blueprint.arguments(location="view_args") fails with "view_args is not a valid location"

Besides standard locations (query, json, form, etc), webargs also supports Flask-specific view_args option to validate the URL path parameters.

Here is the simplified example of my code:

@blueprint.route("/update/<string:item_id>/<string:list>", methods=["POST"])
@blueprint.arguments(ItemInputSchema(), location="view_args")
@blueprint.response(ItemSchema())
def move_item(data: ItemInputArgs, item_id: string, list: string):
    """Move an item to the specified list."""
    return move_item_to_list(data.item, data.list)

In this case, I want to validate item_id (check that this is a valid ID of the object existing in the database) and list (check that this is a string matching per-defined list of values) The schema looks like this:

class ItemInputArgs:
    """Arguments for the POST /update/<item>/<list>."""

    def __init__(self, item: Item, list: string):
        self.item = item
        self.list = list

class ItemInputSchema(Schema):

    item_id = fields.Str(required=True)
    list = fields.Str(required=True)

    @validates("item_id")
    def validate_item_id(self, value: str) -> None:
        """Check if item exists in the DB."""
        item = Item.query.filter_by(id=value).one_or_none()
        if item is None:
            raise ValidationError("Item with this id does not exist", "item")

    @validates("list")
    def validate_list(self, value: str) -> None:
        if value not in ['value1', 'value2', 'value3']:
            raise ValidationError('Wrong list value')

    @post_load
    def make_object(self, data: JSON, **kwargs: Any) -> ItemInputArgs:
        return ItemInputArgs(
            Item.query.filter_by(item_id=data["item_id"]).one(),
        )

This fails with an error:

    def arguments(
            self, schema, *, location='json', required=True,
            example=None, examples=None, **kwargs
    ):
        """Decorator specifying the schema used to deserialize parameters
        ...
        """
        try:
            openapi_location = __location_map__[location]
        except KeyError as exc:
            raise InvalidLocationError(
>               "{} is not a valid location".format(location)) from exc
E           flask_rest_api.exceptions.InvalidLocationError: view_args is not a valid location

venv/lib/python3.7/site-packages/flask_rest_api/arguments.py:47: InvalidLocationError

The __location_map__ itself is taken from the apispec package: from apispec.ext.marshmallow.openapi import __location_map__.

The workaround I use is this (just patch the __location_map__ and add view_args):

from apispec.ext.marshmallow.openapi import __location_map__

# The `view_args` location is supported by FlaskParser in webargs, 
# but flask-rest-api looks into the __location_map__ and raises an error, 
# so we patch the map to allow `view_args` as `location` in @blueprint.arguments.
__location_map__["view_args"] = "view_args"

Not sure though if that affects the schema generation (I am not generating it yet, although planning to).

[lafrech]

Done in 0a70faa115cca26c194a5d8f20f319f4de53f645.

Thanks for the feedback.

[lafrech]

Hi. Thanks for reporting.

(Note you can specify ```py before your code blocks to get syntax highlighting.)

I don’t use webargs to validate path parameters as flask/werkzeug validation is often enough, so I never investigated this, but I knew it might be a little tricky.

This part of the code is removed in master branch (unreleased). Please give it a try.

You may have to keep your patch. Otherwise, you won’t get errors but the doc will be wrong, with the location defaulting to body.

Here’s what I’d try:

from apispec.ext.marshmallow.openapi import __location_map__
__location_map__["view_args"] = "path"

Please tell us how it goes.

We should find a better way to address this.