[wip] Improve reachability in narrowing logic, avoid losing narrowing constraints #20660

hauntsaninja · 2026-01-28T00:45:45Z

By decoupling if_map's and else_map's we don't have to make up empty dictionaries to make them zip well, which enables us to preserve the None's that indicate a lack of reachability better.

This also prevents us from losing narrowing constraints in some cases, see e.g. the new testNarrowingConstrainedTypeVarType test which we previously errored on

Improves #17372
Fixes #17045
Improves #14965
Fixes this comment #9003 (comment)

hauntsaninja · 2026-01-28T00:49:53Z

mypy/typeshed/stubs/mypy-extensions/mypy_extensions.pyi

    def __ge__(self, x: i64) -> bool: ...
    def __gt__(self, x: i64) -> bool: ...
    def __index__(self) -> int: ...
+    def __eq__(self, x: object) -> bool: ...


Without (correctly) annotating the custom equality here, we would consider otherwise reachable code to be unreachable. Specifically the right side of this or clause:

mypy/mypy/build.py

Line 1627 in d475659

if meta[0] != cache_version() or meta[1] != CACHE_VERSION:

hauntsaninja · 2026-01-28T00:56:39Z

@p-sawicki I'm getting AssertionError: Cannot add a traceback entry with a negative line number for op on this PR, seems like this is new to your PR, any advice? If I revert your PR, things pass

https://github.com/python/mypy/actions/runs/21421020108/job/61680277800?pr=20660

hauntsaninja · 2026-01-28T01:46:20Z

Primer needs some work, specifically I need to handle promotions better

This reverts commit 1449c5e.

This reverts commit 0473058.

github-actions · 2026-01-28T07:58:10Z

Diff from mypy_primer, showing the effect of this PR on open source code:

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/server/api/server.py:664: error: Invalid index type "tuple[str, bool, bool]" for "dict[tuple[Settings, bool], Any]"; expected type "tuple[Settings, bool]"  [index]

pydantic (https://github.com/pydantic/pydantic)
+ pydantic/v1/typing.py:236: error: Unused "type: ignore" comment  [unused-ignore]

mypy (https://github.com/python/mypy)
+ mypy/server/aststrip.py:140: error: If condition in comprehension is always true  [redundant-expr]
+ mypy/server/aststrip.py:140: note: Error code "redundant-expr" not covered by "type: ignore" comment
+ mypy/server/aststrip.py:140: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-redundant-expr for more info
+ mypy/stubutil.py:76: error: Statement is unreachable  [unreachable]
+ mypy/stubutil.py:76: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-unreachable for more info

schemathesis (https://github.com/schemathesis/schemathesis)
+ src/schemathesis/specs/openapi/extra_data_source.py: note: In member "weighted_select" of class "VariantUsageTracker":
+ src/schemathesis/specs/openapi/extra_data_source.py:85: error: Statement is unreachable  [unreachable]

operator (https://github.com/canonical/operator)
- ops/_private/harness.py:2563: error: Invalid index type "str" for "dict[int, dict[str, Any]]"; expected type "int"  [index]

core (https://github.com/home-assistant/core)
+ homeassistant/components/homekit/__init__.py:982: error: Right operand of "or" is never evaluated  [unreachable]
+ homeassistant/components/firefly_iii/sensor.py:190: error: Statement is unreachable  [unreachable]

pytest (https://github.com/pytest-dev/pytest)
+ testing/test_capture.py:923: error: Statement is unreachable  [unreachable]

hydra-zen (https://github.com/mit-ll-responsible-ai/hydra-zen)
+ src/hydra_zen/structured_configs/_implementations.py:1075: error: Unused "type: ignore" comment  [unused-ignore]
+ src/hydra_zen/structured_configs/_implementations.py:1322: error: Unused "type: ignore" comment  [unused-ignore]

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/arrays/datetimes.py:2997: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/core/arrays/datetimes.py:3009: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/core/arrays/datetimes.py:3011: error: Redundant cast to "Timestamp"  [redundant-cast]
+ pandas/core/arrays/datetimes.py:3012: error: Redundant cast to "Timestamp"  [redundant-cast]

