PoC Python dataclasses.dataclass reimplementation

structure.py

import contextlib
import copy
import types

class structure:
  def __init__(self, _class):
    self.__class = _class
    self.annotations: list = list(_class.__annotations__.items())

    _class.__class = self.__class
    _class.__repr__ = lambda self: \
      "<structure %s, %s>" % (
        self.__class.__name__,
        self.fields
      )

  def __call__(self, **args):
    _class = copy.deepcopy(self.__class)()
    _class.fields = args

    for field_name, field_value in list(args.items()):
      for (
        annotation_name, 
        annotation_type
      ) in self.annotations:
        if not annotation_name in list(args.keys()):
          if isinstance(annotation_type, types.UnionType) \
             and type(None) in annotation_type.__args__:
               setattr(_class, annotation_name, None)
          else:
            raise Exception("field %s is not nullable, so it must be specified" % annotation_name)
        else:
          if field_name == annotation_name:
            if isinstance(field_value, annotation_type):
              setattr(_class, field_name, field_value)
            else:
              raise TypeError("type mismatch, required %s, got %s" % (
                annotation_type,
                type(field_value)
              ))

    return _class
  
  def __repr__(self):
    return "<structure %s {\n%s\n}>" % (
      self.__class.__name__,
      "".join([
        "  %s: %s%s" % (
          annotated[0],
          annotated[1].__name__,
          "\n" if not annotated == self.annotations[-1] \
               else ""
        )
        for annotated in self.annotations
      ])
    )

structure_example.py

@structure
class Human:
    name: str
    age: int
    born: datetime.datetime
    nullable_field_test: int | None