Dictionary accepting Mapping in a merge operation

[jzazo]

Hi. I would like to propose that a dictionary accepts a mapping to the or operation. Something like:

class dict(MutableMapping[_KT, _VT]):
    ...
        @overload
        def __or__(self, __value: Mapping[_KT, _VT]) -> dict[_KT, _VT]: ...
        @overload
        def __or__(self, __value: Mapping[_T1, _T2]) -> dict[_KT | _T1, _VT | _T2]: ...

It would be useful in scenarios like the following:

from collections.abc import Mapping
from typing import cast

a = cast(Mapping[str, int], {"a": 1})
{"b": 2} | a

At this point, pyright complains with # error: Operator "|" not supported for types "dict[str, int]" and "Mapping[str, int]".
I referenced this problem in this issue. Thanks.

[AlexWaygood]

That would be incorrect, however, because not all mappings can be merged with a dict using the | operator: it's not part of the collections.abc.Mapping interface. We used to define dict.__or__ in the way you propose above, but we changed it after a user complained that their type checker failed to spot a bug in their code. See #10678 and #10679.

[jzazo]

I don't understand what you mean with it's not part of the Mapping interface. dict | Mapping -> dict and this can be guaranteed regardless of the concrete mapping instance. And Mapping | dict -> ? because the concrete implementation of Mapping will take precedence over the dict.__ror__ method. Am I wrong?

I was proposing a change on the __or__ method, not on the __ror__ method. Thanks and let me know if I overlooked something.

[srittau]

As pointed out by Alex in #10678, dict.__or__ explicitly checks whether the other object is a dict:

https://github.com/python/cpython/blob/5d384b0468b35b393f8ae2d3149d13ff607c9501/Objects/dictobject.c#L3713C1-L3728

static PyObject *
dict_or(PyObject *self, PyObject *other)
{
    if (!PyDict_Check(self) || !PyDict_Check(other)) {
        Py_RETURN_NOTIMPLEMENTED;
    }
    /* ... */
}

The following code fails at runtime:

from collections.abc import Mapping

class MyMapping(Mapping):
    def __getitem__(self, i):
        return 42
    def __iter__(self):
        return iter([42])
    def __len__(self):
        return 1

{} | MyMapping()

Traceback (most recent call last):
  File "foo.py", line 11, in <module>
    {} | MyMapping()
    ~~~^~~~~~~~~~~~~
TypeError: unsupported operand type(s) for |: 'dict' and 'MyMapping'

[jzazo]

Thanks @srittau for explaining.

[JelleZijlstra]

And just in case you were looking for a workaround {"b": 2, **a} works on any Mapping and should be accepted by type checkers.