core.py

from __future__ import annotations

import asyncio
import functools
import inspect
import json
import logging
import typing

import marshmallow as ma
from marshmallow import ValidationError
from marshmallow.utils import missing

from webargs.multidictproxy import MultiDictProxy

logger = logging.getLogger(__name__)


__all__ = [
    "ValidationError",
    "Parser",
    "missing",
    "parse_json",
]


Request = typing.TypeVar("Request")
ArgMap = typing.Union[
    ma.Schema,
    typing.Type[ma.Schema],
    typing.Mapping[str, typing.Union[ma.fields.Field, typing.Type[ma.fields.Field]]],
    typing.Callable[[Request], ma.Schema],
]

ValidateArg = typing.Union[None, typing.Callable, typing.Iterable[typing.Callable]]
CallableList = typing.List[typing.Callable]
ErrorHandler = typing.Callable[..., typing.NoReturn]
# generic type var with no particular meaning
T = typing.TypeVar("T")
# type var for callables, to make type-preserving decorators
C = typing.TypeVar("C", bound=typing.Callable)
# type var for multidict proxy classes
MultiDictProxyT = typing.TypeVar("MultiDictProxyT", bound=MultiDictProxy)
# type var for a callable which is an error handler
# used to ensure that the error_handler decorator is type preserving
ErrorHandlerT = typing.TypeVar("ErrorHandlerT", bound=ErrorHandler)

AsyncErrorHandler = typing.Callable[..., typing.Awaitable[typing.NoReturn]]


# a value used as the default for arguments, so that when `None` is passed, it
# can be distinguished from the default value
_UNKNOWN_DEFAULT_PARAM = "_default"

DEFAULT_VALIDATION_STATUS: int = 422


def _record_arg_name(f: typing.Callable[..., typing.Any], argname: str | None) -> None:
    if argname is None:
        return
    if not hasattr(f, "__webargs_argnames__"):
        f.__webargs_argnames__ = ()  # type: ignore[attr-defined]
    f.__webargs_argnames__ += (argname,)  # type: ignore[attr-defined]


def _iscallable(x: typing.Any) -> bool:
    # workaround for
    #   https://github.com/python/mypy/issues/9778
    return callable(x)


def _callable_or_raise(obj: T | None) -> T | None:
    """Makes sure an object is callable if it is not ``None``. If not
    callable, a ValueError is raised.
    """
    if obj and not _iscallable(obj):
        raise ValueError(f"{obj!r} is not callable.")
    return obj


def get_mimetype(content_type: str) -> str:
    return content_type.split(";")[0].strip()


# Adapted from werkzeug:
# https://github.com/mitsuhiko/werkzeug/blob/master/werkzeug/wrappers.py
def is_json(mimetype: str | None) -> bool:
    """Indicates if this mimetype is JSON or not.  By default a request
    is considered to include JSON data if the mimetype is
    ``application/json`` or ``application/*+json``.
    """
    if not mimetype:
        return False
    if ";" in mimetype:  # Allow Content-Type header to be passed
        mimetype = get_mimetype(mimetype)
    if mimetype == "application/json":
        return True
    if mimetype.startswith("application/") and mimetype.endswith("+json"):
        return True
    return False


def parse_json(s: typing.AnyStr, *, encoding: str = "utf-8") -> typing.Any:
    if isinstance(s, str):
        decoded = s
    else:
        try:
            decoded = s.decode(encoding)
        except UnicodeDecodeError as exc:
            raise json.JSONDecodeError(
                f"Bytes decoding error : {exc.reason}",
                doc=str(exc.object),
                pos=exc.start,
            ) from exc
    return json.loads(decoded)


def _ensure_list_of_callables(obj: typing.Any) -> CallableList:
    if obj:
        if isinstance(obj, (list, tuple)):
            validators = typing.cast(CallableList, list(obj))
        elif callable(obj):
            validators = [obj]
        else:
            raise ValueError(f"{obj!r} is not a callable or list of callables.")
    else:
        validators = []
    return validators


class Parser(typing.Generic[Request]):
    """Base parser class that provides high-level implementation for parsing
    a request.

