OrderedDict() needs type annotation but {} doesn't

[kamahen]

The following error message is triggered when by adding -> Dict[Text, Any] to the type signature:

/tmp/pod.py:7: error: Need type annotation for 'result'

This is the contents of /tmp/pod.py:

import collections
from typing import Any, Dict, Text

class PlainOldData:

    def as_json_dict(self) -> Dict[Text, Any]:
        result = collections.OrderedDict()
        for k in self.__slots__:
            value = getattr(self, k)
            if value is not None:
                result[k] = value
        return result

[emmatyping]

Since mypy doesn't know the type of the ordered dict when you create it, you need to add a type annotation.

[kamahen]

Oops ... the problem I was trying to report (about slots) turned out to be due to having __slots__ = () (which gets overridden in subclasses) and I needed to annotate that even though object.slots has an appropriate definition in typeshed.

As for OrderedDict ... shouldn't it work? When I change the code to result = {}, mypy is happy.

[emmatyping]

The issue with __slots__ is #4547 then, while the issue with result is interesting... if I had to guess mypy is inferring the type of {} based on the return type but fails to do so in your real code since it is an OrderedDict.

[JukkaL]

Mypy has custom type inference rules for common collection types such as List and Dict, but they don't apply to OrderedDict. It would be nice if we could generalize this to (almost) arbitrary collection types.

[emmatyping]

The example below (updated to have type information to express subclasses will have __slots__):

import collections
from typing import Any, Dict, Text

class PlainOldData:
    __slots__: tuple[str, ...]
    def as_json_dict(self) -> Dict[Text, Any]:
        result = collections.OrderedDict()
        for k in self.__slots__:
            value = getattr(self, k)
            if value is not None:
                result[k] = value
        return result

passes with the latest mypy, closing.