gistfile1.txt

from typing import Any, Callable, Optional, Type, TypeVar
from mypy.plugin import Plugin, FunctionContext  # pylint: disable=no-name-in-module
from mypy.types import TypedDictType  # pylint: disable=no-name-in-module

A = TypeVar('A')

def safe_typeddict_cast(_: Type[A], data: Any) -> A:
    # This function does nothing on its own, but the types are checked in mypy_plugin.py.
    # The goal is to safely convert the data argument to the type of the first argument.
    # The type will only be converted if it passes type checks. If it doesn't pass,
    # error messages will tell you about missing keys or incompatible key types.
    return data


class TypedDictCastPlugin(Plugin):
    @staticmethod
    def get_function_hook(fullname: str) -> Optional[Callable[[FunctionContext], TypedDictType]]:
        def convert_data_type(context: FunctionContext) -> TypedDictType:
            # Compare the types
            target_type = context.arg_types[0][0].ret_type.type.typeddict_type  # type: ignore
            target_type_name = context.args[0][0].name  # type: ignore
            target_type_items = target_type.items
            input_type = context.arg_types[1][0]
            input_type_items = input_type.items  # type: ignore

            # All keys in the input type must be present and subtyped on the target type
            missing_keys_in_target_type = []
            mistyped_keys_in_target_type = []
            for key, value in input_type_items.items():
                if key not in target_type_items:
                    missing_keys_in_target_type.append(key)
                elif value != target_type_items[key]:
                    # TODO: Recursively support checking nested typed dicts for better errors?
                    mistyped_keys_in_target_type.append(key)

            if missing_keys_in_target_type:
                if len(missing_keys_in_target_type) == 1:
                    context.api.fail(
                        f'Input data type has extra key "{missing_keys_in_target_type[0]}" '
                        f'which is missing on type "{target_type_name}"', context.context
                    )
                else:
                    joined_keys = '"' + '", "'.join(missing_keys_in_target_type) + '"'
                    context.api.fail(
                        f'Input data type has extra keys {joined_keys} which are missing on type "{target_type_name}"',
                        context.context
                    )

            if mistyped_keys_in_target_type:
                if len(mistyped_keys_in_target_type) == 1:
                    context.api.fail(
                        f'Input data type has key "{mistyped_keys_in_target_type[0]}" '
                        f'which has incompatible type on type {target_type_name}', context.context
                    )
                else:
                    joined_keys = '"' + '", "'.join(mistyped_keys_in_target_type) + '"'
                    context.api.fail(
                        f'Input data type has keys {joined_keys} '
                        f'which have incompatible types on type "{target_type_name}"', context.context
                    )

            # TODO: All required keys in the target type must be present and subtyped on the input type

            # If any type checks failed, don't convert the type
            if missing_keys_in_target_type or mistyped_keys_in_target_type:
                return input_type  # type: ignore

            return target_type

        if '.safe_typeddict_cast' in fullname:
            return convert_data_type

        return None


def plugin(_: str) -> Type[TypedDictCastPlugin]:
    return TypedDictCastPlugin