Codestin Search App

charliermarsh · 2026-06-19T16:10:51Z

Summary

We currently retain a function-literal type when reading an ordinary method from a class literal. Because each function definition has its own singleton type, assigning a different function is rejected even when its signature and binding behavior match:

class Foo:
    def method(self, value: int) -> str:
        return str(value)

def replacement(self: Foo, value: int) -> str:
    return f"replacement: {value}"

Foo.method = replacement

This widens the declared method to its function-like callable type only while checking a class attribute assignment. Reads continue to retain the original function-literal type, while replacements must have a compatible signature and ordinary instance-binding behavior.

This is intentionally unsound for function identity: after the assignment, the runtime value of Foo.method is replacement, but its inferred type still identifies the method declared in Foo. staticmethod and classmethod attributes are not widened because replacing either descriptor with a plain function changes how class and instance access bind arguments. Supporting those cases would require a replacement with the corresponding descriptor.

Closes astral-sh/ty#2648.

charliermarsh · 2026-06-19T16:12:28Z

+identity-sensitive code unsound: after the assignment below, `requires_original_add(Foo.add)` is
+accepted even though the function only allows the original `add`. Its assertion fails at runtime
+because `Foo.add` is now `add_replacement`. Calls to the method itself remain safe because both
+functions have compatible signatures and the same instance-binding behavior.


I know this is lengthy but it took me some time to understand it myself so wanted to leave a full explanation.

I'm changing the example to avoid ty_extensions, maybe I'm still misunderstanding though, hmm...

charliermarsh · 2026-06-19T16:12:34Z

+replaced, `DescriptorMethods.class_(1)` would stop passing `DescriptorMethods` as the first
+argument, while `DescriptorMethods().class_(1)` would pass the instance instead of the class.
+Supporting these assignments requires replacements with the corresponding decorator (for example,
+`staticmethod(static_replacement)` or `classmethod(class_replacement)`).


Same here...

astral-sh-bot · 2026-06-19T16:12:46Z

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 94.37%. The percentage of expected errors that received a diagnostic held steady at 89.00%. The number of fully passing files held steady at 94/134.

astral-sh-bot · 2026-06-19T16:13:46Z

Memory usage report

Memory usage unchanged ✅

astral-sh-bot · 2026-06-19T16:15:26Z

`ecosystem-analyzer` results

Lint rule	Added	Removed	Changed
`invalid-assignment`	0	23	12
`unresolved-attribute`	8	0	0
Total	8	23	12

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Raw diff (43 changes)