    Descendant classes must provide lower-level implementations for reading
    data from  different locations, e.g. ``load_json``, ``load_querystring``,
    etc.

    :param str location: Default location to use for data
    :param str unknown: A default value to pass for ``unknown`` when calling the
        schema's ``load`` method. Defaults to ``marshmallow.EXCLUDE`` for non-body
        locations and ``marshmallow.RAISE`` for request bodies. Pass ``None`` to use the
        schema's setting instead.
    :param callable error_handler: Custom error handler function.
    """

    #: Default location to check for data
    DEFAULT_LOCATION: str = "json"
    #: Default value to use for 'unknown' on schema load
    #  on a per-location basis
    DEFAULT_UNKNOWN_BY_LOCATION: dict[str, str | None] = {
        "json": None,
        "form": None,
        "json_or_form": None,
        "querystring": ma.EXCLUDE,
        "query": ma.EXCLUDE,
        "headers": ma.EXCLUDE,
        "cookies": ma.EXCLUDE,
        "files": ma.EXCLUDE,
    }
    #: The marshmallow Schema class to use when creating new schemas
    DEFAULT_SCHEMA_CLASS: type[ma.Schema] = ma.Schema
    #: Default status code to return for validation errors
    DEFAULT_VALIDATION_STATUS: int = DEFAULT_VALIDATION_STATUS
    #: Default error message for validation errors
    DEFAULT_VALIDATION_MESSAGE: str = "Invalid value."
    #: field types which should always be treated as if they set `is_multiple=True`
    KNOWN_MULTI_FIELDS: list[type] = [ma.fields.List, ma.fields.Tuple]
    #: set use_args to use a positional argument (rather than a keyword argument)
    #  defaults to True, but will become False in a future major version
    USE_ARGS_POSITIONAL: bool = True

    #: Maps location => method name
    __location_map__: dict[str, str | typing.Callable] = {
        "json": "load_json",
        "querystring": "load_querystring",
        "query": "load_querystring",
        "form": "load_form",
        "headers": "load_headers",
        "cookies": "load_cookies",
        "files": "load_files",
        "json_or_form": "load_json_or_form",
    }

    def __init__(
        self,
        location: str | None = None,
        *,
        unknown: str | None = _UNKNOWN_DEFAULT_PARAM,
        error_handler: ErrorHandler | None = None,
        schema_class: type[ma.Schema] | None = None,
    ) -> None:
        self.location = location or self.DEFAULT_LOCATION
        self.error_callback: ErrorHandler | None = _callable_or_raise(error_handler)
        self.schema_class = schema_class or self.DEFAULT_SCHEMA_CLASS
        self.unknown = unknown

    def _makeproxy(
        self,
        multidict: typing.Any,
        schema: ma.Schema,
        *,
        cls: type[MultiDictProxyT] | type[MultiDictProxy] = MultiDictProxy,
    ) -> MultiDictProxyT | MultiDictProxy:
        """Create a multidict proxy object with options from the current parser"""
        return cls(multidict, schema, known_multi_fields=tuple(self.KNOWN_MULTI_FIELDS))

    def _get_loader(self, location: str) -> typing.Callable:
        """Get the loader function for the given location.

        :raises: ValueError if a given location is invalid.
        """
        valid_locations = set(self.__location_map__.keys())
        if location not in valid_locations:
            raise ValueError(f"Invalid location argument: {location}")

        # Parsing function to call
        # May be a method name (str) or a function
        func = self.__location_map__[location]
        if isinstance(func, str):
            return getattr(self, func)
        return func

    def _load_location_data(
        self, *, schema: ma.Schema, req: Request, location: str
    ) -> typing.Any:
        """Return a dictionary-like object for the location on the given request.

