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With parameters


Suppose you'd want to add a cast from str (presumably JSON encoded) to a Pydantic model. With plain model described in the previous chapters, that would require you to write quite a lot of boilerplate, like this:

import functools
from typecasts import casts
import pydantic

class Robot(pydantic.BaseModel):

    name: str

class Dinosaur(pydantic.BaseModel):

    species: str

casts[str, Robot] = functools.partial(pydantic.parse_raw_as, Robot)
casts[str, Dinosaur] = functools.partial(pydantic.parse_raw_as, Dinosaur)
...   # and so on, for each of your Pydantic models.


Works like this:

from typing import Type

import pydantic

from typecasts import casts
from typecasts.types import SubclassOf

@casts.register(str, SubclassOf[pydantic.BaseModel])
def json_string_to_pydantic(
    serialized_value: str,
    destination_type: Type[pydantic.BaseModel],
) -> pydantic.BaseModel:
    """Convert a JSON representation of a Pydantic model into model instance."""
    return pydantic.parse_raw_as(destination_type, serialized_value)

In a nutshell

If the destination type of your typecast is a SubclassOf[Something], then:

  • this cast will be applicable when destination type is any subclass of Something including itself;
  • the cast function will be provided with two arguments: the value to convert plus the destination type requested by the casts user.