trycast 0.1.0

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trycast

Status: In development. Current limitations in mypy prevent the examples below from actually typechecking successfully, despite functioning correctly at runtime.

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

Release Notes

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.