dd-trace-py (https://github.com/DataDog/dd-trace-py)
- ddtrace/appsec/_iast/_taint_tracking/_vendor/pybind11/pybind11/setup_helpers.py:492:9 error[invalid-assignment] Object of type `(CCompiler, list[str], str | None, list[tuple[str] | tuple[str, str | None]] | None, list[str] | None, bool, list[str] | None, list[str] | None, list[str] | None, /) -> list[str]` is not assignable to attribute `compile` of type `def compile(self, sources: Sequence[str | PathLike[str]], output_dir: str | None = None, macros: list[tuple[str] | tuple[str, str | None]] | None = None, include_dirs: list[str] | None = None, debug: bool | Literal[0, 1] = 0, extra_preargs: list[str] | None = None, extra_postargs: list[str] | None = None, depends: list[str] | None = None) -> list[str]`
+ ddtrace/appsec/_iast/_taint_tracking/_vendor/pybind11/pybind11/setup_helpers.py:492:9 error[invalid-assignment] Object of type `(CCompiler, list[str], str | None, list[tuple[str] | tuple[str, str | None]] | None, list[str] | None, bool, list[str] | None, list[str] | None, list[str] | None, /) -> list[str]` is not assignable to attribute `compile` of type `(self, sources: Sequence[str | PathLike[str]], output_dir: str | None = None, macros: list[tuple[str] | tuple[str, str | None]] | None = None, include_dirs: list[str] | None = None, debug: bool | Literal[0, 1] = 0, extra_preargs: list[str] | None = None, extra_postargs: list[str] | None = None, depends: list[str] | None = None) -> list[str]`
- ddtrace/appsec/_iast/_taint_tracking/_vendor/pybind11/pybind11/setup_helpers.py:500:9 error[invalid-assignment] Object of type `(CCompiler, list[str], str | None, list[tuple[str] | tuple[str, str | None]] | None, list[str] | None, bool, list[str] | None, list[str] | None, list[str] | None, /) -> list[str]` is not assignable to attribute `compile` of type `def compile(self, sources: Sequence[str | PathLike[str]], output_dir: str | None = None, macros: list[tuple[str] | tuple[str, str | None]] | None = None, include_dirs: list[str] | None = None, debug: bool | Literal[0, 1] = 0, extra_preargs: list[str] | None = None, extra_postargs: list[str] | None = None, depends: list[str] | None = None) -> list[str]`
+ ddtrace/appsec/_iast/_taint_tracking/_vendor/pybind11/pybind11/setup_helpers.py:500:9 error[invalid-assignment] Object of type `(CCompiler, list[str], str | None, list[tuple[str] | tuple[str, str | None]] | None, list[str] | None, bool, list[str] | None, list[str] | None, list[str] | None, /) -> list[str]` is not assignable to attribute `compile` of type `(self, sources: Sequence[str | PathLike[str]], output_dir: str | None = None, macros: list[tuple[str] | tuple[str, str | None]] | None = None, include_dirs: list[str] | None = None, debug: bool | Literal[0, 1] = 0, extra_preargs: list[str] | None = None, extra_postargs: list[str] | None = None, depends: list[str] | None = None) -> list[str]`
- ddtrace/internal/compat.py:90:5 error[invalid-assignment] Object of type `def _register(self, cls, method=None) -> Unknown` is not assignable to attribute `register` of type `Overload[(self, cls: type[Any], method: None = None) -> (((...) -> Unknown, /) -> ((...) -> Unknown)), (self, cls: (...) -> Unknown, method: None = None) -> ((...) -> Unknown), (self, cls: type[Any], method: (...) -> Unknown) -> ((...) -> Unknown)]`
- ddtrace/internal/coverage/multiprocessing_coverage.py:220:5 error[invalid-assignment] Object of type `def join(self, *args, **kwargs) -> Unknown` is not assignable to attribute `join` of type `def join(self, timeout: int | float | None = None) -> None`
- ddtrace/internal/coverage/multiprocessing_coverage.py:219:5 error[invalid-assignment] Object of type `def close(self) -> Unknown` is not assignable to attribute `close` of type `def close(self) -> None`
+ ddtrace/internal/coverage/multiprocessing_coverage.py:219:5 error[invalid-assignment] Object of type `def close(self) -> Unknown` is not assignable to attribute `close` of type `(self) -> None`
- ddtrace/internal/coverage/multiprocessing_coverage.py:221:5 error[invalid-assignment] Object of type `def kill(self) -> Unknown` is not assignable to attribute `kill` of type `def kill(self) -> None`
+ ddtrace/internal/coverage/multiprocessing_coverage.py:221:5 error[invalid-assignment] Object of type `def kill(self) -> Unknown` is not assignable to attribute `kill` of type `(self) -> None`
- ddtrace/internal/coverage/multiprocessing_coverage.py:222:5 error[invalid-assignment] Object of type `def terminate(self) -> Unknown` is not assignable to attribute `terminate` of type `def terminate(self) -> None`
+ ddtrace/internal/coverage/multiprocessing_coverage.py:222:5 error[invalid-assignment] Object of type `def terminate(self) -> Unknown` is not assignable to attribute `terminate` of type `(self) -> None`
- ddtrace/internal/coverage/threading_coverage.py:88:5 error[invalid-assignment] Object of type `def join(self, *args, **kwargs) -> Unknown` is not assignable to attribute `join` of type `def join(self, timeout: int | float | None = None) -> None`
- tests/profiling/collector/test_lock_reflection.py:190:5 error[invalid-assignment] Object of type `def broken_getattr(self: _ProfiledLock, name: str) -> object` is not assignable to attribute `__getattr__` of type `def __getattr__(self, name: str) -> Any`

