Support for default types in TypedDict

[rra]

This problem arose from trying to type the environ argument for a WSGI application. There are specific keys of the environ dict that have non-str types, and then all the other keys are HTTP_ variables (or server-defined variables, which can be ignored for this purpose) with type str (or Text, depending on the version of Python).

I would like to get stronger typing on environ than Dict[str, Any], but there isn't a way to do that with TypedDict currently because the set of possible keys is unbounded (the client can send any HTTP header).

Ideally, I'd like to be able to do something like:

EnvironDict = TypedDict("EnvironDict", {
    "wsgi.version": Tuple[int, int],
    "wsgi.input": IO[str],
# ...
}, default_type=str, total=False)

and have this mean that any key is allowed in the dictionary and all unknown keys get a value type of str.

[ilevkivskyi]

Thanks for the proposal! Unfortunately, I don't think we will have time to work on this in short term, but we may raise priority if this request will appear again.

Also a random note: same functionality is available for protocols using __getattr__().

[JukkaL]

We'd need a way to represent this using the class-based syntax as well.

[JelleZijlstra]

In class-based syntax this could plausibly be supported as:

class EnvironDict(TypedDict, total=False, default_type=str):
    version: Tuple[int, int]

[DMR-coding]

I have a usecase this would be extremely helpful for as well, in the context of a physical modeling app. We have EXTREMELY large dictionaries of physical properties passed to models. They're mostly floating point measurements except for a small minority of values that are strings or nested dicts. Using a default type of float and only specifying the exceptions could improve concision by hundreds of lines.

[gu1p]

Please make this happen

[oren0e]

I join the requests - it will be super convenient if I could write:

class MyClass(TypedDict):
    a: int = 1
    b: int = 10
    v: Optional[int] = None

[gvanrossum]

If anyone has a specific proposal to solve this please post to typing-sig.

[simon-liebehenschel]

@oren0e Why not just use the typing.NamedTuple then? https://docs.python.org/3/library/typing.html#typing.NamedTuple

class Employee(NamedTuple):
    name: str
    id: int = 3

[hellmrf]

@oren0e Why not just use the typing.NamedTuple then? https://docs.python.org/3/library/typing.html#typing.NamedTuple

@AIGeneratedUsername, NamedTuples aren't supposed to work like dicts at runtime. See the following example.

from typing import NamedTuple, TypedDict, Optional


class EmployeeTuple(NamedTuple):
    name: str
    age: Optional[int] = 18


class EmployeeDict(TypedDict):
    name: str
    age: int


def show_employee(age: int, name: str) -> None:
    print(f"{name} is {age} years old.")


empdict = EmployeeDict(name='Alice', age=18)
emptup = EmployeeTuple(name='Bob')


show_employee(**empdict) # "Alice is 18 years old." Works as expected.
show_employee(*emptup) # "18 is Bob years old." Doesn't work.
show_employee(**emptup) # TypeError: show_employee() argument after ** must be a mapping, not EmployeeTuple

[fellnerse]

Is this going to be implemented? Feels a bit weird missing default values here

[gvanrossum]

If you want this you should ask on the typing tracker https://github.com/python/typing/discussions

[fellnerse]

Thanks, maybe it would make sense to close this issue here then
Here is the discussion at python typing: python/typing#1165

[adam-grant-hendry]

> I join the requests - it will be super convenient if I could write: > > ```python > class MyClass(TypedDict): > a: int = 1 > b: int = 10 > v: Optional[int] = None > ```

You might be able to get away with using the Annotated type here, though that was not its intended purpose as stated in PEP 593 and the word "annotated" itself would be a misnomer:

from typing import Annotated, Optional

class MyClass(TypedDict):
    a: Annotated[int, 1]
    b: Annotated[int, 10]
    v: Annotated[Optional[int], None]

NOTE: Variables can't be used in the class definition unless its a type alias, so you can't simply write

DefaultArg = Annotated

and use that because you have to specify the indeces to Annotated to make it an alias (e.g.

`DefaultIntArg = Annotated[int, 1]  # defeats the purpose

so you have to use "Annotated" for now.

Apologies, the discussion is at python/typing#1165

[tommyjcarpenter]

I started migrating all TypedDicts to dataclasses due to this issue, as dataclasses support this feature.

[gvanrossum]

I started migrating all TypedDicts to dataclasses due to this issue, as dataclasses support this feature.

Good for you. A coding style using dataclasses is much more readable than one using dictionaries as records.

[jzazo]

I started migrating all TypedDicts to dataclasses due to this issue, as dataclasses support this feature.

Hi @tommyjcarpenter, how would you do that? How do you add a default type for undefined attributes in a dataclass? Thanks.