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[ty] Enforce Final assignments after conditional bindings#22986

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charliermarsh merged 1 commit intomainfrom
charlie/final
Jan 30, 2026
Merged

[ty] Enforce Final assignments after conditional bindings#22986
charliermarsh merged 1 commit intomainfrom
charlie/final

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@charliermarsh charliermarsh commented Jan 30, 2026

Summary

When a Final symbol had a conditional prior binding, the iterator could yield Undefined (from the control flow path where the condition was false) as the first element.

Closes astral-sh/ty#2674.

@charliermarsh charliermarsh added ty Multi-file analysis & type inference bug Something isn't working labels Jan 30, 2026
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astral-sh-bot bot commented Jan 30, 2026

Typing conformance results

No changes detected ✅

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astral-sh-bot bot commented Jan 30, 2026

mypy_primer results

Changes were detected when running on open source projects 
pip (https://github.com/pypa/pip)
+ src/pip/_vendor/urllib3/util/ssl_.py:75:27: error[invalid-assignment] Reassignment of `Final` symbol `PROTOCOL_TLS` is not allowed: Reassignment of `Final` symbol
+ src/pip/_vendor/urllib3/util/ssl_.py:80:5: error[invalid-assignment] Reassignment of `Final` symbol `PROTOCOL_TLS_CLIENT` is not allowed: Reassignment of `Final` symbol
+ src/pip/_vendor/urllib3/util/ssl_.py:86:5: error[invalid-assignment] Reassignment of `Final` symbol `OP_NO_SSLv2` is not allowed: Reassignment of `Final` symbol
+ src/pip/_vendor/urllib3/util/ssl_.py:86:18: error[invalid-assignment] Reassignment of `Final` symbol `OP_NO_SSLv3` is not allowed: Reassignment of `Final` symbol
+ src/pip/_vendor/urllib3/util/ssl_.py:87:5: error[invalid-assignment] Reassignment of `Final` symbol `OP_NO_COMPRESSION` is not allowed: Reassignment of `Final` symbol
+ src/pip/_vendor/urllib3/util/ssl_.py:93:5: error[invalid-assignment] Reassignment of `Final` symbol `OP_NO_TICKET` is not allowed: Reassignment of `Final` symbol
- Found 625 diagnostics
+ Found 631 diagnostics

prefect (https://github.com/PrefectHQ/prefect)
+ src/integrations/prefect-dbt/prefect_dbt/core/settings.py:94:28: error[invalid-assignment] Object of type `dict[Any, Any] | int | dict[str, Any] | ... omitted 4 union elements` is not assignable to `dict[str, Any]`
+ src/integrations/prefect-dbt/prefect_dbt/core/settings.py:99:28: error[invalid-assignment] Object of type `int | dict[Any, Any] | float | ... omitted 3 union elements` is not assignable to `dict[str, Any]`
+ src/prefect/cli/deploy/_core.py:86:21: error[invalid-assignment] Object of type `dict[Any, Any] | int | dict[str, Any] | ... omitted 4 union elements` is not assignable to `dict[str, Any]`
+ src/prefect/cli/deploy/_core.py:87:21: error[invalid-assignment] Object of type `int | dict[Any, Any] | float | ... omitted 3 union elements` is not assignable to `dict[str, Any]`
+ src/prefect/deployments/steps/core.py:137:38: error[invalid-argument-type] Argument is incorrect: Argument type `dict[Any, Any] | int | dict[str, Any] | ... omitted 4 union elements` does not satisfy constraints (`str`, `int`, `int | float`, `bool`, `dict[Any, Any]`, `list[Any]`, `None`) of type variable `T`
- src/prefect/utilities/templating.py:320:13: error[invalid-assignment] Invalid subscript assignment with key of type `object` and value of type `Unknown | dict[str, Any]` on object of type `dict[str, Any]`
+ src/prefect/utilities/templating.py:320:13: error[invalid-assignment] Invalid subscript assignment with key of type `object` and value of type `Unknown | int | dict[str, Any] | ... omitted 4 union elements` on object of type `dict[str, Any]`
- src/prefect/utilities/templating.py:323:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_block_document_references | dict[str, Any]`, found `list[Unknown | dict[str, Any]]`
+ src/prefect/utilities/templating.py:323:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_block_document_references | dict[str, Any]`, found `list[Unknown | int | dict[str, Any] | ... omitted 4 union elements]`
- src/prefect/utilities/templating.py:437:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_variables`, found `dict[object, Unknown]`
+ src/prefect/utilities/templating.py:437:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_variables`, found `dict[object, Unknown | int | float | ... omitted 4 union elements]`
- src/prefect/utilities/templating.py:442:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_variables`, found `list[Unknown]`
+ src/prefect/utilities/templating.py:442:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_variables`, found `list[Unknown | int | float | ... omitted 4 union elements]`
- src/prefect/workers/base.py:232:13: error[invalid-argument-type] Argument is incorrect: Argument type `str | dict[str, Any]` does not satisfy constraints (`str`, `int`, `int | float`, `bool`, `dict[Any, Any]`, `list[Any]`, `None`) of type variable `T`
+ src/prefect/workers/base.py:232:13: error[invalid-argument-type] Argument is incorrect: Argument type `str | int | dict[str, Any] | ... omitted 3 union elements` does not satisfy constraints (`str`, `int`, `int | float`, `bool`, `dict[Any, Any]`, `list[Any]`, `None`) of type variable `T`
+ src/prefect/workers/base.py:234:22: error[invalid-argument-type] Argument expression after ** must be a mapping type: Found `int | Unknown | float | ... omitted 4 union elements`
- Found 5369 diagnostics
+ Found 5375 diagnostics

