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Merged
AlexWaygood merged 14 commits into from
Nov 28, 2025
Merged

[ty] Implement typing.final for methods #21646

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0e025d6
[ty] Implement `typing.final` for methods
AlexWaygood Nov 26, 2025
b885cb7
fix unrelated mdtest failure
AlexWaygood Nov 26, 2025
3ca2675
another edge case
AlexWaygood Nov 26, 2025
47d07e2
another edge case
AlexWaygood Nov 26, 2025
486caf5
another edge case
AlexWaygood Nov 26, 2025
76ccc48
try to optimize
AlexWaygood Nov 26, 2025
408895e
use a standalone query to check if it's a function definition
AlexWaygood Nov 27, 2025
5cc8e66
check if rules are enabled before doing expensive analysis
AlexWaygood Nov 27, 2025
9e65bb2
Merge branch 'main' into alex/final-method
AlexWaygood Nov 27, 2025
ce8d67e
Update crates/ty_python_semantic/resources/mdtest/final.md
AlexWaygood Nov 28, 2025
6d40d0e
make recommendation clearer and add autofix
AlexWaygood Nov 28, 2025
cb520d5
Add tests for possibly-undefined cases
AlexWaygood Nov 28, 2025
33915c1
Avoid repetition in the primary diagnostic message
AlexWaygood Nov 28, 2025
f8eb0d2
todo a couple more things
AlexWaygood Nov 28, 2025
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177 changes: 105 additions & 72 deletions crates/ty/docs/rules.md
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436 changes: 435 additions & 1 deletion crates/ty_python_semantic/resources/mdtest/final.md
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352 changes: 352 additions & 0 deletions ...napshots/final.md_-_Tests_for_the_`@typi…_-_A_possibly-undefined…_(fc7b496fd1986deb).snap
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Original file line number Diff line number Diff line change
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---
source: crates/ty_test/src/lib.rs
expression: snapshot
---
---
mdtest name: final.md - Tests for the `@typing(_extensions).final` decorator - A possibly-undefined `@final` method is overridden
mdtest path: crates/ty_python_semantic/resources/mdtest/final.md
---

# Python source files

## mdtest_snippet.py

```
1 | from typing import final
2 |
3 | def coinflip() -> bool:
4 | return False
5 |
6 | class A:
7 | if coinflip():
8 | @final
9 | def method1(self) -> None: ...
10 | else:
11 | def method1(self) -> None: ...
12 |
13 | if coinflip():
14 | def method2(self) -> None: ...
15 | else:
16 | @final
17 | def method2(self) -> None: ...
18 |
19 | if coinflip():
20 | @final
21 | def method3(self) -> None: ...
22 | else:
23 | @final
24 | def method3(self) -> None: ...
25 |
26 | if coinflip():
27 | def method4(self) -> None: ...
28 | elif coinflip():
29 | @final
30 | def method4(self) -> None: ...
31 | else:
32 | def method4(self) -> None: ...
33 |
34 | class B(A):
35 | def method1(self) -> None: ... # error: [override-of-final-method]
36 | def method2(self) -> None: ... # error: [override-of-final-method]
37 | def method3(self) -> None: ... # error: [override-of-final-method]
38 | def method4(self) -> None: ... # error: [override-of-final-method]
39 |
40 | # Possible overrides of possibly `@final` methods...
41 | class C(A):
42 | if coinflip():
43 | # TODO: the autofix here introduces invalid syntax because there are now no
44 | # statements inside the `if:` branch
45 | # (but it might still be a useful autofix in an IDE context?)
46 | def method1(self) -> None: ... # error: [override-of-final-method]
47 | else:
48 | pass
49 |
50 | if coinflip():
51 | def method2(self) -> None: ... # error: [override-of-final-method]
52 | def method3(self) -> None: ... # error: [override-of-final-method]
53 | def method4(self) -> None: ... # error: [override-of-final-method]
```

