[ty] Allow `Final` variable assignments in `__post_init__` by charliermarsh · Pull Request #24529 · astral-sh/ruff

charliermarsh · 2026-04-10T02:39:59Z

Summary

As long as the __post_init__ is in a dataclass, e.g., the following is accepted:

from dataclasses import dataclass
from typing import Final

@dataclass
class Test:
    def __post_init__(self):
        self.test_int: Final[int] = 0

Closes astral-sh/ty#3247.

astral-sh-bot · 2026-04-10T02:42:24Z

Typing conformance results

The percentage of diagnostics emitted that were expected errors held steady at 87.72%. The percentage of expected errors that received a diagnostic held steady at 82.85%. The number of fully passing files held steady at 74/132.

Summary

How are test cases classified?

Each test case represents one expected error annotation or a group of annotations sharing a tag. Counts are per test case, not per diagnostic — multiple diagnostics on the same line count as one. Required annotations (E) are true positives when ty flags the expected location and false negatives when it does not. Optional annotations (E?) are true positives when flagged but true negatives (not false negatives) when not. Tagged annotations (E[tag]) require ty to flag exactly one of the tagged lines; tagged multi-annotations (E[tag+]) allow any number up to the tag count. Flagging unexpected locations counts as a false positive.

Metric	Old	New	Diff
True Positives	879	879	+0
False Positives	123	123	+0
False Negatives	182	182	+0
Total Diagnostics	1052	1052	+0
Precision	87.72%	87.72%	+0.00%
Recall	82.85%	82.85%	+0.00%
Passing Files	74/132	74/132	+0

True positives changed (5)

5 diagnostics

Test case	Diff
dataclasses_final.py:27	-error[invalid-assignment] Cannot assign to final attribute `final_classvar` on type `<class 'D'>`: `Final` attributes can only be assigned in the class body or `__init__` +error[invalid-assignment] Cannot assign to final attribute `final_classvar` on type `<class 'D'>`: `Final` attributes can only be assigned in the class body, `__init__`, or `__post_init__` on dataclass-like classes
dataclasses_final.py:35	-error[invalid-assignment] Cannot assign to final attribute `final_no_default` on type `D`: `Final` attributes can only be assigned in the class body or `__init__` +error[invalid-assignment] Cannot assign to final attribute `final_no_default` on type `D`: `Final` attributes can only be assigned in the class body, `__init__`, or `__post_init__` on dataclass-like classes
dataclasses_final.py:36	-error[invalid-assignment] Cannot assign to final attribute `final_with_default` on type `D`: `Final` attributes can only be assigned in the class body or `__init__` +error[invalid-assignment] Cannot assign to final attribute `final_with_default` on type `D`: `Final` attributes can only be assigned in the class body, `__init__`, or `__post_init__` on dataclass-like classes
dataclasses_final.py:37	-error[invalid-assignment] Cannot assign to final attribute `final_no_default` on type `<class 'D'>`: `Final` attributes can only be assigned in the class body or `__init__` +error[invalid-assignment] Cannot assign to final attribute `final_no_default` on type `<class 'D'>`: `Final` attributes can only be assigned in the class body, `__init__`, or `__post_init__` on dataclass-like classes
dataclasses_final.py:38	-error[invalid-assignment] Cannot assign to final attribute `final_with_default` on type `<class 'D'>`: `Final` attributes can only be assigned in the class body or `__init__` +error[invalid-assignment] Cannot assign to final attribute `final_with_default` on type `<class 'D'>`: `Final` attributes can only be assigned in the class body, `__init__`, or `__post_init__` on dataclass-like classes

astral-sh-bot · 2026-04-10T02:42:39Z

Memory usage report

Memory usage unchanged ✅

astral-sh-bot · 2026-04-10T02:43:34Z

`ecosystem-analyzer` results

Lint rule	Added	Removed	Changed
`invalid-assignment`	0	1	0
Total	0	1	0

Raw diff:

meson (https://github.com/mesonbuild/meson)
- mesonbuild/options.py:330:9 error[invalid-assignment] Cannot assign to final attribute `default` on type `Self@__post_init__`: `Final` attributes can only be assigned in the class body or `__init__`

Full report with detailed diff (timing results)

sharkdp

Thanks!

sharkdp · 2026-04-10T07:48:47Z

+@dataclass
+class C:
+    def __post_init__(self):
+        self.x: Final[int] = 1


Can we add another test here where a field is (accidentally) redeclared? Because I think this should probably be an error? (the developer might think that the field is now final, when it's not)

@dataclass class C: an_actual_field: str def __post_init__(self): self.an_actual_field: Final[str] = "bar"

If that's hard to do, it may be fine to leave it as a future-work TODO.

sharkdp · 2026-04-10T08:02:09Z

+        object_ty
+            .nominal_class(db)
+            .or_else(|| object_ty.to_class_type(db))
+            .and_then(|class_ty| class_ty.static_class_literal(db))
+            .is_some_and(|(class_literal, _)| class_literal.is_dataclass_like(db))


What kind of case is covered by .or_else(|| object_ty.to_class_type(db))? Isn't that conflating instance types with class-literal types somehow? I think I would have expected something like

Suggested change

object_ty

.nominal_class(db)

.or_else(|| object_ty.to_class_type(db))

.and_then(|class_ty| class_ty.static_class_literal(db))

