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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"AttributeErrors raised in `.keys()` or `.__getitem__()` during `{**mymapping}`are incorrectly masked","articleBody":"### Bug description:\n\nI have a custom Mapping type. I am unpacking it with eg `{**mymapping}`. If, in either the `keys()` or the the `__getitem__()` method, I raise most kinds of errors, such as a ValueError, these are reported correctly. BUT, if I raise an AttributeError, then this error isn't reported properly, instead I get `TypeError: 'MyMapping' object is not a mapping`, masking the actual error:\n\n```python\nclass MyMapping:\n    def __init__(\n        self,\n        *,\n        raises_on_keys: type[Exception] | None = None,\n        raises_on_getitem: type[Exception] | None = None,\n    ):\n        self.raises_on_keys = raises_on_keys\n        self.raises_on_getitem = raises_on_getitem\n\n    def __getitem__(self, key):\n        if self.raises_on_getitem:\n            raise self.raises_on_getitem(\"error in __getitem__\")\n        return key * 2\n\n    def keys(self):\n        if self.raises_on_keys:\n            raise self.raises_on_keys(\"error in keys\")\n        return [1, 2, 3]\n\n\noptions = [\n    None,\n    ValueError,\n    AttributeError,\n]\noutcomes = []\nfor raises_on_keys in options:\n    for raises_on_getitem in options:\n        try:\n            d = {\n                **MyMapping(\n                    raises_on_keys=raises_on_keys, raises_on_getitem=raises_on_getitem\n                )\n            }\n            outcomes.append((raises_on_keys, raises_on_getitem, \"Success\", d))\n        except Exception as e:\n            outcomes.append((raises_on_keys, raises_on_getitem, \"Exception\", str(e)))\n\n# format to markdown table\nprint(\"| raises_on_keys | raises_on_getitem | outcome | result |\")\nprint(\"| --- | --- | --- | --- |\")\nfor raises_on_keys, raises_on_getitem, outcome, result in outcomes:\n    raises_on_keys_str = raises_on_keys.__name__ if raises_on_keys else \"None\"\n    raises_on_getitem_str = raises_on_getitem.__name__ if raises_on_getitem else \"None\"\n    print(\n        f\"| {raises_on_keys_str} | {raises_on_getitem_str} | {outcome} | `{result}` |\"\n    )\n```\n\nRan with `uv run --python 3.14 bug.py`, which resolves to python `3.14.2`. This gives:\n\n| raises_on_keys | raises_on_getitem | error |\n| --- | --- | --- |\n| None | None | `` |\n| None | ValueError | `ValueError: error in __getitem__` |\n| None | AttributeError | `TypeError: 'MyMapping' object is not a mapping` |\n| ValueError | None | `ValueError: error in keys` |\n| ValueError | ValueError | `ValueError: error in keys` |\n| ValueError | AttributeError | `ValueError: error in keys` |\n| AttributeError | None | `TypeError: 'MyMapping' object is not a mapping` |\n| AttributeError | ValueError | `TypeError: 'MyMapping' object is not a mapping` |\n| AttributeError | AttributeError | `TypeError: 'MyMapping' object is not a mapping` |\n\nWhat I would expect is for all of the `TypeError: 'MyMapping' object is not a mapping` errors to actually be `AttributeError: error in keys` or `AttributeError: error in __getitem__` errors.\n\nI assume this is because in the implementation, it does assumes ducktyping, and the raised attribute error is interpreted as \"the passed object doesn't even have a `keys()`/`__getitem__` method\"\n\neg guessing this is how this is currently implemented:\n```python\ntry:\n    for key in obj.keys():\n        yield key, obj.__getitem__(key)\nexcept AttributeError as e:\n    raise TypeError(f\"'{type(obj).__name__}' object is not a mapping\")\n```\n\nWhat I think SHOULD happen:\n\n```python\ntry:\n    keys = obj.keys\nexcept AttributeError as e:\n    raise TypeError(f\"'{type(obj).__name__}' object is not a mapping\")\nfor key in keys():\n    try:\n        getter = obj.__getitem__\n    except AttributeError as e:\n        raise TypeError(f\"'{type(obj).__name__}' object is not a mapping\")\n    yield key, getter(key)\n```\n\nEDIT: Actually this should be more performant, only 2 checks, instead of N checks, one per key. (Also, for the record, this includes suggestion to improve the error messages, but that should definitely be a separate PR)\n\n```python\ntry:\n    keys = obj.keys\nexcept AttributeError as e:\n    raise TypeError(f\"'{type(obj).__name__}' object requires a .keys() method to be used as a mapping\")\ntry:\n    getter = obj.__getitem__\nexcept AttributeError as e:\n    raise TypeError(f\"'{type(obj).__name__}' object requires a .__getitem__() method to be used as a mapping\")\nfor key in keys():\n    yield key, getter(key)\n```\n\n### CPython versions tested on:\n\n3.14\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-145877\n* gh-145878\n* gh-145906\n* gh-146472\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/NickCrews","@type":"Person","name":"NickCrews"},"datePublished":"2026-03-12T18:03:32.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":16},"url":"https://github.com/145876/cpython/issues/145876"}