Output Fields¶
Flask-RESTful provides an easy way to control what data you actually render in
your response. 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 RFC 822 date string (ISO 8601 is supported as well)
from flask_restful import Resource, fields, marshal_with
resource_fields = {
'name': fields.String,
'address': fields.String,
'date_updated': fields.DateTime(dt_format='rfc822'),
}
class Todo(Resource):
@marshal_with(resource_fields, 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(), resource_fields), 200
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.
fields = {
'name': fields.String(attribute='private_name'),
'address': fields.String,
}
A lambda (or any callable) can also be specified as the attribute
fields = {
'name': fields.String(attribute=lambda x: x._private_name),
'address': fields.String,
}
Nested properties can also be accessed with attribute
fields = {
'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
.
fields = {
'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"
fields = {
'name': fields.String,
'priority': UrgentItem(attribute='flags'),
'status': UnreadItem(attribute='flags'),
}
Url & Other Concrete Fields¶
Flask-RESTful 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()
fields = {
'name': fields.String,
# todo_resource is the endpoint name when you called api.add_resource()
'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:
fields = {
'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_restful 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_restful 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"}'
Advanced : 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_restful 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. 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 = {
'id': fields.Integer,
'name': fields.String,
}
user_list_fields = {
fields.List(fields.Nested(user_fields)),
}