discord.py (https://github.com/Rapptz/discord.py)
+ discord/http.py:274: error: Unused "type: ignore" comment  [unused-ignore]
- discord/http.py:190: error: Incompatible types in assignment (expression has type "str", target has type "list[Embed]")  [assignment]
- discord/http.py:192: error: Incompatible types in assignment (expression has type "None", target has type "list[Embed]")  [assignment]
- discord/http.py:196: error: Incompatible types in assignment (expression has type "list[dict[str, Any]]", target has type "list[Embed]")  [assignment]
- discord/http.py:207: error: Incompatible types in assignment (expression has type "str", target has type "list[Embed]")  [assignment]
- discord/http.py:208: error: Incompatible types in assignment (expression has type "bool", target has type "list[Embed]")  [assignment]
- discord/http.py:211: error: Incompatible types in assignment (expression has type "MessageReference | None", target has type "list[Embed]")  [assignment]
- discord/http.py:215: error: Incompatible types in assignment (expression has type "list[str | int]", target has type "list[Embed]")  [assignment]
- discord/http.py:219: error: Incompatible types in assignment (expression has type "bool", target has type "list[Embed]")  [assignment]
- discord/http.py:221: error: Incompatible types in assignment (expression has type "str", target has type "list[Embed]")  [assignment]
- discord/http.py:223: error: Incompatible types in assignment (expression has type "str", target has type "list[Embed]")  [assignment]
- discord/http.py:226: error: Incompatible types in assignment (expression has type "int", target has type "list[Embed]")  [assignment]
- discord/http.py:229: error: Incompatible types in assignment (expression has type "str", target has type "list[Embed]")  [assignment]
- discord/http.py:233: error: Incompatible types in assignment (expression has type "AllowedMentions", target has type "list[Embed]")  [assignment]
- discord/http.py:235: error: Incompatible types in assignment (expression has type "AllowedMentions", target has type "list[Embed]")  [assignment]
- discord/http.py:237: error: Incompatible types in assignment (expression has type "AllowedMentions", target has type "list[Embed]")  [assignment]
- discord/http.py:241: error: Incompatible types in assignment (expression has type "AllowedMentions", target has type "list[Embed]")  [assignment]
- discord/http.py:242: error: No overload variant of "__setitem__" of "list" matches argument types "str", "bool"  [call-overload]
- discord/http.py:242: note: Possible overload variants:
- discord/http.py:242: note:     def __setitem__(self, SupportsIndex, Embed, /) -> None
- discord/http.py:242: note:     def __setitem__(self, slice[Any, Any, Any], Iterable[Embed], /) -> None
- discord/http.py:257: error: Argument 1 to "append" of "list" has incompatible type "Attachment"; expected "dict[str, Any]"  [arg-type]
- discord/http.py:259: error: Incompatible types in assignment (expression has type "list[dict[str, Any]]", target has type "list[Embed]")  [assignment]
- discord/http.py:263: error: Incompatible types in assignment (expression has type "list[str | int]", target has type "list[Embed]")  [assignment]
- discord/http.py:269: error: Dict entry 0 has incompatible type "str": "dict[str, list[Embed]]"; expected "str": "list[Embed]"  [dict-item]
- discord/http.py:279: error: Incompatible types in assignment (expression has type "None", variable has type "dict[str, list[Embed]]")  [assignment]
- discord/http.py:284: error: Dict entry 1 has incompatible type "str": "BufferedIOBase"; expected "str": "str"  [dict-item]
- discord/webhook/sync.py:1105: error: Argument "applied_tags" to "handle_message_parameters" has incompatible type "str | Snowflake"; expected "list[str | int] | None"  [arg-type]
+ discord/webhook/sync.py:1105: error: Cannot determine type of "applied_tag_ids"  [has-type]
- discord/webhook/async_.py:578: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:582: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:584: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:588: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:590: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:594: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:603: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:606: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:610: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:612: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:614: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:629: error: Argument 1 to "append" of "list" has incompatible type "Attachment"; expected "dict[str, Any]"  [arg-type]
- discord/webhook/async_.py:631: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:634: error: Unsupported target for indexed assignment ("dict[str, Any] | None")  [index]
- discord/webhook/async_.py:645: error: Dict entry 1 has incompatible type "str": "BufferedIOBase"; expected "str": "str"  [dict-item]
- discord/webhook/async_.py:1892: error: Argument "applied_tags" to "handle_message_parameters" has incompatible type "str | Snowflake"; expected "list[str | int] | None"  [arg-type]
+ discord/webhook/async_.py:1892: error: Cannot determine type of "applied_tag_ids"  [has-type]

trio (https://github.com/python-trio/trio)
+ src/trio/_core/_tests/test_run.py:2457: error: Unused "type: ignore" comment  [unused-ignore]

