Haven't seen a reply to this yet, so I'll start the bidding at 2¢. Generally, I like the idea of using Annotated in this fashion. I'm currently using it for expressing validation rules and hints about how parameters are handled in HTTP requests. Handy! If = field(...) weren't already established, I'd say it should be as an annotated value instead of an assignment. Since = field(...) pattern is well established, I'm not so sure what the value would be of changing this in dataclass now. Paul On Sat, 2021-04-03 at 20:57 +0000, brianmarroq@gmail.com wrote:

typing.Annotated could be used to build dataclasses. Using Annotated will allow libraries to add functionality to a dataclass without having to change dataclass creation or behavior. The example below shows how a dataclass could be implemented. It continues the example of struct2 shown in pep593. From a dataclass point of view, the Sample and AnnotatedSample would be equivalent.

```python @dataclass class Sample:     a: int     b: int     c: int = field(default=5)     d: int = 10     e: int = field(default=10)

@packed @dataclass class AnnotatedSample:     a: Annotated[int, ctype("I")]     b: int     c: Annotated[int, field(default=5), ctype("I")]     d: Annotated[int, ctype("h")] = 10     e: Annotated[int, ctype("h")] = field(default=10)

out_bytes = struct2.pack(AnnotatedSample()) ``` When parsing the Annotated parameters, the dataclass decorator will only look at the type parameter and ```field``` parameter if present. If not Annotated, it falls back to existing behavior.

Let me know what you think. Thanks! _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/TI4ZHE... Code of Conduct: http://python.org/psf/codeofconduct/