discord.py (https://github.com/Rapptz/discord.py)
- discord/enums.py:100:9 error[invalid-assignment] Object of type `(self, other) -> Literal[False] | Unknown` is not assignable to attribute `__le__` of type `def __le__(self, value: tuple[Unknown, ...], /) -> bool`
+ discord/enums.py:100:80 error[unresolved-attribute] Object of type `Self@__le__` has no attribute `value`
+ discord/enums.py:100:94 error[unresolved-attribute] Object of type `Self@__le__` has no attribute `value`
- discord/enums.py:101:9 error[invalid-assignment] Object of type `(self, other) -> Literal[False] | Unknown` is not assignable to attribute `__ge__` of type `def __ge__(self, value: tuple[Unknown, ...], /) -> bool`
+ discord/enums.py:101:80 error[unresolved-attribute] Object of type `Self@__ge__` has no attribute `value`
+ discord/enums.py:101:94 error[unresolved-attribute] Object of type `Self@__ge__` has no attribute `value`
- discord/enums.py:102:9 error[invalid-assignment] Object of type `(self, other) -> Literal[False] | Unknown` is not assignable to attribute `__lt__` of type `def __lt__(self, value: tuple[Unknown, ...], /) -> bool`
+ discord/enums.py:102:80 error[unresolved-attribute] Object of type `Self@__lt__` has no attribute `value`
+ discord/enums.py:102:93 error[unresolved-attribute] Object of type `Self@__lt__` has no attribute `value`
- discord/enums.py:103:9 error[invalid-assignment] Object of type `(self, other) -> Literal[False] | Unknown` is not assignable to attribute `__gt__` of type `def __gt__(self, value: tuple[Unknown, ...], /) -> bool`
+ discord/enums.py:103:80 error[unresolved-attribute] Object of type `Self@__gt__` has no attribute `value`
+ discord/enums.py:103:93 error[unresolved-attribute] Object of type `Self@__gt__` has no attribute `value`

ibis (https://github.com/ibis-project/ibis)
- ibis/backends/singlestoredb/tests/test_client.py:327:5 error[invalid-assignment] Object of type `def mock_do_connect(_self, *_args, **kwargs) -> Unknown` is not assignable to attribute `do_connect` of type `def do_connect(self, host: str | None = None, user: str | None = None, password: str | None = None, port: int | None = None, database: str | None = None, driver: str | None = None, autocommit: bool = True, local_infile: bool = True, **kwargs) -> None`

meson (https://github.com/mesonbuild/meson)
- unittests/failuretests.py:50:5 error[invalid-assignment] Object of type `def new_search(self, name, search_dirs, exclude_paths) -> Unknown` is not assignable to attribute `_search` of type `def _search(self, name: str, search_dirs: list[str | None], exclude_paths: list[str] | None) -> list[str]`

mitmproxy (https://github.com/mitmproxy/mitmproxy)
- examples/contrib/ntlm_upstream_proxy.py:150:9 error[invalid-assignment] Object of type `def patched_start_handshake(self) -> Generator[Command, Any, None]` is not assignable to attribute `start_handshake` of type `def start_handshake(self) -> Generator[Command, Any, None]`
- examples/contrib/ntlm_upstream_proxy.py:151:9 error[invalid-assignment] Object of type `def patched_receive_handshake_data(self, data) -> Generator[Command, Any, tuple[bool, str | None]]` is not assignable to attribute `receive_handshake_data` of type `def receive_handshake_data(self, data: bytes) -> Generator[Command, Any, tuple[bool, str | None]]`

mypy (https://github.com/python/mypy)
- mypyc/build.py:514:5 error[invalid-assignment] Object of type `def patched(self: Any) -> None` is not assignable to attribute `copy_extensions_to_source` of type `def copy_extensions_to_source(self) -> None`
- mypyc/build_setup.py:69:1 error[invalid-assignment] Object of type `def spawn(self, cmd, **kwargs) -> None` is not assignable to attribute `spawn` of type `def spawn(self, cmd: Iterable[str]) -> None`
- mypyc/lib-rt/build_setup.py:69:1 error[invalid-assignment] Object of type `def spawn(self, cmd, **kwargs) -> None` is not assignable to attribute `spawn` of type `def spawn(self, cmd: Iterable[str]) -> None`