Expression (https://github.com/cognitedata/Expression)
- README.py:460: error: Incompatible types in assignment (expression has type "Result[int, Exception]", variable has type "Seq[int]")  [assignment]

pyodide (https://github.com/pyodide/pyodide)
+ pyodide-build/pyodide_build/tests/test_xbuildenv.py:88: error: Statement is unreachable  [unreachable]

rotki (https://github.com/rotki/rotki)
+ rotkehlchen/exchanges/binance.py:391: error: Statement is unreachable  [unreachable]
+ rotkehlchen/tests/api/test_evmlike.py:212: error: Statement is unreachable  [unreachable]
+ rotkehlchen/tests/api/test_history_base_entry.py:1263: error: Need type annotation for "ids_per_subtype"  [var-annotated]

static-frame (https://github.com/static-frame/static-frame)
+ static_frame/core/type_blocks.py:1633: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/type_blocks.py:1634: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/type_blocks.py:1759: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/type_blocks.py:1760: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/archive_npy.py:231: error: Statement is unreachable  [unreachable]
+ static_frame/core/interface.py:1041: error: Statement is unreachable  [unreachable]
+ static_frame/core/interface.py:1043: error: Statement is unreachable  [unreachable]
+ static_frame/core/interface.py:1066: error: Item "type" of "type[InterfaceValues[TVContainer_co@InterfaceValues]] | type[InterfaceString[TVContainer_co@InterfaceString]] | type[InterfaceDatetime[TVContainer_co@InterfaceDatetime]] | type[InterfaceTranspose[TVContainer_co@InterfaceTranspose]] | type[InterfaceFillValue[TVContainer_co@InterfaceFillValue]] | type[InterfaceRe[TVContainer_co@InterfaceRe]] | type[ReduceDispatch] | type[<subclass of "static_frame.core.node_selector.Interface" and "typing.Mapping[Any, Any]">]" has no attribute "CLS_DELEGATE"  [union-attr]

cloud-init (https://github.com/canonical/cloud-init)
+ tests/unittests/sources/test_maas.py:269: error: Statement is unreachable  [unreachable]
+ tests/unittests/cmd/test_status.py:260: error: Statement is unreachable  [unreachable]

xarray (https://github.com/pydata/xarray)
+ xarray/core/extension_array.py: note: In function "union_unordered_categorical_and_scalar":
+ xarray/core/extension_array.py:163: error: If condition in comprehension is always true  [redundant-expr]

p-sawicki · 2026-01-28T16:48:19Z

@p-sawicki I'm getting AssertionError: Cannot add a traceback entry with a negative line number for op on this PR, seems like this is new to your PR, any advice? If I revert your PR, things pass

https://github.com/python/mypy/actions/runs/21421020108/job/61680277800?pr=20660

the assertions are meant to catch mistakes when modifying mypyc so it's unfortunate that changes in mypy triggered them. are tests still failing because of them? if they block unrelated PRs then i think we would have to disable them.

Improve reachability in narrowing logic

b7555c7

hauntsaninja mentioned this pull request Jan 28, 2026

Better narrowing #20564

Open

hauntsaninja commented Jan 28, 2026

View reviewed changes

github-actions bot mentioned this pull request Jan 28, 2026

Check #20660 sterliakov/mypy-issues#250

Closed

hauntsaninja mentioned this pull request Jan 28, 2026

Match exhaustion fails incorrectly when matching same option twice. #17372

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hauntsaninja mentioned this pull request Jan 28, 2026

Non-overlapping equality not detected inside of pattern matching case statements. #14965

Closed

add a test

7062598

hauntsaninja changed the title ~~Improve reachability in narrowing logic~~ Improve reachability in narrowing logic, avoid losing narrowing constraints Jan 28, 2026

hauntsaninja marked this pull request as draft January 28, 2026 01:32

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github-actions bot mentioned this pull request Jan 28, 2026

Check #20660 sterliakov/mypy-issues#251

Closed

hauntsaninja added 3 commits January 27, 2026 19:25

ignore promotions

9881335

fix unions

78e256a

update test, sadly

35a3b2d

hauntsaninja force-pushed the narrow37 branch from 4849aaf to 35a3b2d Compare January 28, 2026 03:28

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hauntsaninja added 2 commits January 27, 2026 22:34

revert

0473058

different approach

1449c5e

hauntsaninja changed the title ~~Improve reachability in narrowing logic, avoid losing narrowing constraints~~ [wip] Improve reachability in narrowing logic, avoid losing narrowing constraints Jan 28, 2026

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hauntsaninja added 5 commits January 27, 2026 22:56

more test

ea630b2

Revert "different approach"

4d6be0d

This reverts commit 1449c5e.

Revert "revert"

635a82b

This reverts commit 0473058.

another approach

54512be

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e9c1274

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