Advanced Usage
==============

This section includes guides for advanced usage patterns.

Custom Location Handlers
------------------------

To add your own custom location handler, write a function that receives a request, an argument name, and a :class:`Field <marshmallow.fields.Field>`, then decorate that function with :func:`Parser.location_handler <webargs.core.Parser.location_handler>`.


.. code-block:: python

    from webargs import fields
    from webargs.flaskparser import parser


    @parser.location_handler("data")
    def parse_data(request, name, field):
        return request.data.get(name)


    # Now 'data' can be specified as a location
    @parser.use_args({"per_page": fields.Int()}, locations=("data",))
    def posts(args):
        return "displaying {} posts".format(args["per_page"])


marshmallow Integration
-----------------------

When you need more flexibility in defining input schemas, you can pass a marshmallow `Schema <marshmallow.Schema>` instead of a dictionary to `Parser.parse <webargs.core.Parser.parse>`, `Parser.use_args <webargs.core.Parser.use_args>`, and `Parser.use_kwargs <webargs.core.Parser.use_kwargs>`.


.. code-block:: python

    from marshmallow import Schema, fields
    from webargs.flaskparser import use_args


    class UserSchema(Schema):
        id = fields.Int(dump_only=True)  # read-only (won't be parsed by webargs)
        username = fields.Str(required=True)
        password = fields.Str(load_only=True)  # write-only
        first_name = fields.Str(missing="")
        last_name = fields.Str(missing="")
        date_registered = fields.DateTime(dump_only=True)

        # NOTE: Uncomment below two lines if you're using marshmallow 2
        # class Meta:
        #    strict = True


    @use_args(UserSchema())
    def profile_view(args):
        username = args["userame"]
        # ...


    @use_kwargs(UserSchema())
    def profile_update(username, password, first_name, last_name):
        update_profile(username, password, first_name, last_name)
        # ...


    # You can add additional parameters
    @use_kwargs({"posts_per_page": fields.Int(missing=10, location="query")})
    @use_args(UserSchema())
    def profile_posts(args, posts_per_page):
        username = args["username"]
        # ...

.. warning::
    If you're using marshmallow 2, you should always set ``strict=True`` (either as a ``class Meta`` option or in the Schema's constructor) when passing a schema to webargs. This will ensure that the parser's error handler is invoked when expected.

.. warning::
    Any `Schema <marshmallow.Schema>` passed to `use_kwargs <webargs.core.Parser.use_kwargs>` MUST deserialize to a dictionary of data. Keep this in mind when writing `post_load <marshmallow.decorators.post_load>` methods.


Schema Factories
----------------

If you need to parametrize a schema based on a given request, you can use a "Schema factory": a callable that receives the current `request` and returns a `marshmallow.Schema` instance.

Consider the following use cases:

- Filtering via a query parameter by passing ``only`` to the Schema.
- Handle partial updates for PATCH requests using marshmallow's `partial loading <https://marshmallow.readthedocs.io/en/latest/quickstart.html#partial-loading>`_ API.

.. code-block:: python

    from flask import Flask
    from marshmallow import Schema, fields
    from webargs.flaskparser import use_args

    app = Flask(__name__)


    class UserSchema(Schema):
        id = fields.Int(dump_only=True)
        username = fields.Str(required=True)
        password = fields.Str(load_only=True)
        first_name = fields.Str(missing="")
        last_name = fields.Str(missing="")
        date_registered = fields.DateTime(dump_only=True)


    def make_user_schema(request):
        # Filter based on 'fields' query parameter
        fields = request.args.get("fields", None)
        only = fields.split(",") if fields else None
        # Respect partial updates for PATCH requests
        partial = request.method == "PATCH"
        # Add current request to the schema's context
        return UserSchema(only=only, partial=partial, context={"request": request})


    # Pass the factory to .parse, .use_args, or .use_kwargs
    @app.route("/profile/", methods=["GET", "POST", "PATCH"])
    @use_args(make_user_schema)
    def profile_view(args):
        username = args.get("username")
        # ...



Reducing Boilerplate
++++++++++++++++++++

We can reduce boilerplate and improve [re]usability with a simple helper function:

.. code-block:: python

    from webargs.flaskparser import use_args


    def use_args_with(schema_cls, schema_kwargs=None, **kwargs):
        schema_kwargs = schema_kwargs or {}

        def factory(request):
            # Filter based on 'fields' query parameter
            only = request.args.get("fields", None)
            # Respect partial updates for PATCH requests
            partial = request.method == "PATCH"
            return schema_cls(
                only=only, partial=partial, context={"request": request}, **schema_kwargs
            )

        return use_args(factory, **kwargs)


Now we can attach input schemas to our view functions like so:

.. code-block:: python

    @use_args_with(UserSchema)
    def profile_view(args):
        # ...
        get_profile(**args)


Custom Fields
-------------

See the "Custom Fields" section of the marshmallow docs for a detailed guide on defining custom fields which you can pass to webargs parsers: https://marshmallow.readthedocs.io/en/latest/custom_fields.html.