# Diagnostics

```
error[override-of-final-method]: Cannot override `A.method1`
--> src/mdtest_snippet.py:35:9
|
34 | class B(A):
35 | def method1(self) -> None: ... # error: [override-of-final-method]
| ^^^^^^^ Override of `method1` from superclass `A`
36 | def method2(self) -> None: ... # error: [override-of-final-method]
37 | def method3(self) -> None: ... # error: [override-of-final-method]
|
info: `A.method1` is decorated with `@final`, forbidding overrides
--> src/mdtest_snippet.py:8:9
|
6 | class A:
7 | if coinflip():
8 | @final
| ------
9 | def method1(self) -> None: ...
| ------- `A.method1` defined here
10 | else:
11 | def method1(self) -> None: ...
|
help: Remove the override of `method1`
info: rule `override-of-final-method` is enabled by default
32 | def method4(self) -> None: ...
33 |
34 | class B(A):
- def method1(self) -> None: ... # error: [override-of-final-method]
35 + # error: [override-of-final-method]
36 | def method2(self) -> None: ... # error: [override-of-final-method]
37 | def method3(self) -> None: ... # error: [override-of-final-method]
38 | def method4(self) -> None: ... # error: [override-of-final-method]
note: This is an unsafe fix and may change runtime behavior

```

```
error[override-of-final-method]: Cannot override `A.method2`
--> src/mdtest_snippet.py:36:9
|
34 | class B(A):
35 | def method1(self) -> None: ... # error: [override-of-final-method]
36 | def method2(self) -> None: ... # error: [override-of-final-method]
| ^^^^^^^ Override of `method2` from superclass `A`
37 | def method3(self) -> None: ... # error: [override-of-final-method]
38 | def method4(self) -> None: ... # error: [override-of-final-method]
|
info: `A.method2` is decorated with `@final`, forbidding overrides
--> src/mdtest_snippet.py:16:9
|
14 | def method2(self) -> None: ...
15 | else:
16 | @final
| ------
17 | def method2(self) -> None: ...
| ------- `A.method2` defined here
18 |
19 | if coinflip():
|
help: Remove the override of `method2`
info: rule `override-of-final-method` is enabled by default
33 |
34 | class B(A):
35 | def method1(self) -> None: ... # error: [override-of-final-method]
- def method2(self) -> None: ... # error: [override-of-final-method]
36 + # error: [override-of-final-method]
37 | def method3(self) -> None: ... # error: [override-of-final-method]
38 | def method4(self) -> None: ... # error: [override-of-final-method]
39 |
note: This is an unsafe fix and may change runtime behavior

```

```
error[override-of-final-method]: Cannot override `A.method3`
--> src/mdtest_snippet.py:37:9
|
35 | def method1(self) -> None: ... # error: [override-of-final-method]
36 | def method2(self) -> None: ... # error: [override-of-final-method]
37 | def method3(self) -> None: ... # error: [override-of-final-method]
| ^^^^^^^ Override of `method3` from superclass `A`
38 | def method4(self) -> None: ... # error: [override-of-final-method]
|
info: `A.method3` is decorated with `@final`, forbidding overrides
--> src/mdtest_snippet.py:20:9
|
19 | if coinflip():
20 | @final
| ------
21 | def method3(self) -> None: ...
| ------- `A.method3` defined here
22 | else:
23 | @final
|
help: Remove the override of `method3`
info: rule `override-of-final-method` is enabled by default
34 | class B(A):
35 | def method1(self) -> None: ... # error: [override-of-final-method]
36 | def method2(self) -> None: ... # error: [override-of-final-method]
- def method3(self) -> None: ... # error: [override-of-final-method]
37 + # error: [override-of-final-method]
38 | def method4(self) -> None: ... # error: [override-of-final-method]
39 |
40 | # Possible overrides of possibly `@final` methods...
note: This is an unsafe fix and may change runtime behavior

