trycast

This module provides a single function trycast which can be used to parse a JSON-like value.

Here is an example of parsing a Point2D object defined as a TypedDict:

from bottle import HTTPResponse, request, route
from trycast import trycast
from typing import TypedDict

class Point2D(TypedDict):
    x: float
    y: float
    name: str

@route('/draw_point')
def draw_point_endpoint() -> None:
    request_json = request.json  # type: object
    if (point := trycast(Point2D, request_json)) is not None:
        draw_point(point)  # type is narrowed to Point2D
    else:
        return HTTPResponse(status=400)  # Bad Request

def draw_point(point: Point2D) -> None:
    # ...

In this example the trycast function is asked to parse a request_json into a Point2D object, returning the original object (with its type narrowed appropriately) if parsing was successful.

More complex types can be parsed as well, such as the Shape in the following example, which is a tagged union that can be either a Circle or Rect value:

from bottle import HTTPResponse, request, route
from trycast import trycast
from typing import Literal, TypedDict, Union

class Point2D(TypedDict):
    x: float
    y: float

class Circle(TypedDict):
    type: Literal['circle']
    center: Point2D  # a nested TypedDict!
    radius: float

class Rect(TypedDict):
    type: Literal['rect']
    x: float
    y: float
    width: float
    height: float

Shape = Union[Circle, Rect]  # a Tagged Union!

@route('/draw_shape')
def draw_shape_endpoint() -> None:
    request_json = request.json  # type: object
    if (shape := trycast(Shape, request_json)) is not None:
        draw_shape(shape)  # type is narrowed to Shape
    else:
        return HTTPResponse(status=400)  # Bad Request

Important: Current limitations in the mypy typechecker require that you add an extra cast(Optional[Shape], ...) around the call to trycast in the example so that it is accepted by the typechecker without complaining. These limitations are in the process of being resolved by introducing TypeForm support to mypy.

Recommendations when using trycast

• So that trycast() can recognize TypedDicts with mixed required and optional keys correctly:
  • Use Python 3.9+ if possible.
  • Prefer using typing.TypedDict, unless you must use Python 3.8. In Python 3.8 prefer typing_extensions.TypedDict instead.
  • Avoid using mypy_extensions.TypedDict in general.

Release Notes

Future

• See the Roadmap.

v0.2.0

• TypedDict improvements & fixes:
  • Fix trycast() to recognize TypedDicts from mypy_extensions.
  • Extend trycast() to recognize TypedDicts that contain forward-references to other types.
    • Unfortunately there appears to be no easy way to support arbitrary kinds of types that contain forward-references.
    • In particular {Union, Optional} types and collection types (List, Dict) with forward-references remain unsupported by trycast().
  • Recognize TypedDicts that have mixed required and optional keys correctly.
    • Exception: Does not work for mypy_extensions.TypedDict or Python 3.8's typing.TypedDict due to insufficient runtime type annotation information.
  • Fix recognition of a total=False TypedDict so that extra keys are disallowed.
• Alter typing_extensions to be an optional dependency of trycast.

v0.1.0

• Add support for Python 3.6, 3.7, and 3.9, in addition to 3.8.

v0.0.2

• Fix README to appear on PyPI.
• Add other package metadata, such as the supported Python versions.

v0.0.1

• Initial release.
• Supports typechecking all types found in JSON.