12:54:21 -04:00
tests [syntax-errors] Async comprehension in sync comprehension (#17177) 2025-04-08 12:50:52 -04:00
Cargo.toml Allow passing ParseOptions to inline tests (#16357) 2025-02-27 10:23:15 -05:00
CONTRIBUTING.md Update pre-commit dependencies (#17073) 2025-03-31 07:42:15 +00:00
README.md Add basic docs for the parser crate (#11199) 2024-04-29 17:08:07 +00:00

README.md

Ruff Python Parser

Ruff's Python parser is a hand-written recursive descent parser which can parse Python source code into an Abstract Syntax Tree (AST). It also utilizes the Pratt parsing technique to parse expressions with different precedence.

Try out the parser in the playground.

Python version support

The parser supports the latest Python syntax, which is currently Python 3.12. It does not throw syntax errors if it encounters a syntax feature that is not supported by the target-version. This will be fixed in a future release (see https://github.com/astral-sh/ruff/issues/6591).

Contributing

Refer to the contributing guidelines to get started and GitHub issues with the parser label for issues that need help.