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Avoid narrowing to NewType by hauntsaninja · Pull Request #20766 · python/mypy · GitHub

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Avoid narrowing to NewType - #20766

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hauntsaninja merged 1 commit into
python:masterfrom
hauntsaninja:narrow90
Feb 14, 2026
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Avoid narrowing to NewType#20766
hauntsaninja merged 1 commit into
python:masterfrom
hauntsaninja:narrow90

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hauntsaninja commented Feb 8, 2026 •
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Fixes #20733

I might make this type munging look a little nicer in a future refactor

This introduces new errors in two primer projects

hauntsaninja mentioned this pull request Feb 8, 2026
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github-actions Bot commented Feb 8, 2026

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

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]

rotki (https://github.com/rotki/rotki)
- rotkehlchen/chain/evm/decoding/zerox/decoder.py:146: error: Unused "type: ignore" comment  [unused-ignore]
- rotkehlchen/chain/ethereum/modules/sky/decoder.py:147: error: Unused "type: ignore" comment  [unused-ignore]

hauntsaninja commented Feb 8, 2026

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rotki was fixed in #20492 and is regressed here
steam was fixed in #20602 and is regressed here

JukkaL reviewed Feb 11, 2026
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Are you planning to address the regressions in this PR, or should we merge this as is, since this fixes a more important regression?

Comment thread test-data/unit/check-narrowing.test

def f2(whatever: object, uid: UserId | Other):
if whatever == uid:
reveal_type(whatever) # N: Revealed type is "builtins.int | __main__.Other"

JukkaL Feb 11, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

This is a bit questionable, since whatever could be float or another numeric type that happens to compare equal to int (not directly related to the changes in this PR).

hauntsaninja commented Feb 11, 2026 •
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The regressions are from what the PR does (no longer narrowing to a NewType), so nothing to address beyond deciding what behaviour we want mypy to have.

(I don't have a strong opinion / am not a big NewType user myself. Allowing narrowing is good on primer and the implementation is simpler and consistent with subclassing, but I can see the argument made in #20733 about having to be explicit)

JukkaL approved these changes Feb 12, 2026
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I like that NewTypes require an explicit constructor call.

hauntsaninja changed the title Avoid narrowing NewType Avoid narrowing to NewType Feb 12, 2026
hauntsaninja merged commit a2d715a into python:master Feb 14, 2026
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hauntsaninja deleted the narrow90 branch February 14, 2026 01:40
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Regression: Type of literal narrowed when asserting equality to a NewType

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