Anonymous and inline declaration of TypedDict types

[jetzhou]

Feature
Currently, TypedDict must be declared and used as so:

MyDict = TypedDict('MyDict', {'foo': int, 'bar': bool})
def fn() -> MyDict:
    pass

In many situations, the type doesn't actually get re-used outside the context of this one local function, so a much more concise declaration would be:

def fn() -> TypedDict({'foo': int, 'bar': bool}):
    pass

or alternatively:

def fn() -> TypedDict(foo=int, bar=bool):
    pass

This syntax makes the TypedDict definition both anonymous and inline, which are two separate features I guess, even though they are pretty much hand in hand. Both are desirable in situations where the return type isn't re-used in multiple places so there is no need for the separate MyDict definition.

Pitch

I've read through #985 and python/typing#28. Both issues had a little bit of discussion on anonymous TypedDict and dismissed the feature as not being useful enough.

In my case, we run a GraphQL server and many of our mutation types return dictionaries/JSON objects with the shape {'ok': bool, 'error': str, 'item': <ItemType>}. These objects have no relevance outside of where their mutation function is defined, so it is extremely verbose and pointless to type:

ReturnType = TypedDict('ReturnedType', {'ok': bool, 'error': str, 'item': <ItemType>})
def mutation() -> ReturnType:
    return {'ok', True, 'error', '', 'item': <item>}

When everything can be succinctly expressed as:

def mutation() -> TypedDict({'ok': bool, 'error': str, 'item': <ItemType>}):
    return {'ok', True, 'error', '', 'item': <item>}

The simplified syntax has better readability and loses none of the type safety that's achieved with the type hints.

Outside of my use cases, I can also imagine that inline anonymous TypedDict can be pretty useful when they are used in a nested fashion, something like:

MyType = TypedDict({
    'foo': int,
    'bar': TypedDict({
        'baz': int,
    })
})

Happy to provide more context on the use case that I have or to come up with more use cases. If this seems like a worthy direction to pursue, I would definitely love to dig into implementation and figure out what needs to be done to make this happen.

[bennettdams]

Just to give a perspective from another language: One of TypeScript's main features is exactly this "object typing". It's even a main bullet point in their documentation and one of the first examples.
This way of "inline typing" is useful for many use cases, but especially for return values, as @jetzhou described.

https://www.typescriptlang.org/docs/handbook/2/objects.html

[zuckerruebe]

I'd prefer the following syntax:

def fn() -> {"foo": int, "bar": bool}:
    pass

That would be so much better than having to do

def fn() -> Tuple[it, bool]:
    pass

(as is the case now) and then having to remember whether [0] or [1] was foo.

[amh4r]

I'd love this feature! I'm writing a library that codegens JSON Schema into TypedDicts, but JSON schema has nested object schemas.

For example, I'd like to convert this JSON schema:

{
    "id": "#root",
    "properties": {
        "Foo": {
            "properties": {
                "bar": {
                    "type": "object",
                    "properties": {
                        "baz": {"type": "integer"}
                    }
                }
            }
        }
    }
}

Into something like this:

Foo = TypedDict(
    {
        "bar": {
            {"baz": int},
        },
    },
)

I could write logic that creates something like this:

class FooBar(TypedDict):
    baz: int

class Foo(TypedDict):
    bar: FooBar

But that adds complexity to my recursive functions:

• They need to have extra context to generate the path-based name (i.e. Foo.bar -> FooBar).
• They need to handle name conflicts. Like if the schema explicitly declares FooBar somewhere, then the generated name would need to be something like FooBar2.

TypeScript supports nested interfaces and it's made JSON Schema codegen much easier. Hopefully TypedDict can adopt some of that flexibility

[mahmoudajawad]

Does this require a PEP or it can be implemented on mypy as feature?

[JelleZijlstra]

Mypy can implement its own extensions if it wants, but standardizing this feature would require a new PEP.

[JukkaL]

TypedDict({'foo': int, 'bar': bool}) generates a TypeError at runtime. To properly support this we'd need changes to typing.TypedDict.

[erictraut]

If you want to pursue this, I recommend discussing it in the typing-sig email list or the python/typing discussion forum. It deserves broader discussion and input before it is implemented in type checkers.

[jetzhou]

@erictraut, @JelleZijlstra thanks for the pointers! I'm glad that this thread/feature is gaining some traction here. I have some time on my hands so I will look into starting the discussion on the mailing list and the PEP process. Will update here if it gets somewhere. Thanks all!

[andersk]

We could represent an anonymous TypedDict with an empty string for the name: TypedDict("", {'foo': int, 'bar': bool}). This already works at runtime.

[emmatyping]

I suggest we close this in favor of discussing this on typing-sig and having it standardized in a PEP, I'd love to see something like {'foo': int} be shorter syntax for an anonymous TypedDict, similar to how we now allow | for union, dict from builtins etc.

[michaeloliverx]

It would be great if type checkers were able to infer the return types of functions as typed dicts if returning literal dicts e.g.

def foo():
    return {
        "foo: "bar"
    }

TypeScript shines in this regard.

[DetachHead]

pyright supports this using the following syntax:

foo: dict[{"a": str}] = {"a": "asdf"}

[MrLoh]

@DetachHead is this pyright feature documented somewhere?

[DetachHead]

it's an experimental feature that i don't think is documented anywhere except this thread python/typing#1391

[erictraut]

Yes, this feature is experimental in pyright. The idea was discussed in a typing forum thread, but it was never fully fleshed out in a specification. I will likely remove from pyright since no one has stepped up to write a PEP.

If you would like to see such a feature added to the type system, please consider starting a new thread in the typing forum, gather feedback and ideas, and write a draft PEP.

[ilevkivskyi]

An experimental support for this has been merged to master, you can play with it using --enable-incomplete-feature=InlineTypedDict.

[layday]

Support for inline dicts was removed from Pyright in June, by the way, which seems rather unfortunate.

[erictraut]

Yes, I removed support for this experimental feature because no one stepped up to formally specify it, and there was little or no feedback (positive or negative) from pyright users.

If someone is interested in spearheading the formal specification for such a feature, the Python typing forum is a good place to start the discussion. This feature would require a PEP.

[JelleZijlstra]

We had a typing meetup presentation from @sobolevn about this some time ago, and as I remember, there was significant disagreement over how inheritance should work, with the result that Nikita stopped pursuing the proposal.

[DetachHead]

this feature is still supported in basedpyright