Using ``Method`` and ``Function`` Fields with webargs
+++++++++++++++++++++++++++++++++++++++++++++++++++++

Using the :class:`Method <marshmallow.fields.Method>` and :class:`Function <marshmallow.fields.Function>` fields requires that you pass the ``deserialize`` parameter.


.. code-block:: python

    @use_args({"cube": fields.Function(deserialize=lambda x: int(x) ** 3)})
    def math_view(args):
        cube = args["cube"]
        # ...

.. _custom-parsers:

Custom Parsers
--------------

To add your own parser, extend :class:`Parser <webargs.core.Parser>` and implement the `parse_*` method(s) you need to override. For example, here is a custom Flask parser that handles nested query string arguments.


.. code-block:: python

    import re

    from webargs import core
    from webargs.flaskparser import FlaskParser


    class NestedQueryFlaskParser(FlaskParser):
        """Parses nested query args

        This parser handles nested query args. It expects nested levels
        delimited by a period and then deserializes the query args into a
        nested dict.

        For example, the URL query params `?name.first=John&name.last=Boone`
        will yield the following dict:

            {
                'name': {
                    'first': 'John',
                    'last': 'Boone',
                }
            }
        """

        def parse_querystring(self, req, name, field):
            return core.get_value(_structure_dict(req.args), name, field)


    def _structure_dict(dict_):
        def structure_dict_pair(r, key, value):
            m = re.match(r"(\w+)\.(.*)", key)
            if m:
                if r.get(m.group(1)) is None:
                    r[m.group(1)] = {}
                structure_dict_pair(r[m.group(1)], m.group(2), value)
            else:
                r[key] = value

        r = {}
        for k, v in dict_.items():
            structure_dict_pair(r, k, v)
        return r

Returning HTTP 400 Responses
----------------------------

If you'd prefer validation errors to return status code ``400`` instead
of ``422``, you can override ``DEFAULT_VALIDATION_STATUS`` on a :class:`Parser <webargs.core.Parser>`.


.. code-block:: python

    from webargs.falconparser import FalconParser


    class Parser(FalconParser):
        DEFAULT_VALIDATION_STATUS = 400


    parser = Parser()
    use_args = parser.use_args
    use_kwargs = parser.use_kwargs

Bulk-type Arguments
-------------------

In order to parse a JSON array of objects, pass ``many=True`` to your input ``Schema`` .

For example, you might implement JSON PATCH according to `RFC 6902 <https://tools.ietf.org/html/rfc6902>`_ like so:


.. code-block:: python

    from webargs import fields
    from webargs.flaskparser import use_args
    from marshmallow import Schema, validate


    class PatchSchema(Schema):
        op = fields.Str(
            required=True,
            validate=validate.OneOf(["add", "remove", "replace", "move", "copy"]),
        )
        path = fields.Str(required=True)
        value = fields.Str(required=True)


    @app.route("/profile/", methods=["patch"])
    @use_args(PatchSchema(many=True), locations=("json",))
    def patch_blog(args):
        """Implements JSON Patch for the user profile

        Example JSON body:

        [
            {"op": "replace", "path": "/email", "value": "mynewemail@test.org"}
        ]
        """
        # ...

Mixing Locations
----------------

Arguments for different locations can be specified by passing ``location`` to each field individually:

.. code-block:: python

    @app.route("/stacked", methods=["POST"])
    @use_args(
        {
            "page": fields.Int(location="query"),
            "q": fields.Str(location="query"),
            "name": fields.Str(location="json"),
        }
    )
    def viewfunc(args):
        page = args["page"]
        # ...

Alternatively, you can pass multiple locations to `use_args <webargs.core.Parser.use_args>`:

.. code-block:: python

    @app.route("/stacked", methods=["POST"])
    @use_args(
        {"page": fields.Int(), "q": fields.Str(), "name": fields.Str()},
        locations=("query", "json"),
    )
    def viewfunc(args):
        page = args["page"]
        # ...

However, this allows ``page`` and ``q`` to be passed in the request body and ``name`` to be passed as a query parameter.

To restrict the arguments to single locations without having to pass ``location`` to every field, you can call the `use_args <webargs.core.Parser.use_args>` multiple times:

.. code-block:: python

    query_args = {"page": fields.Int(), "q": fields.Int()}
    json_args = {"name": fields.Str()}


    @app.route("/stacked", methods=["POST"])
    @use_args(query_args, locations=("query",))
    @use_args(json_args, locations=("json",))
    def viewfunc(query_parsed, json_parsed):
        page = query_parsed["page"]
        name = json_parsed["name"]
        # ...

To reduce boilerplate, you could create shortcuts, like so:

.. code-block:: python

    import functools

    query = functools.partial(use_args, locations=("query",))
    body = functools.partial(use_args, locations=("json",))


    @query(query_args)
    @body(json_args)
    def viewfunc(query_parsed, json_parsed):
        page = query_parsed["page"]
        name = json_parsed["name"]
        # ...

Next Steps
----------

- See the :doc:`Framework Support <framework_support>` page for framework-specific guides.
- For example applications, check out the `examples <https://github.com/marshmallow-code/webargs/tree/dev/examples>`_ directory.
