The docs give very clear guidance on how to write code using unions of TypedDicts:

https://mypy.readthedocs.io/en/stable/typed_dict.html#unions-of-typeddicts

This makes sense!

However, a common use case (for me) of a TypedDict is to allow external code (e.g. REST API input) to be deserialized into a dict and refined at runtime into a TypedDict -- which implies the the design requirement to have a branded field (e.g. tag) must now exist in our public facing API.

This could work in some cases, but consider something like this:

from __future__ import annotations
from typing import List, TypedDict

class Policy(TypedDict):
    name: str

class CombinedORPolicy(TypedDict):
    OR: List[Policy]

class CombinedANDPolicy(TypedDict):
    AND: List[Policy]


def print_policy(policy: CombinedANDPolicy | CombinedORPolicy | Policy) -> None:
    if 'OR' in policy:
        allowed_policy_names = ' '.join([p['name'] for p in policy['OR']])
        print(f"Can match _any_ of the following: {allowed_policy_names}")
    elif 'AND' in policy:
        allowed_policy_names = ' '.join([p['name'] for p in policy['AND']])
        print(f"Must match _all_ the following: {allowed_policy_names}")
    else:
        print(f"Must be exactly {policy['name']}")


# pretend this is an API endpoint or something
print_policy({'name': 'role:is_admin'})

#13838 being closed seems to imply this is possible but i'm still having trouble.

Here's the current mypy (1.14.1) output:

test.py: note: In function "print_policy":
test.py:16:68: error: TypedDict "CombinedANDPolicy" has no key "OR"  [typeddict-item]
test.py:16:68: error: TypedDict "Policy" has no key "OR"  [typeddict-item]
test.py:19:68: error: TypedDict "Policy" has no key "AND"  [typeddict-item]
Found 3 errors in 1 file (checked 1 source file)

The solution as I understand is either:

• force an API change (e.g. print_policy({'name': 'role:is_admin', 'tag': 'root-policy'}))
• use cast (e.g. policy = cast(CombinedANDPolicy, policy)

Is there anything more idiomatic i'm missing?

(I searched the tracker but wasn't able to find discission of this specifically, but I think this is related: #7981 #12098 #11080)

Is there a stance on this yet? Is this is a goal or non-goal of mypy? Thanks!

FWIW -- here's a version of this working in

• TypeScript
• Pyright

thanks!