Add attributes option to from_builtins

[jcrist]

This adds a new attributes option to from_builtins. If enabled, arbitrary objects may be coerced to Struct/dataclasses/attrs types by extracting attributes from the input object matching fields in the output type. One use case is converting database query results (ORM or otherwise) to msgspec structured types.

Fixes #416.

[jcrist]

A quick example:

import msgspec


class User(msgspec.Struct):
    name: str
    groups: list[str] = []
    email: str | None = None


class UserORM:
    """An example ORM-like input object"""
    def __init__(self, name, groups):
        self.name = name
        self.groups = groups
        # No `email` present on ORM object, default will be used when converting


data = [UserORM("alice", ["admin"]), UserORM("ben", [])]

converted = msgspec.from_builtins(data, list[User], attributes=True)
print(converted)
#> [User(name='alice', groups=['admin'], email=None), User(name='ben', groups=[], email=None)]

[dmckeone]

I've been playing with this a little and it's been working well for every case I've tried.

[jcrist]

Glad to hear it!