        Needs to have the schema in hand in order to correctly handle loading
        lists from multidict objects and `many=True` schemas.
        """
        loader_func = self._get_loader(location)
        return loader_func(req, schema)

    async def _async_load_location_data(
        self, schema: ma.Schema, req: Request, location: str
    ) -> typing.Any:
        # an async variant of the _load_location_data method
        # the loader function itself may or may not be async
        loader_func = self._get_loader(location)

        data = loader_func(req, schema)

        if inspect.isawaitable(data):
            return await data

        return data

    def _on_validation_error(
        self,
        error: ValidationError,
        req: Request,
        schema: ma.Schema,
        location: str,
        *,
        error_status_code: int | None,
        error_headers: typing.Mapping[str, str] | None,
    ) -> typing.NoReturn:
        # rewrite messages to be namespaced under the location which created
        # them
        # e.g. {"json":{"foo":["Not a valid integer."]}}
        #      instead of
        #      {"foo":["Not a valid integer."]}
        error.messages = {location: error.messages}
        error_handler: ErrorHandler = self.error_callback or self.handle_error
        error_handler(
            error,
            req,
            schema,
            error_status_code=error_status_code,
            error_headers=error_headers,
        )

    async def _async_on_validation_error(
        self,
        error: ValidationError,
        req: Request,
        schema: ma.Schema,
        location: str,
        *,
        error_status_code: int | None,
        error_headers: typing.Mapping[str, str] | None,
    ) -> typing.NoReturn:
        # an async-aware variant of the _on_validation_error method
        error.messages = {location: error.messages}
        error_handler = self.error_callback or self.handle_error
        # an async error handler was registered, await it
        if asyncio.iscoroutinefunction(error_handler):
            async_error_handler = typing.cast(AsyncErrorHandler, error_handler)
            await async_error_handler(
                error,
                req,
                schema,
                error_status_code=error_status_code,
                error_headers=error_headers,
            )
        # the error handler was synchronous (e.g. Parser.handle_error) so it
        # will raise an error
        else:
            error_handler(
                error,
                req,
                schema,
                error_status_code=error_status_code,
                error_headers=error_headers,
            )

    def _validate_arguments(self, data: typing.Any, validators: CallableList) -> None:
        # although `data` is typically a Mapping, nothing forbids a `schema.load`
        # from returning an arbitrary object subject to validators
        for validator in validators:
            if validator(data) is False:
                msg = self.DEFAULT_VALIDATION_MESSAGE
                raise ValidationError(msg, data=data)

    def _get_schema(self, argmap: ArgMap, req: Request) -> ma.Schema:
        """Return a `marshmallow.Schema` for the given argmap and request.

        :param argmap: Either a `marshmallow.Schema`, `dict`
            of argname -> `marshmallow.fields.Field` pairs, or a callable that returns
            a `marshmallow.Schema` instance.
        :param req: The request object being parsed.
        :rtype: marshmallow.Schema
        """
        if isinstance(argmap, ma.Schema):
            schema = argmap
        elif isinstance(argmap, type) and issubclass(argmap, ma.Schema):
            schema = argmap()
        elif callable(argmap):
            schema = argmap(req)
        elif isinstance(argmap, typing.Mapping):
            if not isinstance(argmap, dict):
                argmap = dict(argmap)
            schema = self.schema_class.from_dict(argmap)()
        else:
            raise TypeError(f"argmap was of unexpected type {type(argmap)}")
        return schema

    def _prepare_for_parse(
        self,
        argmap: ArgMap,
        req: Request | None = None,
        location: str | None = None,
        unknown: str | None = _UNKNOWN_DEFAULT_PARAM,
        validate: ValidateArg = None,
    ) -> tuple[None, Request, str, CallableList, ma.Schema]:
        # validate parse() arguments and handle defaults
        # (shared between sync and async variants)
        req = req if req is not None else self.get_default_request()
        if req is None:
            raise ValueError("Must pass req object")
        location = location or self.location
        validators = _ensure_list_of_callables(validate)
        schema = self._get_schema(argmap, req)
        return (None, req, location, validators, schema)

    def _process_location_data(
        self,
        location_data: typing.Any,
        schema: ma.Schema,
        req: Request,
        location: str,
        unknown: str | None,
        validators: CallableList,
    ) -> typing.Any:
        # after the data has been fetched from a registered location,
        # this is how it is processed
        # (shared between sync and async variants)