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
- src/scikit_build_core/build/wheel.py:99:20: error[no-matching-overload] No overload of bound method `__init__` matches arguments
- Found 47 diagnostics
+ Found 46 diagnostics

dd-trace-py (https://github.com/DataDog/dd-trace-py)
+ ddtrace/vendor/psutil/_common.py:29:5: error[invalid-assignment] Reassignment of `Final` symbol `AF_INET6` is not allowed: Reassignment of `Final` symbol
+ ddtrace/vendor/psutil/_common.py:33:5: error[invalid-assignment] Reassignment of `Final` symbol `AF_UNIX` is not allowed: Reassignment of `Final` symbol
- Found 8567 diagnostics
+ Found 8569 diagnostics

No memory usage changes detected ✅

@charliermarsh
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charliermarsh commented Jan 30, 2026

The new ecosystem diagnostics are false positives, but they may be considered "correct" given our handling of try-except:

try:
    from ssl import PROTOCOL_TLS
    PROTOCOL_SSLv23 = PROTOCOL_TLS
except ImportError:
    try:
        from ssl import PROTOCOL_SSLv23 as PROTOCOL_TLS
        PROTOCOL_SSLv23 = PROTOCOL_TLS
    except ImportError:
        PROTOCOL_SSLv23 = PROTOCOL_TLS = 2

We're flagging the "reassignment" to PROTOCOL_TLS which is declared as Final in ssl.pyi...

@charliermarsh charliermarsh marked this pull request as ready for review January 30, 2026 18:54
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AlexWaygood commented Jan 30, 2026

The new ecosystem diagnostics are false positives, but they may be considered "correct" given our handling of try-except:

I disagree that Final type qualifiers should travel across module boundaries like that, but the consensus was against me in https://discuss.python.org/t/imported-final-variable/82429

@charliermarsh charliermarsh merged commit 0a7bd6f into main Jan 30, 2026
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@charliermarsh charliermarsh deleted the charlie/final branch January 30, 2026 18:58
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charliermarsh commented Jan 30, 2026

I was also surprised by this but am significantly less learned in the discourse.

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@AlexWaygood AlexWaygood AlexWaygood approved these changes

@carljm carljm Awaiting requested review from carljm carljm is a code owner

@sharkdp sharkdp Awaiting requested review from sharkdp sharkdp is a code owner

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no error when reassigning to Final name after conditional assignment

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@charliermarsh @AlexWaygood