trycast

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Trycast helps parses JSON-like values whose shape is defined by typed dictionaries (TypedDicts) and other standard Python type hints.

You can use the trycast(), checkcast(), or isassignable() functions below for parsing:

trycast()

Here is an example of parsing a Point2D object defined as a TypedDict using trycast():

from bottle import HTTPResponse, request, route  # Bottle is a web framework
from trycast import trycast
from typing import TypedDict

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

@route('/draw_point')
def draw_point_endpoint() -> HTTPResponse:
    request_json = request.json  # type: object
    if (point := trycast(Point2D, request_json)) is None:
        return HTTPResponse(status=400)  # Bad Request
    draw_point(point)  # type is narrowed to Point2D
    return HTTPResponse(status=200)

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

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 = Circle | Rect  # a Tagged Union!

@route('/draw_shape')
def draw_shape_endpoint() -> HTTPResponse:
    request_json = request.json  # type: object
    if (shape := trycast(Shape, request_json)) is None:
        return HTTPResponse(status=400)  # Bad Request
    draw_shape(shape)  # type is narrowed to Shape
    return HTTPResponse(status=200)  # OK

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:

shape = cast(Optional[Shape], trycast(Shape, request_json))
if shape is None:
    ...

These limitations are in the process of being resolved by introducing TypeForm support to mypy.

checkcast()

checkcast() is similar to trycast() but instead of returning None when parsing fails it raises an exception explaining why and where the parsing failed.

Here is an example of parsing a Circle object using checkcast():

>>> from typing import Literal, TypedDict
>>> from trycast import checkcast
>>> 
>>> class Point2D(TypedDict):
...     x: float
...     y: float
... 
>>> class Circle(TypedDict):
...     type: Literal['circle']
...     center: Point2D  # a nested TypedDict!
...     radius: float
... 
>>> checkcast(Circle, {"type": "circle", "center": {"x": 1}, "radius": 10})
Traceback (most recent call last):
  ...
trycast.ValidationError: Expected Circle but found {'type': 'circle', 'center': {'x': 1}, 'radius': 10}
  At key 'center': Expected Point2D but found {'x': 1}
    Required key 'y' is missing
>>> 

ValidationError only spends time generating a message if you try to print it or stringify it, so can be cheaply caught if you only want to use it for control flow purposes.

isassignable()

Here is an example of parsing a Shape object defined as a union of TypedDicts using isassignable():

class Circle(TypedDict):
    type: Literal['circle']
    ...

class Rect(TypedDict):
    type: Literal['rect']
    ...

Shape = Circle | Rect  # a Tagged Union!

@route('/draw_shape')
def draw_shape_endpoint() -> HTTPResponse:
    request_json = request.json  # type: object
    if not isassignable(request_json, Shape):
        return HTTPResponse(status=400)  # Bad Request
    draw_shape(request_json)  # type is narrowed to Shape
    return HTTPResponse(status=200)  # OK

Important: Current limitations in the mypy typechecker prevent the automatic narrowing of the type of request_json in the above example to Shape, so you must add an additional cast() to narrow the type manually:

if not isassignable(request_json, Shape):
    ...
shape = cast(Shape, request_json)  # type is manually narrowed to Shape
draw_shape(shape)

These limitations are in the process of being resolved by introducing TypeForm support to mypy.

A better isinstance()

isassignable(value, T) is similar to Python's builtin isinstance() but additionally supports checking against arbitrary type annotation objects including TypedDicts, Unions, Literals, and many others.

Formally, isassignable(value, T) checks whether value is consistent with a variable of type T (using PEP 484 static typechecking rules), but at runtime.

Motivation & Alternatives

Why use trycast?

The trycast module is primarily designed for recognizing JSON-like structures that can be described by Python's typing system. Secondarily, it can be used for recognizing arbitrary structures that can be described by Python's typing system.

Please see Philosophy for more information about how trycast differs from similar libraries like pydantic.

Why use TypedDict?

Typed dictionaries are the natural form that JSON data comes in over the wire. They can be trivially serialized and deserialized without any additional logic. For applications that use a lot of JSON data - such as web applications - using typed dictionaries is very convenient for representing data structures.

If you just need a lightweight class structure that doesn't need excellent support for JSON-serialization you might consider other alternatives for representing data structures in Python such as dataclasses (recommended), named tuples, attrs, or plain classes.

Installation

python -m pip install trycast

Recommendations while using trycast

• So that trycast() can recognize TypedDicts with mixed required and not-required keys correctly:
  • Prefer using typing.TypedDict instead of typing_extensions.TypedDict.

Presentations & Videos

A presentation about using trycast to parse JSON was given at the 2021 PyCon US Typing Summit:

A presentation describing tools that use Python type annotations at runtime, including trycast, was given at the 2022 PyCon US Typing Summit:

Contributing

Pull requests are welcome! The Python Community Code of Conduct does apply.

You can checkout the code locally using:

git clone git@github.com:davidfstr/trycast.git
cd trycast

Create your local virtual environment to develop in using Poetry:

poetry shell
poetry install

You can run the existing automated tests in the current version of Python with:

make test

You can also run the tests against all supported Python versions with:

make testall

See additional development commands by running:

make help

License

MIT

Feature Reference

Typing Features Supported

• Scalars
  • bool
  • int
  • float
  • None, type(None)
• Strings
  • str
• Raw Collections
  • list, List
  • tuple, Tuple
  • Sequence, MutableSequence
  • dict, Dict
  • Mapping, MutableMapping
• Generic Collections (including PEP 585)
  • list[T], List[T]
  • tuple[T, ...], Tuple[T, ...]
  • Sequence[T], MutableSequence[T]
  • dict[K, V], Dict[K, V]
  • Mapping[K, V], MutableMapping[K, V]
• TypedDict
  • typing.TypedDict, typing_extensions.TypedDict (PEP 589)
  • –––
  • Required, NotRequired (PEP 655)
  • ReadOnly (PEP 705)
• Tuples (Heterogeneous)
  • tuple[T1], tuple[T1, T2], tuple[T1, T2, T3], etc
  • Tuple[T1], Tuple[T1, T2], Tuple[T1, T2, T3], etc
• Unions
  • Union[X, Y]
  • Optional[T]
  • X | Y (PEP 604)
• Literals
  • Literal[value] (PEP 586)
• Callables
  • Callable
  • Callable[P, R] (where P=[Any]*N and R=Any)
• NewTypes (when strict=False)
• Special Types
  • Any
  • Never
  • NoReturn

Type Checkers Supported

Trycast does type check successfully with the following type checkers:

• [Mypy]
• [Pyright] / [Pylance]
• [Pyre]

API Reference

<a name="trycast-api"></a>

trycast API

def trycast(
    tp: TypeForm[T]† | TypeFormString[T]‡,
    value: object,
    /, failure: F = None,
    *, strict: bool = True,
    eval: bool = True
) -> T | F: ...

If value i