```

```
error[override-of-final-method]: Cannot override `A.method4`
--> src/mdtest_snippet.py:38:9
|
36 | def method2(self) -> None: ... # error: [override-of-final-method]
37 | def method3(self) -> None: ... # error: [override-of-final-method]
38 | def method4(self) -> None: ... # error: [override-of-final-method]
| ^^^^^^^ Override of `method4` from superclass `A`
39 |
40 | # Possible overrides of possibly `@final` methods...
|
info: `A.method4` is decorated with `@final`, forbidding overrides
--> src/mdtest_snippet.py:29:9
|
27 | def method4(self) -> None: ...
28 | elif coinflip():
29 | @final
| ------
30 | def method4(self) -> None: ...
| ------- `A.method4` defined here
31 | else:
32 | def method4(self) -> None: ...
|
help: Remove the override of `method4`
info: rule `override-of-final-method` is enabled by default
35 | def method1(self) -> None: ... # error: [override-of-final-method]
36 | def method2(self) -> None: ... # error: [override-of-final-method]
37 | def method3(self) -> None: ... # error: [override-of-final-method]
- def method4(self) -> None: ... # error: [override-of-final-method]
38 + # error: [override-of-final-method]
39 |
40 | # Possible overrides of possibly `@final` methods...
41 | class C(A):
note: This is an unsafe fix and may change runtime behavior

```

```
error[override-of-final-method]: Cannot override `A.method1`
--> src/mdtest_snippet.py:46:13
|
44 | # statements inside the `if:` branch
45 | # (but it might still be a useful autofix in an IDE context?)
46 | def method1(self) -> None: ... # error: [override-of-final-method]
| ^^^^^^^ Override of `method1` from superclass `A`
47 | else:
48 | pass
|
info: `A.method1` is decorated with `@final`, forbidding overrides
--> src/mdtest_snippet.py:8:9
|
6 | class A:
7 | if coinflip():
8 | @final
| ------
9 | def method1(self) -> None: ...
| ------- `A.method1` defined here
10 | else:
11 | def method1(self) -> None: ...
|
help: Remove the override of `method1`
info: rule `override-of-final-method` is enabled by default
43 | # TODO: the autofix here introduces invalid syntax because there are now no
44 | # statements inside the `if:` branch
45 | # (but it might still be a useful autofix in an IDE context?)
- def method1(self) -> None: ... # error: [override-of-final-method]
46 + # error: [override-of-final-method]
47 | else:
48 | pass
49 |
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@AlexWaygood AlexWaygood Nov 28, 2025 •
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@MichaReiser, I feel like I remember Ruff having this issue in the past -- can you remember what the solution was? Ideally we'd mark the autofix as display-only if we can see that it's going to introduce invalid syntax. (Or use a range_replacement that replaces the definition with pass.)

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@MichaReiser MichaReiser Nov 28, 2025 •
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We have a DisplayOnly Applicability. I don't think they are shown anywhere by default (ever?). The IDE also filters out manual only edits.

I don't think we should add fixes that introduce syntax errors. I'm also not convinced that it's the right fix. It's as likely that the proper fix is to remove the @final

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Right, I was more wondering if Ruff had a nice little routine somewhere to easily detect that this was the only statement in the if: branch (and therefore that a naive deletion would introduce an unsafe fix)

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@MichaReiser MichaReiser Nov 28, 2025

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ruff/crates/ruff_linter/src/fix/edits.rs

Lines 25 to 63 in 096e539

/// Return the [`Edit`] to use when deleting a [`Stmt`].
///
/// In some cases, this is as simple as deleting the [`TextRange`] of the [`Stmt`]
/// itself. However, there are a few exceptions:
/// - If the [`Stmt`] is _not_ the terminal statement in a multi-statement line,
/// we need to delete up to the start of the next statement (and avoid
/// deleting any content that precedes the statement).
/// - If the [`Stmt`] is the terminal statement in a multi-statement line, we need
/// to avoid deleting any content that precedes the statement.
/// - If the [`Stmt`] has no trailing and leading content, then it's convenient to
/// remove the entire start and end lines.
/// - If the [`Stmt`] is the last statement in its parent body, replace it with a
/// `pass` instead.
pub(crate) fn delete_stmt(
stmt: &Stmt,
parent: Option<&Stmt>,
locator: &Locator,
indexer: &Indexer,
) -> Edit {
if parent.is_some_and(|parent| is_lone_child(stmt, parent)) {
// If removing this node would lead to an invalid syntax tree, replace
// it with a `pass`.
Edit::range_replacement("pass".to_string(), stmt.range())
} else {
if let Some(semicolon) = trailing_semicolon(stmt.end(), locator) {
let next = next_stmt_break(semicolon, locator);
Edit::deletion(stmt.start(), next)
} else if has_leading_content(stmt.start(), locator.contents()) {
Edit::range_deletion(stmt.range())
} else if let Some(start) =
indexer.preceded_by_continuations(stmt.start(), locator.contents())
{
Edit::deletion(start, stmt.end())
} else {
let range = locator.full_lines_range(stmt.range());
Edit::range_deletion(range)
}
}
}

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@AlexWaygood AlexWaygood Nov 28, 2025

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ty

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@AlexWaygood AlexWaygood Nov 28, 2025

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seems complicated, I'll leave this for a followup

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@MichaReiser MichaReiser Nov 28, 2025

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haha yeah

note: This is an unsafe fix and may change runtime behavior