.is_some_and(|(class_literal, _)| class_literal.is_dataclass_like(db))

object_ty.nominal_class(db).is_some_and(|cls| {

cls.static_class_literal(db)

.is_some_and(|(class_literal, _)| class_literal.is_dataclass_like(db))

})

Ah ok, after changing this, it looks like this was necessary to catch these kinds of cases in the conformance suite (just for the diagnostic message in this case): https://github.com/python/typing/blob/1df1565c69730d88ce6877009d268ba1d602af1e/conformance/tests/dataclasses_final.py#L27

sharkdp · 2026-04-10T08:03:59Z

+            diagnostic.set_primary_message(if allows_post_init {
+                "`Final` attributes can only be assigned in the class body, `__init__`, or `__post_init__` on dataclass-like classes"
+            } else {
+                "`Final` attributes can only be assigned in the class body or `__init__`"
+            });


carljm · 2026-04-10T08:18:09Z

Haven't reviewed the PR and don't know if it makes anything easier here, but FWIW I don't think we need to restrict this to dataclass-likes. __post_init__ is a dunder, so it's invalid for anyone to use that name for a purpose other than what CPython uses it for -- which is for dataclass-likes. That is to say, if anyone used that method name for any other purpose, and complained that we allow Final in it, it would be their bug not ours.

sharkdp · 2026-04-10T08:41:47Z

Haven't reviewed the PR and don't know if it makes anything easier here, but FWIW I don't think we need to restrict this to dataclass-likes.

Thanks, yes. I still think it's valuable to distinguish dataclass-likes and normal classes if only for purposes of special-casing the diagnostic message. I'd rather not mention __post_init__ in the diagnostic message for Final violations on "normal" classes.

* main: [ty] Fix bad diagnostic range for incorrect implicit `__init_subclass__` calls (#24541) [ty] Add a `SupportedPythonVersion` enum (#24412) [ty] Ignore unsupported editor-selected Python versions (#24498) [ty] Add snapshots for `__init_subclass__` diagnostics (#24539) [ty] Minor fix in tests (#24538) [ty] Allow `Final` variable assignments in `__post_init__` (#24529) [ty] Expand test suite for assignment errors (#24537) [ty] Use `map`, not `__map`, as the name of the mapping parameter in `TypedDict` `__init__` methods (#24535) [ty] Rework logic for synthesizing `TypedDict` methods (#24534) [flake8-bandit] Fix S103 false positives and negatives in mask analysis (#24424) [ty] mdtest.py: update dependencies (#24533) Rename patterns and arguments source order iterator method (#24532) [ty] Omit invalid keyword arguments from `TypedDict` signature (#24522) [ty] support super() in metaclass methods (#24483) [ty] Synthesize `__init__` for `TypedDict` (#24476)

* main: Bump typing conformance suite commit to latest upstream (#24553) [ty] Reject deleting`Final` attributes (#24508) [ty] Respect property deleters in attribute deletion checks (#24500) [ty] stop unioning Unknown into types of un-annotated attributes (#24531) [ty] Fix bad diagnostic range for incorrect implicit `__init_subclass__` calls (#24541) [ty] Add a `SupportedPythonVersion` enum (#24412) [ty] Ignore unsupported editor-selected Python versions (#24498) [ty] Add snapshots for `__init_subclass__` diagnostics (#24539) [ty] Minor fix in tests (#24538) [ty] Allow `Final` variable assignments in `__post_init__` (#24529) [ty] Expand test suite for assignment errors (#24537) [ty] Use `map`, not `__map`, as the name of the mapping parameter in `TypedDict` `__init__` methods (#24535) [ty] Rework logic for synthesizing `TypedDict` methods (#24534) [flake8-bandit] Fix S103 false positives and negatives in mask analysis (#24424) [ty] mdtest.py: update dependencies (#24533) Rename patterns and arguments source order iterator method (#24532) [ty] Omit invalid keyword arguments from `TypedDict` signature (#24522) [ty] support super() in metaclass methods (#24483) [ty] Synthesize `__init__` for `TypedDict` (#24476)

astral-sh-bot Bot added the ty Multi-file analysis & type inference label Apr 10, 2026

charliermarsh marked this pull request as ready for review April 10, 2026 02:43

charliermarsh requested review from AlexWaygood, carljm, dcreager, ibraheemdev and sharkdp as code owners April 10, 2026 02:43

astral-sh-bot Bot assigned sharkdp Apr 10, 2026

sharkdp approved these changes Apr 10, 2026

View reviewed changes

charliermarsh added 5 commits April 10, 2026 08:03

[ty] Allow Final variable assignments in __post_init__

7a6cff3

Split tests

39f309b

Add TODO

d806916

Simplify query

4b43f3c

Limit to other dunders

4aa98bf

charliermarsh force-pushed the charlie/post-init branch from b15914c to 4aa98bf Compare April 10, 2026 12:16

AlexWaygood removed their request for review April 10, 2026 12:20

Cover classes

55e79d3

charliermarsh merged commit 5da2161 into main Apr 10, 2026
55 checks passed

charliermarsh deleted the charlie/post-init branch April 10, 2026 12:35

This was referenced Apr 27, 2026

Always include panic payload in panic diagnostic message #24873

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[ty] Allow `Final` variable assignments in `__post_init__`#24529

[ty] Allow `Final` variable assignments in `__post_init__`#24529
charliermarsh merged 6 commits intomainfrom
charlie/post-init

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charliermarsh commented Apr 10, 2026

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Typing conformance results

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