pybind11 (https://github.com/pybind/pybind11)
- pybind11/setup_helpers.py:488:9 error[invalid-assignment] Object of type `(CCompiler, list[str], str | None, list[tuple[str] | tuple[str, str | None]] | None, list[str] | None, bool, list[str] | None, list[str] | None, list[str] | None, /) -> list[str]` is not assignable to attribute `compile` of type `def compile(self, sources: Sequence[str | PathLike[str]], output_dir: str | None = None, macros: list[tuple[str] | tuple[str, str | None]] | None = None, include_dirs: list[str] | None = None, debug: bool | Literal[0, 1] = 0, extra_preargs: list[str] | None = None, extra_postargs: list[str] | None = None, depends: list[str] | None = None) -> list[str]`
+ pybind11/setup_helpers.py:488:9 error[invalid-assignment] Object of type `(CCompiler, list[str], str | None, list[tuple[str] | tuple[str, str | None]] | None, list[str] | None, bool, list[str] | None, list[str] | None, list[str] | None, /) -> list[str]` is not assignable to attribute `compile` of type `(self, sources: Sequence[str | PathLike[str]], output_dir: str | None = None, macros: list[tuple[str] | tuple[str, str | None]] | None = None, include_dirs: list[str] | None = None, debug: bool | Literal[0, 1] = 0, extra_preargs: list[str] | None = None, extra_postargs: list[str] | None = None, depends: list[str] | None = None) -> list[str]`
- pybind11/setup_helpers.py:496:9 error[invalid-assignment] Object of type `(CCompiler, list[str], str | None, list[tuple[str] | tuple[str, str | None]] | None, list[str] | None, bool, list[str] | None, list[str] | None, list[str] | None, /) -> list[str]` is not assignable to attribute `compile` of type `def compile(self, sources: Sequence[str | PathLike[str]], output_dir: str | None = None, macros: list[tuple[str] | tuple[str, str | None]] | None = None, include_dirs: list[str] | None = None, debug: bool | Literal[0, 1] = 0, extra_preargs: list[str] | None = None, extra_postargs: list[str] | None = None, depends: list[str] | None = None) -> list[str]`
+ pybind11/setup_helpers.py:496:9 error[invalid-assignment] Object of type `(CCompiler, list[str], str | None, list[tuple[str] | tuple[str, str | None]] | None, list[str] | None, bool, list[str] | None, list[str] | None, list[str] | None, /) -> list[str]` is not assignable to attribute `compile` of type `(self, sources: Sequence[str | PathLike[str]], output_dir: str | None = None, macros: list[tuple[str] | tuple[str, str | None]] | None = None, include_dirs: list[str] | None = None, debug: bool | Literal[0, 1] = 0, extra_preargs: list[str] | None = None, extra_postargs: list[str] | None = None, depends: list[str] | None = None) -> list[str]`

pywin32 (https://github.com/mhammond/pywin32)
- pythonwin/pywin/framework/intpyapp.py:31:1 error[invalid-assignment] Object of type `def _SetupSharedMenu_(self) -> Unknown` is not assignable to attribute `_SetupSharedMenu_` of type `def _SetupSharedMenu_(self) -> Unknown`

scikit-learn (https://github.com/scikit-learn/scikit-learn)
- sklearn/utils/_mocking.py:349:1 error[invalid-assignment] Object of type `RequestMethod` is not assignable to attribute `set_fit_request` of type `def set_fit_request(self, **kwargs) -> Unknown`
+ sklearn/utils/_mocking.py:349:1 error[invalid-assignment] Object of type `RequestMethod` is not assignable to attribute `set_fit_request` of type `(self, **kwargs) -> Unknown`

