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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, and a :class:`Schema <marshmallow.Schema>`, then decorate that function with :func:`Parser.location_loader <webargs.core.Parser.location_loader>`.
from webargs import fields from webargs.flaskparser import parser @parser.location_loader("data") def load_data(request, schema): return request.data # Now 'data' can be specified as a location @parser.use_args({"per_page": fields.Int()}, location="data") def posts(args): return "displaying {} posts".format(args["per_page"])
Note
The schema is passed so that it can be used to wrap multidict types and unpack List fields correctly. If you are writing a loader for a multidict type, consider looking at :class:`MultiDictProxy <webargs.multidictproxy.MultiDictProxy>` for an example of how to do this.
"meta" Locations
You can define your own locations which mix data from several existing locations.
The json_or_form location does this -- first trying to load data as JSON and then falling back to a form body -- and its implementation is quite simple:
def load_json_or_form(self, req, schema): """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)
You can imagine your own locations with custom behaviors like this. For example, to mix query parameters and form body data, you might write the following:
from webargs import fields from webargs.multidictproxy import MultiDictProxy from webargs.flaskparser import parser @parser.location_loader("query_and_form") def load_data(request, schema): # relies on the Flask (werkzeug) MultiDict type's implementation of # these methods, but when you're extending webargs, you may know things # about your framework of choice newdata = request.args.copy() newdata.update(request.form) return MultiDictProxy(newdata, schema) # Now 'query_and_form' means you can send these values in either location, # and they will be *mixed* together into a new dict to pass to your schema @parser.use_args({"favorite_food": fields.String()}, location="query_and_form") def set_favorite_food(args): ... # do stuff return "your favorite food is now set to {}".format(args["favorite_food"])
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>.
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) @use_args(UserSchema()) def profile_view(args): username = args["username"] # ... @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"] # ...
Setting unknown
webargs supports several ways of setting and passing the unknown parameter for handling unknown fields.
You can pass unknown=... as a parameter to any of Parser.parse <webargs.core.Parser.parse>, Parser.use_args <webargs.core.Parser.use_args>, and Parser.use_kwargs <webargs.core.Parser.use_kwargs>.
Note
The unknown value is passed to the schema's load() call. It therefore only applies to the top layer when nesting is used. To control unknown at multiple layers of a nested schema, you must use other mechanisms, like the unknown argument to fields.Nested.
Default unknown
By default, webargs will pass unknown=marshmallow.EXCLUDE except when the location is json, form, json_or_form, or path. In those cases, it uses unknown=marshmallow.RAISE instead.
You can change these defaults by overriding DEFAULT_UNKNOWN_BY_LOCATION. This is a mapping of locations to values to pass.
For example,
from flask import Flask from marshmallow import EXCLUDE, fields from webargs.flaskparser import FlaskParser app = Flask(__name__) class Parser(FlaskParser): DEFAULT_UNKNOWN_BY_LOCATION = {"query": EXCLUDE} parser = Parser() # location is "query", which is listed in DEFAULT_UNKNOWN_BY_LOCATION, # so EXCLUDE will be used @app.route("/", methods=["GET"]) @parser.use_args({"foo": fields.Int()}, location="query") def get(args): return f"foo x 2 = {args['foo'] * 2}" # location is "json", which is not in DEFAULT_UNKNOWN_BY_LOCATION, # so no value will be passed for `unknown` @app.route("/", methods=["POST"]) @parser.use_args({"foo": fields.Int(), "bar": fields.Int()}, location="json") def post(args): return f"foo x bar = {args['foo'] * args['bar']}"
You can also define a default at parser instantiation, which will take precedence over these defaults, as in
from marshmallow import INCLUDE parser = Parser(unknown=INCLUDE) # because `unknown` is set on the parser, `DEFAULT_UNKNOWN_BY_LOCATION` has # effect and `INCLUDE` will always be used @app.route("/", methods=["POST"]) @parser.use_args({"foo": fields.Int(), "bar": fields.Int()}, location="json") def post(args): unexpected_args = [k for k in args.keys() if k not in ("foo", "bar")] return f"foo x bar = {args['foo'] * args['bar']}; unexpected args={unexpected_args}"
Using Schema-Specfied unknown
If you wish to use the value of unknown specified by a schema, simply pass unknown=None. This will disable webargs' automatic passing of values for unknown. For example,
from flask import Flask from marshmallow import Schema, fields, EXCLUDE, missing from webargs.flaskparser import use_args class RectangleSchema(Schema): length = fields.Float() width = fields.Float() class Meta: unknown = EXCLUDE app = Flask(__name__) # because unknown=None was passed, no value is passed during schema loading # as a result, the schema's behavior (EXCLUDE) is used @app.route("/", methods=["POST"]) @use_args(RectangleSchema(), location="json", unknown=None) def get(args): return f"area = {args['length'] * args['width']}"
You can also set unknown=None when instantiating a parser to make this behavior the default for a parser.
When to avoid use_kwargs
Any Schema <marshmallow.Schema> passed to use_kwargs <webargs.core.Parser.use_kwargs> MUST deserialize to a dictionary of data. If your schema has a post_load <marshmallow.decorators.post_load> method that returns a non-dictionary, you should use use_args <webargs.core.Parser.use_args> instead.
from marshmallow import Schema, fields, post_load from webargs.flaskparser import use_args class Rectangle: def __init__(self, length, width): self.length = length self.width = width class RectangleSchema(Schema): length = fields.Float() width = fields.Float() @post_load def make_object(self, data, **kwargs): return Rectangle(**data) @use_args(RectangleSchema) def post(rect: Rectangle): return f"Area: {rect.length * rect.width}"
Packages such as marshmallow-sqlalchemy and marshmallow-dataclass generate schemas that deserialize to non-dictionary objects. Therefore, use_args <webargs.core.Parser.use_args> should be used with those schemas.
