Support for generic TypedDict

[dgoldstein0]

R = TypeVar("R")
FileCachedData = TypedDict('FileCachedData', {'mtime': float, 'size': int, 'value': R})

mypy complains about R in the TypedDict declaration. I would expect that I could declare a generic TypedDict like this.

[gvanrossum]

Generic TypedDicts are not supported yet. (Though if there were, this would be a reasonable notations -- after all this is how you declare generic aliases.)

[dgoldstein0]

Well then, consider this a feature request.

…

On Tue, Aug 22, 2017, 5:21 PM Guido van Rossum ***@***.***> wrote: Generic TypedDicts are not supported yet. (Though if there were, this would be a reasonable notations -- after all this is how you declare generic aliases.) — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#3863 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABBndywOD43OsdDFu1fawtNkYW1ZpFDAks5sa3BpgaJpZM4O_UuC> .

[gwax]

To add a justification note, this feature would be useful for handling polymorphic JSON responses, such as a shared pager:

class Item(TypedDict):
    type: str

class ValueItem(Item):
    value: int

T = TypeVar('T', bound=Item)

class ItemPager(TypedDict, Generic[T]):
    next_page: Optional[str]
    items: List[T]

def get_value_items() -> List[ValueItem]:
    value_items: List[ValueItem] = []
    url = "https://example.com/value_items"
    while url:
        response = requests.get(url)
        pager: ItemPager[ValueItem] = response.json()
        url = pager['next_page']
        value_items += pager['items']
    return value_items

[pawelrubin]

Hi!
Are Generic TypedDicts/NamedTuples planned to be supported in the future versions of Python/mypy?
As @gwax mentioned, they would be super useful for annotating polymorphic JSON responses.

[gvanrossum]

I suggest filing a bpo issue for that.

[peterschutt]

@pawelrubin was a bpo issue created for this?

[AlexWaygood]

There's now a BPO issue for this here

[graingert]

See also python/typing_extensions#7