scipy (https://github.com/scipy/scipy)
- scipy/stats/_distribution_infrastructure.py:4293:9 error[invalid-assignment] Object of type `def _moment_raw_formula(self, order, **kwargs) -> Unknown` is not assignable to attribute `_moment_raw_formula` of type `def _moment_raw_formula(self, order, **params) -> Unknown`
- scipy/stats/_distribution_infrastructure.py:4294:9 error[invalid-assignment] Object of type `def _moment_central_formula(self, order, **kwargs) -> Unknown` is not assignable to attribute `_moment_central_formula` of type `def _moment_central_formula(self, order, **params) -> Unknown`
- scipy/stats/_distribution_infrastructure.py:4295:9 error[invalid-assignment] Object of type `def _moment_standardized_formula(self, order, **kwargs) -> Unknown` is not assignable to attribute `_moment_standardized_formula` of type `def _moment_standardized_formula(self, order, **params) -> Unknown`

spack (https://github.com/spack/spack)
- lib/spack/docs/conf.py:137:1 error[invalid-assignment] Object of type `(self, instance, owner) -> Unknown` is not assignable to attribute `__get__` of type `def __get__(self, instance, owner) -> Unknown`
- lib/spack/spack/test/conftest.py:2409:5 error[invalid-assignment] Object of type `def _libc_from_python(self) -> Unknown` is not assignable to attribute `default_libc` of type `def default_libc(self) -> Unknown | None`

spark (https://github.com/apache/spark)
- python/pyspark/sql/session.py:140:9 error[invalid-assignment] Object of type `def toDF(self, schema=None, sampleRatio=None) -> Unknown` is not assignable to attribute `toDF` of type `Overload[[RowLike](self: RDD[RowLike], schema: list[str] | tuple[str, ...] | None = None, sampleRatio: int | float | None = None) -> DataFrame, [RowLike](self: RDD[RowLike], schema: StructType | str | None = None) -> DataFrame, [AtomicValue](self: RDD[AtomicValue], schema: AtomicType | str) -> DataFrame]`
- python/pyspark/sql/tests/connect/client/test_client.py:251:5 error[invalid-assignment] Object of type `(_) -> None` is not assignable to attribute `_cleanup_ml_cache` of type `def _cleanup_ml_cache(self) -> None`
+ python/pyspark/sql/tests/connect/client/test_client.py:251:5 error[invalid-assignment] Object of type `(_: SparkConnectClient) -> None` is not assignable to attribute `_cleanup_ml_cache` of type `(self) -> None`
- python/pyspark/sql/tests/connect/client/test_client_call_stack_trace.py:33:5 error[invalid-assignment] Object of type `(_) -> None` is not assignable to attribute `_cleanup_ml_cache` of type `def _cleanup_ml_cache(self) -> None`
+ python/pyspark/sql/tests/connect/client/test_client_call_stack_trace.py:33:5 error[invalid-assignment] Object of type `(_: SparkConnectClient) -> None` is not assignable to attribute `_cleanup_ml_cache` of type `(self) -> None`

sympy (https://github.com/sympy/sympy)
- sympy/interactive/printing.py:283:9 error[invalid-assignment] Object of type `def _repr_disabled(self) -> Unknown` is not assignable to attribute `_repr_latex_` of type `def _repr_latex_(self) -> Unknown`
- sympy/interactive/printing.py:275:9 error[invalid-assignment] Object of type `def _print_latex_text(o) -> Unknown` is not assignable to attribute `_repr_latex_` of type `def _repr_latex_(self) -> Unknown`
+ sympy/interactive/printing.py:275:9 error[invalid-assignment] Object of type `def _print_latex_text(o) -> Unknown` is not assignable to attribute `_repr_latex_` of type `(self) -> Unknown`

werkzeug (https://github.com/pallets/werkzeug)
- tests/live_apps/run.py:35:5 error[invalid-assignment] Object of type `(_) -> Any` is not assignable to attribute `address_string` of type `def address_string(self) -> str`
+ tests/live_apps/run.py:35:5 error[invalid-assignment] Object of type `(_: WSGIRequestHandler) -> Any` is not assignable to attribute `address_string` of type `(self) -> str`

Full report with detailed diff (timing results)

carljm

With #26253 fixed, this looks fine.

I'm still a bit on the fence about whether we should even do this. The mypy/zuban approach of erroring on this also seems pretty OK to me -- it's an odd thing to do, you can suppress the type error if you insist on doing it.

