Narrow types based on collection containment #20602

hauntsaninja · 2026-01-16T21:44:43Z

~~Test cases come from #17344 authored by Jordandev678~~

We've wanted to do this for a long time, but previous attempt was reverted due to issues like #17841 , #17864 , #17869

Following #20492 we should now be in a position to do this narrowing

Fixes #3229
Fixes #20234
Fixes #18208
Fixes #16774

hauntsaninja · 2026-01-16T22:16:29Z

The setuptools regression is:

import os
from typing import Callable, TypeVar
T = TypeVar("T")
def auto_chmod(func: Callable[..., T], arg: str) -> T:
    if func in [os.unlink, os.remove]:
        return func(arg)
    raise

The xarray regression is just a redefinition issue, now that we have narrowed to Literal

hauntsaninja · 2026-01-17T06:02:31Z

mypy/checker.py

+                            narrowable_indices={0},
+                        )
+
+                        # We only try and narrow away 'None' for now


I think this is actually not really needed anymore. I will clean it up, there is one test case affected, a later PR in my stack changes it

hauntsaninja · 2026-01-21T20:43:54Z

Planning on merging this in the next few days

JukkaL

Nice to finally have this working! Left a few comments about tests.

JukkaL · 2026-01-22T16:52:56Z

test-data/unit/check-narrowing.test

+if x in typ:
+    reveal_type(x)  # N: Revealed type is "Literal['a'] | Literal['b']"
+else:
+    reveal_type(x)  # N: Revealed type is "builtins.str"


Is this a duplicate test case?

Thanks! I'd copied these tests from the first PR that attempted to make this change, but now I've just rewritten them completely from scratch

JukkaL · 2026-01-22T16:53:11Z

test-data/unit/check-narrowing.test

+typeAlpha: Tuple[Literal['a', 'b', 'c'], ...] = ('a', 'b', 'c')
+strIn: Literal['c'] = "c"
+reveal_type(strIn)  # N: Revealed type is "Literal['c']"
+#Check we don't expand a Literal into the Union type


Style nit: missing space after #.

JukkaL · 2026-01-22T16:54:43Z

test-data/unit/check-narrowing.test

+[builtins fixtures/narrowing.pyi]
+[typing fixtures/typing-medium.pyi]
+
+[case testTypeNarrowingStringInList]


Test type narrowing with an unsupported collection type (Sequence?). Test unsupported equality narrowing with collection, i.e. a case where normal equality doesn't narrow, so it shouldn't narrow in the context of a collection either (I think there was a bug like this in the past -- it's possible that there is an existing test case, though).

Can this be used for enum type narrowing, e.g. if there is a union with enum type and non-enum type?

Yeah, I'd added a test for the consistency bug as a regression test the last time round. That is testConsistentNarrowingEqAndIn and you can see the change in this PR is desirable.

The implementation in this PR kind of guarantees that the narrowing we get from in and from == are basically the same

Yes, can be used for enum type narrowing when non-enum type is not ambiguous. I will add a test in a future PR (a later commit in my series rewrites some of those tests)

github-actions · 2026-01-22T22:12:58Z

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

spark (https://github.com/apache/spark)
+ python/pyspark/pandas/utils.py:756: error: Unused "type: ignore" comment  [unused-ignore]

mypy (https://github.com/python/mypy)
+ mypy/constraints.py:265: error: Argument "arg_kinds" to "Parameters" has incompatible type "list[Literal[ArgKind.ARG_POS, ArgKind.ARG_STAR, ArgKind.ARG_STAR2]]"; expected "list[ArgKind]"  [arg-type]
+ mypy/constraints.py:265: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
+ mypy/constraints.py:265: note: Consider using "Sequence" instead, which is covariant
+ mypyc/irbuild/util.py:141: error: Redundant cast to "Literal['native_class', 'allow_interpreted_subclasses', 'serializable', 'free_list_len']"  [redundant-cast]
+ mypyc/irbuild/util.py:141: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-redundant-cast for more info

Tanjun (https://github.com/FasterSpeeding/Tanjun)
- tanjun/context/menu.py:144: error: Incompatible return value type (got "CommandType", expected "Literal[CommandType.USER, CommandType.MESSAGE]")  [return-value]

altair (https://github.com/vega/altair)
+ tools/versioning.py:258: error: Unused "type: ignore" comment  [unused-ignore]

mitmproxy (https://github.com/mitmproxy/mitmproxy)
+ mitmproxy/addons/core.py:172: error: Unused "type: ignore" comment  [unused-ignore]

steam.py (https://github.com/Gobot1234/steam.py)
- steam/ext/csgo/state.py:213: error: Argument 1 to "__delitem__" of "dict" has incompatible type "int"; expected "AssetID"  [arg-type]
- steam/ext/csgo/state.py:214: error: Argument 1 to "__delitem__" of "dict" has incompatible type "int"; expected "AssetID"  [arg-type]