        # when the desired location is empty (no data), provide an empty
        # dict as the default so that optional arguments in a location
        # (e.g. optional JSON body) work smoothly
        if location_data is missing:
            location_data = {}

        # precedence order: explicit, instance setting, default per location
        unknown = (
            unknown
            if unknown != _UNKNOWN_DEFAULT_PARAM
            else (
                self.unknown
                if self.unknown != _UNKNOWN_DEFAULT_PARAM
                else self.DEFAULT_UNKNOWN_BY_LOCATION.get(location)
            )
        )
        load_kwargs: dict[str, typing.Any] = {"unknown": unknown} if unknown else {}
        preprocessed_data = self.pre_load(
            location_data, schema=schema, req=req, location=location
        )
        data = schema.load(preprocessed_data, **load_kwargs)
        self._validate_arguments(data, validators)
        return data

    def parse(
        self,
        argmap: ArgMap,
        req: Request | None = None,
        *,
        location: str | None = None,
        unknown: str | None = _UNKNOWN_DEFAULT_PARAM,
        validate: ValidateArg = None,
        error_status_code: int | None = None,
        error_headers: typing.Mapping[str, str] | None = None,
    ) -> typing.Any:
        """Main request parsing method.

        :param argmap: Either a `marshmallow.Schema`, a `dict`
            of argname -> `marshmallow.fields.Field` pairs, or a callable
            which accepts a request and returns a `marshmallow.Schema`.
        :param req: The request object to parse.
        :param str location: Where on the request to load values.
            Can be any of the values in :py:attr:`~__location_map__`. By
            default, that means one of ``('json', 'query', 'querystring',
            'form', 'headers', 'cookies', 'files', 'json_or_form')``.
        :param str unknown: A value to pass for ``unknown`` when calling the
            schema's ``load`` method. Defaults to ``marshmallow.EXCLUDE`` for non-body
            locations and ``marshmallow.RAISE`` for request bodies. Pass ``None`` to use
            the schema's setting instead.
        :param callable validate: Validation function or list of validation functions
            that receives the dictionary of parsed arguments. Validator either returns a
            boolean or raises a :exc:`ValidationError`.
        :param int error_status_code: Status code passed to error handler functions when
            a `ValidationError` is raised.
        :param dict error_headers: Headers passed to error handler functions when a
            a `ValidationError` is raised.

         :return: A dictionary of parsed arguments
        """
        data, req, location, validators, schema = self._prepare_for_parse(
            argmap, req, location, unknown, validate
        )
        try:
            location_data = self._load_location_data(
                schema=schema, req=req, location=location
            )
            data = self._process_location_data(
                location_data, schema, req, location, unknown, validators
            )
        except ma.exceptions.ValidationError as error:
            self._on_validation_error(
                error,
                req,
                schema,
                location,
                error_status_code=error_status_code,
                error_headers=error_headers,
            )
            raise ValueError(
                "_on_validation_error hook did not raise an exception"
            ) from error
        return data

    async def async_parse(
        self,
        argmap: ArgMap,
        req: Request | None = None,
        *,
        location: str | None = None,
        unknown: str | None = _UNKNOWN_DEFAULT_PARAM,
        validate: ValidateArg = None,
        error_status_code: int | None = None,
        error_headers: typing.Mapping[str, str] | None = None,
    ) -> typing.Any:
        """Coroutine variant of `webargs.core.Parser.parse`.

        Receives the same arguments as `webargs.core.Parser.parse`.
        """
        data, req, location, validators, schema = self._prepare_for_parse(
            argmap, req, location, unknown, validate
        )
        try:
            location_data = await self._async_load_location_data(
                schema=schema, req=req, location=location
            )
            data = self._process_location_data(
                location_data, schema, req, location, unknown, validators
            )
        except ma.exceptions.ValidationError as error:
            await self._async_on_validation_error(
                error,
                req,
                schema,
                location,
                error_status_code=error_status_code,
                error_headers=error_headers,
            )
            raise ValueError(
                "_on_validation_error hook did not raise an exception"
            ) from error
        return data

    def get_default_request(self) -> Request | None:
        """Optional override. Provides a hook for frameworks that use thread-local
        request objects.
        """
        return None

    def get_request_from_view_args(
        self,
        view: typing.Callable,
        args: tuple,
        kwargs: typing.Mapping[str, typing.Any],
    ) -> Request | None:
        """Optional override. Returns the request object to be parsed, given a view
        function's args and kwargs.