But if we are going to do it, this implementation looks good.

carljm · 2026-06-24T19:42:41Z

I guess I'm the one who put the issue into Stable milestone, so clearly I'm not thinking consistently over time about this...

astral-sh-bot Bot added the ty Multi-file analysis & type inference label Jun 19, 2026

charliermarsh requested a review from carljm June 19, 2026 16:12

charliermarsh marked this pull request as ready for review June 19, 2026 16:12

charliermarsh requested a review from a team as a code owner June 19, 2026 16:12

charliermarsh commented Jun 19, 2026

View reviewed changes

charliermarsh mentioned this pull request Jun 19, 2026

Monkey patching with a function of the same signature not supported astral-sh/ty#2648

Closed

charliermarsh force-pushed the charlie/fix-method-monkeypatch-assignment branch from 4ffbc7e to 8212ec3 Compare June 19, 2026 18:18

carljm mentioned this pull request Jun 23, 2026

[ty] Preserve regular kind for callable instances #26253

Merged

charliermarsh added 14 commits June 24, 2026 10:33

[ty] Allow compatible method monkeypatching

4ba8f66

[ty] Preserve descriptor kinds in method monkeypatches

57cf5e6

[ty] Support descriptor-safe method mocks

8748185

[ty] Check all method-like monkeypatch targets

7fe5e91

[ty] Bind union monkeypatch arms independently

3ea9de7

[ty] Validate classmethod monkeypatch receivers

5becf82

[ty] Simplify method monkeypatch validation

1ce5a60

[ty] Check method monkeypatches by signature

397138f

[ty] Simplify method monkeypatch coverage

0574164

[ty] Tighten method replacement coverage

40ed83e

[ty] Limit method replacements to ordinary methods

38f2696

[ty] Document method replacement tradeoffs

b5927fb

[ty] Document method assignment widening

bba3b99

[ty] Use Literal in method identity example

58a206d

charliermarsh force-pushed the charlie/fix-method-monkeypatch-assignment branch from 8212ec3 to 58a206d Compare June 24, 2026 14:33

carljm approved these changes Jun 24, 2026

View reviewed changes

charliermarsh merged commit a47a6d6 into main Jun 24, 2026
99 of 100 checks passed

charliermarsh deleted the charlie/fix-method-monkeypatch-assignment branch June 24, 2026 20:26

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

[ty] Allow replacing ordinary methods with compatible functions#26158

[ty] Allow replacing ordinary methods with compatible functions#26158
charliermarsh merged 14 commits into
mainfrom
charlie/fix-method-monkeypatch-assignment

charliermarsh commented Jun 19, 2026

Uh oh!

charliermarsh Jun 19, 2026

Uh oh!

charliermarsh Jun 19, 2026

Uh oh!

charliermarsh Jun 19, 2026

Uh oh!

astral-sh-bot Bot commented Jun 19, 2026 •

edited

Loading

Uh oh!

astral-sh-bot Bot commented Jun 19, 2026 •

edited

Loading

Uh oh!

astral-sh-bot Bot commented Jun 19, 2026 •

edited

Loading

Uh oh!

carljm left a comment

Uh oh!

carljm commented Jun 24, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Uh oh!

Conversation

charliermarsh commented Jun 19, 2026

Summary

Uh oh!

charliermarsh Jun 19, 2026

Choose a reason for hiding this comment

Uh oh!

charliermarsh Jun 19, 2026

Choose a reason for hiding this comment

Uh oh!

charliermarsh Jun 19, 2026

Choose a reason for hiding this comment

Uh oh!

astral-sh-bot Bot commented Jun 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Typing conformance results

No changes detected ✅

Uh oh!

astral-sh-bot Bot commented Jun 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Memory usage report

Uh oh!

astral-sh-bot Bot commented Jun 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

ecosystem-analyzer results

Uh oh!

carljm left a comment

Choose a reason for hiding this comment

Uh oh!

carljm commented Jun 24, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

astral-sh-bot Bot commented Jun 19, 2026 •

edited

Loading

astral-sh-bot Bot commented Jun 19, 2026 •

edited

Loading

astral-sh-bot Bot commented Jun 19, 2026 •

edited

Loading

`ecosystem-analyzer` results