René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"1.17.0 regression: dict literals incompatible with Sequence[TypedDict]","articleBody":"## Bug Report\n\n(yes I used claude code to sum the issue up)\n\n### Summary\nmypy 1.17.0 incorrectly reports type errors when using dict literals that should be compatible with `Sequence[TypedDict]` fields. This worked correctly in mypy 1.16.1 and represents a regression in TypedDict structural typing.\n\n### Expected Behavior\nDict literals with the correct structure should be accepted when assigned to fields expecting `Sequence[TypedDict]`, following structural typing principles that TypedDict is supposed to provide.\n\n### Actual Behavior\nmypy 1.17.0 reports:\n```\nerror: Incompatible types (expression has type \"list[dict[str, str]]\", TypedDict item \"inputs\" has type \"Sequence[Component]\")  [typeddict-item]\n```\n\n### Environment\n- **mypy version**: 1.17.0 (fails) vs 1.16.1 (works)\n- **Python version**: 3.11.9\n- **Platform**: Linux\n- **Dependencies**: None (pure `typing` module)\n\n### Minimal Reproducible Example\n\nSave as `test_regression.py`:\n\n```python\nclass Component(TypedDict):\n    \"\"\"Simulates ABIComponent from eth_typing\"\"\"\n    type: str\n    name: NotRequired[str]\n    components: NotRequired[Sequence['Component']]\n\nclass Function(TypedDict):\n    \"\"\"Simulates ABIFunction from eth_typing\"\"\"\n    type: Literal['function']\n    name: str\n    stateMutability: Literal['pure', 'view', 'nonpayable', 'payable']\n    inputs: NotRequired[Sequence[Component]]\n    outputs: NotRequired[Sequence[Component]]\n\n# This should work according to TypedDict structural typing rules\n# Works in mypy 1.16.1, fails in mypy 1.17.0\n\n# Simple case\nsimple_function: Function = {\n    'type': 'function',\n    'name': 'test',\n    'stateMutability': 'view',\n    'inputs': [{'type': 'uint256', 'name': 'value'}],  # Error in 1.17.0\n    'outputs': [{'type': 'bool'}]  # Error in 1.17.0\n}\n\n# Nested case (components within components)\ncomplex_function: Function = {\n    'type': 'function', \n    'name': 'complex',\n    'stateMutability': 'view',\n    'inputs': [\n        {'type': 'tuple', 'components': [  # Error in 1.17.0\n            {'type': 'uint256', 'name': 'amount'},\n            {'type': 'address', 'name': 'recipient'}\n        ]}\n    ]\n}\n\n# Direct assignment to Sequence should also fail\ncomponents_list: Sequence[Component] = [\n    {'type': 'uint256', 'name': 'value'},  # Error in 1.17.0\n    {'type': 'address'}  # Error in 1.17.0  \n]\n\n# List of functions\nfunction_list: list[Function] = [\n    {\n        'type': 'function',\n        'name': 'func1', \n        'stateMutability': 'pure',\n        'inputs': [{'type': 'uint256'}]  # Error in 1.17.0\n    }\n]\n```\n\n### Steps to Reproduce\n1. Save the code above as `test_regression.py`\n2. Run with mypy 1.16.1: `mypy test_regression.py` → ✅ Success\n3. Run with mypy 1.17.0: `mypy test_regression.py` → ❌ Fails\n\n### Error Output (mypy 1.17.0)\n```\nstandalone_bug_test.py:43: error: Incompatible types (expression has type \"list[dict[str, str]]\", TypedDict item \"components\" has type \"Sequence[Component]\")  [typeddict-item]\nstandalone_bug_test.py:62: error: Incompatible types (expression has type \"list[dict[str, str]]\", TypedDict item \"inputs\" has type \"Sequence[Component]\")  [typeddict-item]\n```\n\n### Analysis\nThis is a clear regression where mypy 1.17.0 is no longer properly recognizing dict literals as structurally compatible with TypedDict types when used in `Sequence[TypedDict]` contexts. The fundamental principle of TypedDict structural typing is broken.\n\n### Root Cause\nThe issue appears to be in mypy's type inference logic where:\n1. Dict literals inside lists are inferred as `dict[str, str]` instead of the contextually expected TypedDict\n2. This inference happens before structural compatibility is checked\n3. The resulting `list[dict[str, str]]` is then (correctly) rejected as incompatible with `Sequence[TypedDict]`\n\n### Expected Fix\nDict literals should be contextually typed when the expected type is `Sequence[TypedDict]`, allowing structural compatibility to work as intended.\n\n### Workarounds\n- Use `# type: ignore[typeddict-item]` or `# type: ignore[assignment]` comments\n- Use explicit `cast(list[Component], [...])` \n- Use explicit TypedDict construction: `Component(type='uint256', name='value')`\n- Downgrade to mypy 1.16.1","author":{"url":"https://github.com/LefterisJP","@type":"Person","name":"LefterisJP"},"datePublished":"2025-07-20T14:40:13.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":7},"url":"https://github.com/19483/mypy/issues/19483"}