        Used by the `use_args` and `use_kwargs` to get a request object from a
        view's arguments.

        :param callable view: The view function or method being decorated by
            `use_args` or `use_kwargs`
        :param tuple args: Positional arguments passed to ``view``.
        :param dict kwargs: Keyword arguments passed to ``view``.
        """
        return None

    @staticmethod
    def _update_args_kwargs(
        args: tuple,
        kwargs: dict[str, typing.Any],
        parsed_args: dict[str, typing.Any],
        as_kwargs: bool,
        arg_name: str | None,
    ) -> tuple[tuple, dict[str, typing.Any]]:
        """Update args or kwargs with parsed_args depending on as_kwargs"""
        if as_kwargs:
            # expand parsed_args into kwargs
            kwargs.update(parsed_args)
        else:
            if arg_name:
                # add parsed_args as a specific kwarg
                kwargs[arg_name] = parsed_args
            else:
                # Add parsed_args after other positional arguments
                args += (parsed_args,)
        return args, kwargs

    def use_args(
        self,
        argmap: ArgMap,
        req: Request | None = None,
        *,
        location: str | None = None,
        unknown: str | None = _UNKNOWN_DEFAULT_PARAM,
        as_kwargs: bool = False,
        arg_name: str | None = None,
        validate: ValidateArg = None,
        error_status_code: int | None = None,
        error_headers: typing.Mapping[str, str] | None = None,
    ) -> typing.Callable[..., typing.Callable]:
        """Decorator that injects parsed arguments into a view function or method.

        Example usage with Flask: ::

            @app.route('/echo', methods=['get', 'post'])
            @parser.use_args({'name': fields.Str()}, location="querystring")
            def greet(querystring_args):
                return 'Hello ' + querystring_args['name']

        :param argmap: Either a `marshmallow.Schema`, a `dict`
            of argname -> `marshmallow.fields.Field` pairs, or a callable
            which accepts a request and returns a `marshmallow.Schema`.
        :param str location: Where on the request to load values.
        :param str unknown: A value to pass for ``unknown`` when calling the
            schema's ``load`` method.
        :param bool as_kwargs: Whether to insert arguments as keyword arguments.
        :param str arg_name: Keyword argument name to use for arguments. Mutually
            exclusive with as_kwargs.
        :param callable validate: Validation function that receives the dictionary
            of parsed arguments. If the function returns ``False``, the parser
            will raise a :exc:`ValidationError`.
        :param int error_status_code: Status code passed to error handler functions when
            a `ValidationError` is raised.
        :param dict error_headers: Headers passed to error handler functions when a
            a `ValidationError` is raised.
        """
        location = location or self.location
        request_obj = req

        # Optimization: If argmap is passed as a dictionary, we only need
        # to generate a Schema once
        if isinstance(argmap, typing.Mapping):
            if not isinstance(argmap, dict):
                argmap = dict(argmap)
            argmap = self.schema_class.from_dict(argmap)()

        if arg_name is not None and as_kwargs:
            raise ValueError("arg_name and as_kwargs are mutually exclusive")
        if arg_name is None and not self.USE_ARGS_POSITIONAL:
            arg_name = self.get_default_arg_name(location, argmap)

        def decorator(func: typing.Callable) -> typing.Callable:
            req_ = request_obj

            # check at decoration time that a unique name is being used
            # (no arg_name conflicts)
            if arg_name is not None and not as_kwargs:
                existing_arg_names = getattr(func, "__webargs_argnames__", ())
                if arg_name in existing_arg_names:
                    raise ValueError(
                        f"Attempted to pass `arg_name='{arg_name}'` via use_args() but "
                        "that name was already used. If this came from stacked webargs "
                        "decorators, try setting `arg_name` to distinguish usages."
                    )

            if asyncio.iscoroutinefunction(func):