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 API.
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:
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:
@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.
@use_args({"cube": fields.Function(deserialize=lambda x: int(x) ** 3)}) def math_view(args): cube = args["cube"] # ...
Custom Parsers
To add your own parser, extend :class:`Parser <webargs.core.Parser>` and implement the load_* method(s) you need to override. For example, here is a custom Flask parser that handles nested query string arguments.
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 load_querystring(self, req, schema): return _structure_dict(req.args) 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
Parser pre_load
Similar to @pre_load decorated hooks on marshmallow Schemas, :class:`Parser <webargs.core.Parser>` classes define a method, pre_load <webargs.core.Parser.pre_load> which can be overridden to provide per-parser transformations of data. The only way to make use of pre_load <webargs.core.Parser.pre_load> is to subclass a :class:`Parser <webargs.core.Parser>` and provide an implementation.
pre_load <webargs.core.Parser.pre_load> is given the data fetched from a location, the schema which will be used, the request object, and the location name which was requested. For example, to define a FlaskParser which strips whitespace from form and query data, one could write the following:
from webargs.flaskparser import FlaskParser import typing def _strip_whitespace(value): if isinstance(value, str): value = value.strip() elif isinstance(value, typing.Mapping): return {k: _strip_whitespace(value[k]) for k in value} elif isinstance(value, (list, tuple)): return type(value)(map(_strip_whitespace, value)) return value class WhitspaceStrippingFlaskParser(FlaskParser): def pre_load(self, location_data, *, schema, req, location): if location in ("query", "form"): return _strip_whitespace(location_data) return location_data
Note that Parser.pre_load <webargs.core.Parser.pre_load> is run after location loading but before Schema.load is called. It can therefore be called on multiple types of mapping objects, including :class:`MultiDictProxy <webargs.MultiDictProxy>`, depending on what the location loader returns.
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>`.
Sublcass the parser for your framework to do so. For example, using Falcon:
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 like so:
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)) def patch_blog(args): """Implements JSON Patch for the user profile Example JSON body: [ {"op": "replace", "path": "/email", "value": "mynewemail@test.org"} ] """ # ...
Multi-Field Detection
If a List field is used to parse data from a location like query parameters -- where one or multiple values can be passed for a single parameter name -- then webargs will automatically treat that field as a list and parse multiple values if present.
To implement this behavior, webargs will examine schemas for marshmallow.fields.List fields. List fields get unpacked to list values when data is loaded, and other fields do not. This also applies to fields which inherit from List.
Note
In webargs v8, Tuple will be treated this way as well, in addition to List.
What if you have a list which should be treated as a "multi-field" but which does not inherit from List? webargs offers two solutions. You can add the custom attribute is_multiple=True to your field or you can add your class to your parser's list of KNOWN_MULTI_FIELDS.
First, let's define a "multiplexing field" which takes a string or list of strings to serve as an example:
# a custom field class which can accept values like List(String()) or String() class CustomMultiplexingField(fields.String): def _deserialize(self, value, attr, data, **kwargs): if isinstance(value, str): return super()._deserialize(value, attr, data, **kwargs) return [ self._deserialize(v, attr, data, **kwargs) for v in value if isinstance(v, str) ] def _serialize(self, value, attr, **kwargs): if isinstance(value, str): return super()._serialize(value, attr, **kwargs) return [self._serialize(v, attr, **kwargs) for v in value if isinstance(v, str)]
If you control the definition of CustomMultiplexingField, you can just add is_multiple=True to it:
# option 1: define the field with is_multiple = True from webargs.flaskparser import parser class CustomMultiplexingField(fields.Field): is_multiple = True # <----- this marks this as a multi-field ... # as above
If you don't control the definition of CustomMultiplexingField, for example because it comes from a library, you can add it to the list of known multifields:
# option 2: add the field to the parer's list of multi-fields class MyParser(FlaskParser): KNOWN_MULTI_FIELDS = list(FlaskParser.KNOWN_MULTI_FIELDS) + [ CustomMultiplexingField ] parser = MyParser()
In either case, the end result is that you can use the multifield and it will be detected as a list when unpacking query string data:
# gracefully handles # ...?foo=a # ...?foo=a&foo=b # and treats them as ["a"] and ["a", "b"] respectively @parser.use_args({"foo": CustomMultiplexingField()}, location="query") def show_foos(foo): ...
Mixing Locations
Arguments for different locations can be specified by passing location to each use_args <webargs.core.Parser.use_args> call:
# "json" is the default, used explicitly below @app.route("/stacked", methods=["POST"]) @use_args({"page": fields.Int(), "q": fields.Str()}, location="query") @use_args({"name": fields.Str()}, location="json") def viewfunc(query_parsed, json_parsed): page = query_parsed["page"] name = json_parsed["name"] # ...
To reduce boilerplate, you could create shortcuts, like so:
import functools query = functools.partial(use_args, location="query") body = functools.partial(use_args, location="json") @query({"page": fields.Int(), "q": fields.Int()}) @body({"name": fields.Str()}) 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 directory.