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AttributeErrors raised in `.keys()` or `.__getitem__()` during `{**mymapping}`are incorrectly masked

NickCrews

Bug description:

I 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:

class MyMapping:
    def __init__(
        self,
        *,
        raises_on_keys: type[Exception] | None = None,
        raises_on_getitem: type[Exception] | None = None,
    ):
        self.raises_on_keys = raises_on_keys
        self.raises_on_getitem = raises_on_getitem

    def __getitem__(self, key):
        if self.raises_on_getitem:
            raise self.raises_on_getitem("error in __getitem__")
        return key * 2

    def keys(self):
        if self.raises_on_keys:
            raise self.raises_on_keys("error in keys")
        return [1, 2, 3]


options = [
    None,
    ValueError,
    AttributeError,
]
outcomes = []
for raises_on_keys in options:
    for raises_on_getitem in options:
        try:
            d = {
                **MyMapping(
                    raises_on_keys=raises_on_keys, raises_on_getitem=raises_on_getitem
                )
            }
            outcomes.append((raises_on_keys, raises_on_getitem, "Success", d))
        except Exception as e:
            outcomes.append((raises_on_keys, raises_on_getitem, "Exception", str(e)))

# format to markdown table
print("| raises_on_keys | raises_on_getitem | outcome | result |")
print("| --- | --- | --- | --- |")
for raises_on_keys, raises_on_getitem, outcome, result in outcomes:
    raises_on_keys_str = raises_on_keys.__name__ if raises_on_keys else "None"
    raises_on_getitem_str = raises_on_getitem.__name__ if raises_on_getitem else "None"
    print(
        f"| {raises_on_keys_str} | {raises_on_getitem_str} | {outcome} | `{result}` |"
    )

Ran with uv run --python 3.14 bug.py, which resolves to python 3.14.2. This gives:

raises_on_keys raises_on_getitem error
None None ``
None ValueError ValueError: error in __getitem__
None AttributeError TypeError: 'MyMapping' object is not a mapping
ValueError None ValueError: error in keys
ValueError ValueError ValueError: error in keys
ValueError AttributeError ValueError: error in keys
AttributeError None TypeError: 'MyMapping' object is not a mapping
AttributeError ValueError TypeError: 'MyMapping' object is not a mapping
AttributeError AttributeError TypeError: 'MyMapping' object is not a mapping

What 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.

I 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"

eg guessing this is how this is currently implemented:

try:
    for key in obj.keys():
        yield key, obj.__getitem__(key)
except AttributeError as e:
    raise TypeError(f"'{type(obj).__name__}' object is not a mapping")

What I think SHOULD happen:

try:
    keys = obj.keys
except AttributeError as e:
    raise TypeError(f"'{type(obj).__name__}' object is not a mapping")
for key in keys():
    try:
        getter = obj.__getitem__
    except AttributeError as e:
        raise TypeError(f"'{type(obj).__name__}' object is not a mapping")
    yield key, getter(key)

EDIT: 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)

try:
    keys = obj.keys
except AttributeError as e:
    raise TypeError(f"'{type(obj).__name__}' object requires a .keys() method to be used as a mapping")
try:
    getter = obj.__getitem__
except AttributeError as e:
    raise TypeError(f"'{type(obj).__name__}' object requires a .__getitem__() method to be used as a mapping")
for key in keys():
    yield key, getter(key)

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs