Response marshalling¶
Flask-RESTPlus provides an easy way to control what data you actually render in
your response or expect as in input payload.
With the fields
module, you can use whatever objects (ORM
models/custom classes/etc.) you want in your resource.
fields
also lets you format and filter the response
so you don’t have to worry about exposing internal data structures.
It’s also very clear when looking at your code what data will be rendered and how it will be formatted.
Basic Usage¶
You can define a dict or OrderedDict of fields whose keys are names of attributes or keys on the object to render,
and whose values are a class that will format & return the value for that field.
This example has three fields:
two are String
and one is a DateTime
,
formatted as an ISO 8601 datetime string (RFC 822 is supported as well):
from flask_restplus import Resource, fields
model = api.model('Model', {
'name': fields.String,
'address': fields.String,
'date_updated': fields.DateTime(dt_format='rfc822'),
})
@api.route('/todo')
class Todo(Resource):
@api.marshal_with(model, envelope='resource')
def get(self, **kwargs):
return db_get_todo() # Some function that queries the db
This example assumes that you have a custom database object (todo
) that
has attributes name
, address
, and date_updated
.
Any additional attributes on the object are considered private and won’t be rendered in the output.
An optional envelope
keyword argument is specified to wrap the resulting output.
The decorator marshal_with()
is what actually takes your data object and applies the field filtering.
The marshalling can work on single objects, dicts, or lists of objects.
Note
marshal_with()
is a convenience decorator, that is functionally
equivalent to:
class Todo(Resource):
def get(self, **kwargs):
return marshal(db_get_todo(), model), 200
The @api.marshal_with
decorator add the swagger documentation ability.
This explicit expression can be used to return HTTP status codes other than 200
along with a successful response (see abort()
for errors).
Renaming Attributes¶
Often times your public facing field name is different from your internal field name.
To configure this mapping, use the attribute
keyword argument.
model = {
'name': fields.String(attribute='private_name'),
'address': fields.String,
}
A lambda (or any callable) can also be specified as the attribute
model = {
'name': fields.String(attribute=lambda x: x._private_name),
'address': fields.String,
}
Nested properties can also be accessed with attribute
:
model = {
'name': fields.String(attribute='people_list.0.person_dictionary.name'),
'address': fields.String,
}
Default Values¶
If for some reason your data object doesn’t have an attribute in your fields list,
you can specify a default value to return instead of None
.
model = {
'name': fields.String(default='Anonymous User'),
'address': fields.String,
}
Custom Fields & Multiple Values¶
Sometimes you have your own custom formatting needs.
You can subclass the fields.Raw
class and implement the format function.
This is especially useful when an attribute stores multiple pieces of information.
e.g. a bit-field whose individual bits represent distinct values.
You can use fields to multiplex a single attribute to multiple output values.
This example assumes that bit 1 in the flags
attribute signifies a
“Normal” or “Urgent” item, and bit 2 signifies “Read” or “Unread”.
These items might be easy to store in a bitfield,
but for a human readable output it’s nice to convert them to seperate string fields.
class UrgentItem(fields.Raw):
def format(self, value):
return "Urgent" if value & 0x01 else "Normal"
class UnreadItem(fields.Raw):
def format(self, value):
return "Unread" if value & 0x02 else "Read"
model = {
'name': fields.String,
'priority': UrgentItem(attribute='flags'),
'status': UnreadItem(attribute='flags'),
}
Url & Other Concrete Fields¶
Flask-RESTPlus includes a special field, fields.Url
,
that synthesizes a uri for the resource that’s being requested.
This is also a good example of how to add data to your response that’s not actually present on your data object.
class RandomNumber(fields.Raw):
def output(self, key, obj):
return random.random()
model = {
'name': fields.String,
# todo_resource is the endpoint name when you called api.route()
'uri': fields.Url('todo_resource'),
'random': RandomNumber,
}
By default fields.Url
returns a relative uri.
To generate an absolute uri that includes the scheme, hostname and port,
pass the keyword argument absolute=True
in the field declaration.
To override the default scheme, pass the scheme
keyword argument:
model = {
'uri': fields.Url('todo_resource', absolute=True)
'https_uri': fields.Url('todo_resource', absolute=True, scheme='https')
}
Complex Structures¶
You can have a flat structure that marshal()
will transform to a nested structure:
>>> from flask_restplus import fields, marshal
>>> import json
>>>
>>> resource_fields = {'name': fields.String}
>>> resource_fields['address'] = {}
>>> resource_fields['address']['line 1'] = fields.String(attribute='addr1')
>>> resource_fields['address']['line 2'] = fields.String(attribute='addr2')
>>> resource_fields['address']['city'] = fields.String
>>> resource_fields['address']['state'] = fields.String
>>> resource_fields['address']['zip'] = fields.String
>>> data = {'name': 'bob', 'addr1': '123 fake street', 'addr2': '', 'city': 'New York', 'state': 'NY', 'zip': '10468'}
>>> json.dumps(marshal(data, resource_fields))
'{"name": "bob", "address": {"line 1": "123 fake street", "line 2": "", "state": "NY", "zip": "10468", "city": "New York"}}'
Note
The address field doesn’t actually exist on the data object, but any of the sub-fields can access attributes directly from the object as if they were not nested.
