Odd interaction with `$slice`

[forana]

I am encountering an issue when passing nesting a function inside $slice calls, but only with mongomock_motor in use (works fine when regular motor is used). Example code is removing an item from a nested list by index:

Anonymized code:

await db.baskets.update_one(
    {"_id": basket_id},
    [
        {
            "$set": {
                "items": {
                    "$concatArrays": [
                        {"$slice": ["$items", index]},
                        {"$slice": ["$items", index + 1, {"$size": "$items"}]},
                    ]
                }
            }
        }
    ],
)

AsyncIOMotorClient handles this just fine. However, AsyncMongoMockClient encounters an error:

../snip/lib/python3.13/site-packages/mongomock_motor/__init__.py:43: in wrapper
    return getattr(proxy_source, method_name)(*args, **kwargs)
../snip/lib/python3.13/site-packages/mongomock/collection.py:616: in update_one
    self._update(
../snip/lib/python3.13/site-packages/mongomock/collection.py:718: in _update
    self._apply_update_pipeline(existing_document, document, session)
../snip/lib/python3.13/site-packages/mongomock/collection.py:765: in _apply_update_pipeline
    [new_document] = aggregate.process_pipeline(
../snip/lib/python3.13/site-packages/mongomock/aggregate.py:1658: in process_pipeline
    collection = handler(collection, database, options)
../snip/lib/python3.13/site-packages/mongomock/aggregate.py:1558: in _handle_add_fields_stage
    out_value = _parse_expression(value, in_doc, ignore_missing_keys=True)
../snip/lib/python3.13/site-packages/mongomock/aggregate.py:1067: in _parse_expression
    return _Parser(doc_dict, ignore_missing_keys=ignore_missing_keys).parse(expression)
../snip/lib/python3.13/site-packages/mongomock/aggregate.py:255: in parse
    return self._handle_array_operator(k, v)
../snip/lib/python3.13/site-packages/mongomock/aggregate.py:685: in _handle_array_operator
    parsed_list = list(self.parse_many(value))
../snip/lib/python3.13/site-packages/mongomock/aggregate.py:288: in parse_many
    yield self.parse(value)
../snip/lib/python3.13/site-packages/mongomock/aggregate.py:255: in parse
    return self._handle_array_operator(k, v)

pymongo.errors.OperationFailure: Third argument to $slice must be numeric, but is of type: <class 'dict'>

[michaelkryukov]

It looks like this issue stems from mongomock's implementation. You might consider reporting it upstream or modifying your implementation to reference inner documents by their IDs rather than using indexes.