                @functools.wraps(func)
                async def wrapper(
                    *args: typing.Any, **kwargs: typing.Any
                ) -> typing.Any:
                    req_obj = req_

                    if not req_obj:
                        req_obj = self.get_request_from_view_args(func, args, kwargs)
                    # NOTE: At this point, argmap may be a Schema, callable, or dict
                    parsed_args = await self.async_parse(
                        argmap,
                        req=req_obj,
                        location=location,
                        unknown=unknown,
                        validate=validate,
                        error_status_code=error_status_code,
                        error_headers=error_headers,
                    )
                    args, kwargs = self._update_args_kwargs(
                        args, kwargs, parsed_args, as_kwargs, arg_name
                    )
                    return await func(*args, **kwargs)

            else:

                @functools.wraps(func)
                def wrapper(*args: typing.Any, **kwargs: typing.Any) -> typing.Any:
                    req_obj = req_

                    if not req_obj:
                        req_obj = self.get_request_from_view_args(func, args, kwargs)
                    # NOTE: At this point, argmap may be a Schema, callable, or dict
                    parsed_args = self.parse(
                        argmap,
                        req=req_obj,
                        location=location,
                        unknown=unknown,
                        validate=validate,
                        error_status_code=error_status_code,
                        error_headers=error_headers,
                    )
                    args, kwargs = self._update_args_kwargs(
                        args, kwargs, parsed_args, as_kwargs, arg_name
                    )
                    return func(*args, **kwargs)

            wrapper.__wrapped__ = func
            _record_arg_name(wrapper, arg_name)
            return wrapper

        return decorator

    def use_kwargs(
        self,
        argmap: ArgMap,
        req: Request | None = None,
        *,
        location: str | None = None,
        unknown: str | None = _UNKNOWN_DEFAULT_PARAM,
        validate: ValidateArg = None,
        error_status_code: int | None = None,
        error_headers: typing.Mapping[str, str] | None = None,
    ) -> typing.Callable[..., typing.Callable]:
        """Decorator that injects parsed arguments into a view function or method
        as keyword arguments.

        This is a shortcut to :meth:`use_args` with ``as_kwargs=True``.

        Example usage with Flask: ::

            @app.route('/echo', methods=['get', 'post'])
            @parser.use_kwargs({'name': fields.Str()})
            def greet(name):
                return 'Hello ' + name

        Receives the same ``args`` and ``kwargs`` as :meth:`use_args`.
        """
        return self.use_args(
            argmap,
            req=req,
            as_kwargs=True,
            location=location,
            unknown=unknown,
            validate=validate,
            error_status_code=error_status_code,
            error_headers=error_headers,
        )

    def get_default_arg_name(self, location: str, schema: ArgMap) -> str:
        """This method provides the rule by which an argument name is derived for
        :meth:`use_args` if no explicit ``arg_name`` is provided.

        By default, the format used is ``{location}_args``. Users may override this method
        to customize the default argument naming scheme.

        ``schema`` will be the argument map or schema passed to :meth:`use_args` unless a
        dict was used, in which case it will be the schema derived from that dict.
        """
        return f"{location}_args"

    def location_loader(self, name: str) -> typing.Callable[[C], C]:
        """Decorator that registers a function for loading a request location.
        The wrapped function receives a schema and a request.

        The schema will usually not be relevant, but it's important in some
        cases -- most notably in order to correctly load multidict values into
        list fields. Without the schema, there would be no way to know whether
        to simply `.get()` or `.getall()` from a multidict for a given value.

        Example: ::

            from webargs import core
            parser = core.Parser()

            @parser.location_loader("name")
            def load_data(request, schema):
                return request.data

        :param str name: The name of the location to register.
        """

        def decorator(func: C) -> C:
            self.__location_map__[name] = func
            return func

        return decorator

    def error_handler(self, func: ErrorHandlerT) -> ErrorHandlerT:
        """Decorator that registers a custom error handling function. The
        function should receive the raised error, request object,
        `marshmallow.Schema` instance used to parse the request, error status code,
        and headers to use for the error response. Overrides
        the parser's ``handle_error`` method.