List Field¶
You can also unmarshal fields as lists
>>> from flask_restplus import fields, marshal
>>> import json
>>>
>>> resource_fields = {'name': fields.String, 'first_names': fields.List(fields.String)}
>>> data = {'name': 'Bougnazal', 'first_names' : ['Emile', 'Raoul']}
>>> json.dumps(marshal(data, resource_fields))
>>> '{"first_names": ["Emile", "Raoul"], "name": "Bougnazal"}'
Nested Field¶
While nesting fields using dicts can turn a flat data object into a nested
response, you can use Nested
to unmarshal nested data
structures and render them appropriately.
>>> from flask_restplus import fields, marshal
>>> import json
>>>
>>> address_fields = {}
>>> address_fields['line 1'] = fields.String(attribute='addr1')
>>> address_fields['line 2'] = fields.String(attribute='addr2')
>>> address_fields['city'] = fields.String(attribute='city')
>>> address_fields['state'] = fields.String(attribute='state')
>>> address_fields['zip'] = fields.String(attribute='zip')
>>>
>>> resource_fields = {}
>>> resource_fields['name'] = fields.String
>>> resource_fields['billing_address'] = fields.Nested(address_fields)
>>> resource_fields['shipping_address'] = fields.Nested(address_fields)
>>> address1 = {'addr1': '123 fake street', 'city': 'New York', 'state': 'NY', 'zip': '10468'}
>>> address2 = {'addr1': '555 nowhere', 'city': 'New York', 'state': 'NY', 'zip': '10468'}
>>> data = { 'name': 'bob', 'billing_address': address1, 'shipping_address': address2}
>>>
>>> json.dumps(marshal_with(data, resource_fields))
'{"billing_address": {"line 1": "123 fake street", "line 2": null, "state": "NY", "zip": "10468", "city": "New York"}, "name": "bob", "shipping_address": {"line 1": "555 nowhere", "line 2": null, "state": "NY", "zip": "10468", "city": "New York"}}'
This example uses two Nested
fields.
The Nested
constructor takes a dict of fields to render as sub-fields.input.
The important difference between the Nested
constructor and nested dicts (previous example),
is the context for attributes.
In this example,
billing_address
is a complex object that has its own fields and
the context passed to the nested field is the sub-object instead of the original data
object.
In other words:
data.billing_address.addr1
is in scope here,
whereas in the previous example data.addr1
was the location attribute.
Remember: Nested
and List
objects create a new scope for attributes.
Use Nested
with List
to marshal lists of more complex objects:
user_fields = api.model('User', {
'id': fields.Integer,
'name': fields.String,
})
user_list_fields = api.model('UserList', {
'users': fields.List(fields.Nested(user_fields)),
})
The api.model()
factory¶
The model()
factory allows you to instanciate
and register models to your API
or Namespace
.
my_fields = api.model('MyModel', {
'name': fields.String,
'age': fields.Integer(min=0)
})
# Equivalent to
my_fields = Model('MyModel', {
'name': fields.String,
'age': fields.Integer(min=0)
})
api.models[my_fields.name] = my_fields
Duplicating with clone
¶
The Model.clone()
method allows you to instanciate an augmented model.
It saves you duplicating all fields.
parent = Model('Parent', {
'name': fields.String
})
child = parent.clone('Child', {
'age': fields.Integer
})
The Api/Namespace.clone
also register it on the API.
parent = api.model('Parent', {
'name': fields.String
})
child = api.clone('Child', parent, {
'age': fields.Integer
})
Polymorphism with api.inherit
¶
The Model.inherit()
method allows to extend a model in the “Swagger way”
and to start handling polymorphism.
parent = api.model('Parent', {
'name': fields.String,
'class': fields.String(discriminator=True)
})
child = api.inherit('Child', parent, {
'extra': fields.String
})
The Api/Namespace.clone
will register both the parent and the child
in the Swagger models definitions.
parent = Model('Parent', {
'name': fields.String,
'class': fields.String(discriminator=True)
})
child = parent.inherit('Child', {
'extra': fields.String
})
The class
field in this example will be populated with the serialized model name
only if the property does not exists in the serialized object.
The Polymorph
field allows you to specify a mapping between Python classes
and fields specifications.
mapping = {
Child1: child1_fields,
Child2: child2_fields,
}
fields = api.model('Thing', {
owner: fields.Polymorph(mapping)
})
Custom fields¶
Custom output fields let you perform your own output formatting without having
to modify your internal objects directly.
All you have to do is subclass Raw
and implement the format()
method:
class AllCapsString(fields.Raw):
def format(self, value):
return value.upper()
# example usage
fields = {
'name': fields.String,
'all_caps_name': AllCapsString(attribute='name'),
}
You can also use the __schema_format__
, __schema_type__
and
__schema_example__
to specify the produced types and examples:
class MyIntField(fields.Integer):
__schema_format__ = 'int64'
class MySpecialField(fields.Raw):
__schema_type__ = 'some-type'
__schema_format__ = 'some-format'
class MyVerySpecialField(fields.Raw):
__schema_example__ = 'hello, world'
Define model using JSON Schema¶
You can define models using JSON Schema (Draft v4).
address = api.schema_model('Address', {
'properties': {
'road': {
'type': 'string'
},
},
'type': 'object'
})
person = address = api.schema_model('Person', {
'required': ['address'],
'properties': {
'name': {
'type': 'string'
},
'age': {
'type': 'integer'
},
'birthdate': {
'type': 'string',
'format': 'date-time'
},
'address': {
'$ref': '#/definitions/Address',
}
},
'type': 'object'
})