```

```
error[override-of-final-method]: Cannot override `A.method2`
--> src/mdtest_snippet.py:51:13
|
50 | if coinflip():
51 | def method2(self) -> None: ... # error: [override-of-final-method]
| ^^^^^^^ Override of `method2` from superclass `A`
52 | def method3(self) -> None: ... # error: [override-of-final-method]
53 | def method4(self) -> None: ... # error: [override-of-final-method]
|
info: `A.method2` is decorated with `@final`, forbidding overrides
--> src/mdtest_snippet.py:16:9
|
14 | def method2(self) -> None: ...
15 | else:
16 | @final
| ------
17 | def method2(self) -> None: ...
| ------- `A.method2` defined here
18 |
19 | if coinflip():
|
help: Remove the override of `method2`
info: rule `override-of-final-method` is enabled by default
48 | pass
49 |
50 | if coinflip():
- def method2(self) -> None: ... # error: [override-of-final-method]
51 + # error: [override-of-final-method]
52 | def method3(self) -> None: ... # error: [override-of-final-method]
53 | def method4(self) -> None: ... # error: [override-of-final-method]
note: This is an unsafe fix and may change runtime behavior

```

```
error[override-of-final-method]: Cannot override `A.method3`
--> src/mdtest_snippet.py:52:13
|
50 | if coinflip():
51 | def method2(self) -> None: ... # error: [override-of-final-method]
52 | def method3(self) -> None: ... # error: [override-of-final-method]
| ^^^^^^^ Override of `method3` from superclass `A`
53 | def method4(self) -> None: ... # error: [override-of-final-method]
|
info: `A.method3` is decorated with `@final`, forbidding overrides
--> src/mdtest_snippet.py:20:9
|
19 | if coinflip():
20 | @final
| ------
21 | def method3(self) -> None: ...
| ------- `A.method3` defined here
22 | else:
23 | @final
|
help: Remove the override of `method3`
info: rule `override-of-final-method` is enabled by default
49 |
50 | if coinflip():
51 | def method2(self) -> None: ... # error: [override-of-final-method]
- def method3(self) -> None: ... # error: [override-of-final-method]
52 + # error: [override-of-final-method]
53 | def method4(self) -> None: ... # error: [override-of-final-method]
note: This is an unsafe fix and may change runtime behavior

```

```
error[override-of-final-method]: Cannot override `A.method4`
--> src/mdtest_snippet.py:53:13
|
51 | def method2(self) -> None: ... # error: [override-of-final-method]
52 | def method3(self) -> None: ... # error: [override-of-final-method]
53 | def method4(self) -> None: ... # error: [override-of-final-method]
| ^^^^^^^ Override of `method4` from superclass `A`
|
info: `A.method4` is decorated with `@final`, forbidding overrides
--> src/mdtest_snippet.py:29:9
|
27 | def method4(self) -> None: ...
28 | elif coinflip():
29 | @final
| ------
30 | def method4(self) -> None: ...
| ------- `A.method4` defined here
31 | else:
32 | def method4(self) -> None: ...
|
help: Remove the override of `method4`
info: rule `override-of-final-method` is enabled by default
50 | if coinflip():
51 | def method2(self) -> None: ... # error: [override-of-final-method]
52 | def method3(self) -> None: ... # error: [override-of-final-method]
- def method4(self) -> None: ... # error: [override-of-final-method]
53 + # error: [override-of-final-method]
note: This is an unsafe fix and may change runtime behavior

```
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