        Example: ::

            from webargs import flaskparser

            parser = flaskparser.FlaskParser()


            class CustomError(Exception):
                pass


            @parser.error_handler
            def handle_error(error, req, schema, *, error_status_code, error_headers):
                raise CustomError(error.messages)

        :param callable func: The error callback to register.
        """
        self.error_callback = func
        return func

    def pre_load(
        self,
        location_data: typing.Mapping,
        *,
        schema: ma.Schema,
        req: Request,
        location: str,
    ) -> typing.Mapping:
        """A method of the parser which can transform data after location
        loading is done. By default it does nothing, but users can subclass
        parsers and override this method.
        """
        return location_data

    def _handle_invalid_json_error(
        self,
        error: json.JSONDecodeError | UnicodeDecodeError,
        req: Request,
        *args: typing.Any,
        **kwargs: typing.Any,
    ) -> typing.NoReturn:
        """Internal hook for overriding treatment of JSONDecodeErrors.

        Invoked by default `load_json` implementation.

        External parsers can just implement their own behavior for load_json ,
        so this is not part of the public parser API.
        """
        raise error

    def load_json(self, req: Request, schema: ma.Schema) -> typing.Any:
        """Load JSON from a request object or return `missing` if no value can
        be found.
        """
        # NOTE: although this implementation is real/concrete and used by
        # several of the parsers in webargs, it relies on the internal hooks
        # `_handle_invalid_json_error` and `_raw_load_json`
        # these methods are not part of the public API and are used to simplify
        # code sharing amongst the built-in webargs parsers
        try:
            return self._raw_load_json(req)
        except json.JSONDecodeError as exc:
            if exc.doc == "":
                return missing
            return self._handle_invalid_json_error(exc, req)
        except UnicodeDecodeError as exc:
            return self._handle_invalid_json_error(exc, req)

    def load_json_or_form(self, req: Request, schema: ma.Schema) -> typing.Any:
        """Load data from a request, accepting either JSON or form-encoded
        data.

        The data will first be loaded as JSON, and, if that fails, it will be
        loaded as a form post.
        """
        data = self.load_json(req, schema)
        if data is not missing:
            return data
        return self.load_form(req, schema)

    # Abstract Methods

    def _raw_load_json(self, req: Request) -> typing.Any:
        """Internal hook method for implementing load_json()

        Get a request body for feeding in to `load_json`, and parse it either
        using core.parse_json() or similar utilities which raise
        JSONDecodeErrors.
        Ensure consistent behavior when encountering decoding errors.

        The default implementation here simply returns `missing`, and the default
        implementation of `load_json` above will pass that value through.
        However, by implementing a "mostly concrete" version of load_json with
        this as a hook for getting data, we consolidate the logic for handling
        those JSONDecodeErrors.
        """
        return missing

    def load_querystring(self, req: Request, schema: ma.Schema) -> typing.Any:
        """Load the query string of a request object or return `missing` if no
        value can be found.
        """
        return missing

    def load_form(self, req: Request, schema: ma.Schema) -> typing.Any:
        """Load the form data of a request object or return `missing` if no
        value can be found.
        """
        return missing

    def load_headers(self, req: Request, schema: ma.Schema) -> typing.Any:
        """Load the headers or return `missing` if no value can be found."""
        return missing

    def load_cookies(self, req: Request, schema: ma.Schema) -> typing.Any:
        """Load the cookies from the request or return `missing` if no value
        can be found.
        """
        return missing

    def load_files(self, req: Request, schema: ma.Schema) -> typing.Any:
        """Load files from the request or return `missing` if no values can be
        found.
        """
        return missing

    def handle_error(
        self,
        error: ValidationError,
        req: Request,
        schema: ma.Schema,
        *,
        error_status_code: int | None,
        error_headers: typing.Mapping[str, str] | None,
    ) -> typing.NoReturn:
        """Called if an error occurs while parsing args. By default, just logs and
        raises ``error``.
        """
        logger.error(error)
        raise error