setuptools (https://github.com/pypa/setuptools)
+ setuptools/_shutil.py:39: error: Incompatible return value type (got "None", expected "_T")  [return-value]

pandas (https://github.com/pandas-dev/pandas)
+ pandas/io/stata.py:1437: error: Redundant cast to "str"  [redundant-cast]
+ pandas/io/stata.py:1797: error: Redundant cast to "str"  [redundant-cast]
+ pandas/io/stata.py:1803: error: Redundant cast to "str"  [redundant-cast]

discord.py (https://github.com/Rapptz/discord.py)
+ discord/components.py:1464: error: Unused "type: ignore" comment  [unused-ignore]
+ discord/app_commands/models.py:900: error: Unused "type: ignore" comment  [unused-ignore]
+ discord/threads.py:286: error: Unused "type: ignore" comment  [unused-ignore]
+ discord/state.py:693: error: Unused "type: ignore" comment  [unused-ignore]

archinstall (https://github.com/archlinux/archinstall)
+ archinstall/lib/menu/list_manager.py:84: error: Redundant cast to "str"  [redundant-cast]

rotki (https://github.com/rotki/rotki)
+ rotkehlchen/types.py:1278: error: Unused "type: ignore" comment  [unused-ignore]
+ rotkehlchen/types.py:1286: error: Unused "type: ignore" comment  [unused-ignore]
+ rotkehlchen/chain/ethereum/utils.py:90: error: Unused "type: ignore" comment  [unused-ignore]
+ rotkehlchen/inquirer.py:976: error: Redundant cast to "Literal[ChainID.ETHEREUM, ChainID.POLYGON_POS, ChainID.OPTIMISM, ChainID.ARBITRUM_ONE, ChainID.GNOSIS, ChainID.BASE, ChainID.BINANCE_SC]"  [redundant-cast]
+ rotkehlchen/tasks/assets.py:459: error: Unused "type: ignore[arg-type]" comment  [unused-ignore]
+ rotkehlchen/chain/aggregator.py:1187: error: Unused "type: ignore" comment  [unused-ignore]
+ rotkehlchen/rotkehlchen.py:739: error: Unused "type: ignore" comment  [unused-ignore]
+ rotkehlchen/rotkehlchen.py:744: error: Unused "type: ignore" comment  [unused-ignore]
+ rotkehlchen/rotkehlchen.py:1017: error: Redundant cast to "Literal[SupportedBlockchain.ETHEREUM, SupportedBlockchain.OPTIMISM, SupportedBlockchain.POLYGON_POS, SupportedBlockchain.ARBITRUM_ONE, SupportedBlockchain.BASE, SupportedBlockchain.GNOSIS, SupportedBlockchain.SCROLL, SupportedBlockchain.BINANCE_SC]"  [redundant-cast]
+ rotkehlchen/api/services/transactions.py:125: error: Unused "type: ignore" comment  [unused-ignore]
+ rotkehlchen/api/services/transactions.py:126: error: "ChainManagerWithTransactions[Any]" has no attribute "node_inquirer"  [attr-defined]
+ rotkehlchen/api/services/transactions.py:240: error: Redundant cast to "Literal[SupportedBlockchain.ETHEREUM, SupportedBlockchain.OPTIMISM, SupportedBlockchain.POLYGON_POS, SupportedBlockchain.ARBITRUM_ONE, SupportedBlockchain.BASE, SupportedBlockchain.GNOSIS, SupportedBlockchain.SCROLL, SupportedBlockchain.BINANCE_SC] | Literal[SupportedBlockchain.SOLANA]"  [redundant-cast]
+ rotkehlchen/api/services/transactions.py:589: error: Unused "type: ignore" comment  [unused-ignore]

xarray (https://github.com/pydata/xarray)
+ xarray/core/dataset.py: note: In member "pad" of class "Dataset":
+ xarray/core/dataset.py:9090: error: Incompatible types in assignment (expression has type "Literal['constant']", variable has type "Literal['edge', 'reflect', 'symmetric', 'wrap']")  [assignment]

Narrow types based on collection containment

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hauntsaninja mentioned this pull request Jan 16, 2026

Better narrowing #20564

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github-actions bot mentioned this pull request Jan 16, 2026

Check #20602 sterliakov/mypy-issues#242

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hauntsaninja commented Jan 17, 2026

View reviewed changes

hauntsaninja mentioned this pull request Jan 22, 2026

Regression with narrowing with TypeVar intersection #20628

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JukkaL reviewed Jan 22, 2026

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rewrite test cases

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fixtures

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hauntsaninja merged commit b91f53c into python:master Jan 22, 2026
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hauntsaninja deleted the narrow4 branch January 22, 2026 22:15

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