[ty] prefer declared type on invalid TypedDict creation #21168
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Diagnostic diff on typing conformance testsChanges were detected when running ty on typing conformance tests--- old-output.txt 2025-10-31 14:56:30.742757677 +0000
+++ new-output.txt 2025-10-31 14:56:33.878763195 +0000
@@ -927,6 +927,7 @@
typeddicts_operations.py:29:42: error[invalid-argument-type] Invalid argument to key "year" with declared type `int` on TypedDict `Movie`: value of type `float`
typeddicts_operations.py:32:36: error[invalid-key] Invalid key for TypedDict `Movie`: Unknown key "other"
typeddicts_operations.py:37:20: error[missing-typed-dict-key] Missing required key 'name' in TypedDict `Movie` constructor
+typeddicts_operations.py:47:1: error[unresolved-attribute] Object of type `Movie` has no attribute `clear`
typeddicts_operations.py:62:1: error[unresolved-attribute] Object of type `MovieOptional` has no attribute `clear`
typeddicts_readonly.py:24:4: error[invalid-assignment] Cannot assign to key "members" on TypedDict `Band`: key is marked read-only
typeddicts_readonly.py:50:4: error[invalid-assignment] Cannot assign to key "title" on TypedDict `Movie1`: key is marked read-only
@@ -946,5 +947,5 @@
typeddicts_usage.py:28:17: error[missing-typed-dict-key] Missing required key 'name' in TypedDict `Movie` constructor
typeddicts_usage.py:28:18: error[invalid-key] Invalid key for TypedDict `Movie`: Unknown key "title"
typeddicts_usage.py:40:24: error[invalid-type-form] The special form `typing.TypedDict` is not allowed in type expressions. Did you mean to use a concrete TypedDict or `collections.abc.Mapping[str, object]` instead?
-Found 948 diagnostics
+Found 949 diagnostics
WARN A fatal error occurred while checking some files. Not all project files were analyzed. See the diagnostics list above for details. |
AlexWaygood
approved these changes
Oct 31, 2025
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Conformance suite change is positive (we are supposed to error on that line). Ecosystem change looks like we have a TypedDict type where we previously didn't. |
carljm
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Oct 31, 2025
…21169) ## Summary Discussion with @ibraheemdev clarified that #21168 was incorrect. In a case of failed inference of a dict literal as a `TypedDict`, we should store the context-less inferred type of the dict literal as the type of the dict literal expression itself; the fallback to declared type should happen at the level of the overall assignment definition. The reason the latter isn't working yet is because currently we (wrongly) consider a homogeneous dict type as assignable to a `TypedDict`, so we don't actually consider the assignment itself as failed. So the "bug" I observed (and tried to fix) will naturally be fixed by implementing TypedDict assignability rules. Rollback #21168 except for the tests, and modify the tests to include TODOs as needed. ## Test Plan Updated mdtests.
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Summary
In general, when we have an invalid assignment (inferred assigned type is not assignable to declared type), we fall back to inferring the declared type, since the declared type is a more explicit declaration of the programmer's intent. This also maintains the invariant that our inferred type for a name is always assignable to the declared type for that same name. For example:
We weren't following this pattern for dictionary literals inferred (via type context) as a typed dictionary; if the literal was not valid for the annotated TypedDict type, we would just fall back to the normal inferred type of the dict literal, effectively ignoring the annotation, and resulting in inferred type not assignable to declared type.
Test Plan
Added mdtest assertions.