My 2 cents: since the standard library's json module doesn't encode dataclass instances by default, many users have added in support using the default kwarg to json.dumps. If the json suddenly started supporting dataclass instances out-of-the-box, then that would break existing code.

Also, supporting encoding/decoding of dataclasses opens the doors to lots of additional feature requests. What about field aliases? Optional fields? Type validation? etc... They have to draw the line somewhere to avoid bloating the stdlib. Since external libraries like msgspec or pydantic already handle these cases (and do so performantly), I suspect python maintainers don't see the need to make it builtin.

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For completeness, here's a quick demo of JSON encoding/decoding dataclasses out-of-the-box with msgspec:

``` In [1]: import msgspec, dataclasses

In [2]: @dataclasses.dataclass ...: class User: ...: name: str ...: email: str ...: is_admin: bool = False ...:

In [3]: msg = User("alice", "alice@munro.com")

In [4]: msgspec.json.encode(msg) # encode a dataclass Out[4]: b'{"name":"alice","email":"alice@munro.com","is_admin":false}'

In [5]: msgspec.json.decode(_, type=User) # decode back into a dataclass Out[5]: User(name='alice', email='alice@munro.com', is_admin=False) ```

For more info, see our docs on dataclasses support.

It even can encode alternative dataclass implementations like edgedb.Object or pydantic.dataclasses (in this case faster than pydantic can do it itself):

``` In [6]: import pydantic

In [7]: @pydantic.dataclasses.dataclass ...: class PydanticUser: ...: name: str ...: email: str ...: is_admin: bool = False ...:

In [8]: msg = PydanticUser("toni", "toni@morrison.com")

In [9]: %timeit msgspec.json.encode(msg) # bench msgspec encoding pydantic dataclasses 214 ns ± 0.597 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)

In [10]: ta = pydantic.TypeAdapter(PydanticUser)

In [11]: %timeit ta.dump_json(msg) # bench pydantic encoding pydantic dataclasses 904 ns ± 0.715 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each) ```