schemas.py

Schemas

A machine-readable [schema] describes what resources are available via the API, what their URLs are, how they are represented and what operations they support.

— Heroku, JSON Schema for the Heroku Platform API

API schemas are a useful tool that allow for a range of use cases, including generating reference documentation, or driving dynamic client libraries that can interact with your API.

Representing schemas internally

REST framework uses Core API in order to model schema information in a format-independent representation. This information can then be rendered into various different schema formats, or used to generate API documentation.

When using Core API, a schema is represented as a Document which is the top-level container object for information about the API. Available API interactions are represented using Link objects. Each link includes a URL, HTTP method, and may include a list of Field instances, which describe any parameters that may be accepted by the API endpoint. The Link and Field instances may also include descriptions, that allow an API schema to be rendered into user documentation.

Here's an example of an API description that includes a single search endpoint:

coreapi.Document(
    title='Flight Search API',
    url='https://api.example.org/',
    content={
        'search': coreapi.Link(
            url='/search/',
            action='get',
            fields=[
                coreapi.Field(
                    name='from',
                    required=True,
                    location='query',
                    description='City name or airport code.'
                ),
                coreapi.Field(
                    name='to',
                    required=True,
                    location='query',
                    description='City name or airport code.'
                ),
                coreapi.Field(
                    name='date',
                    required=True,
                    location='query',
                    description='Flight date in "YYYY-MM-DD" format.'
                )
            ],
            description='Return flight availability and prices.'
        )
    }
)

Schema output formats

In order to be presented in an HTTP response, the internal representation has to be rendered into the actual bytes that are used in the response.

Core JSON is designed as a canonical format for use with Core API. REST framework includes a renderer class for handling this media type, which is available as renderers.CoreJSONRenderer.

Other schema formats such as Open API ("Swagger"), JSON HyperSchema, or API Blueprint can also be supported by implementing a custom renderer class.

Schemas vs Hypermedia

It's worth pointing out here that Core API can also be used to model hypermedia responses, which present an alternative interaction style to API schemas.

With an API schema, the entire available interface is presented up-front as a single endpoint. Responses to individual API endpoints are then typically presented as plain data, without any further interactions contained in each response.

With Hypermedia, the client is instead presented with a document containing both data and available interactions. Each interaction results in a new document, detailing both the current state and the available interactions.

Further information and support on building Hypermedia APIs with REST framework is planned for a future version.

Adding a schema

You'll need to install the coreapi package in order to add schema support for REST framework.

pip install coreapi

REST framework includes functionality for auto-generating a schema, or allows you to specify one explicitly. There are a few different ways to add a schema to your API, depending on exactly what you need.

Using DefaultRouter

If you're using DefaultRouter then you can include an auto-generated schema, simply by adding a schema_title argument to the router.

router = DefaultRouter(schema_title='Server Monitoring API')

The schema will be included at the root URL, /, and presented to clients that include the Core JSON media type in their Accept header.

$ http http://127.0.0.1:8000/ Accept:application/vnd.coreapi+json
HTTP/1.0 200 OK
Allow: GET, HEAD, OPTIONS
Content-Type: application/vnd.coreapi+json

{
    "_meta": {
        "title": "Server Monitoring API"
    },
    "_type": "document",
    ...
}

This is a great zero-configuration option for when you want to get up and running really quickly.

The other available options to DefaultRouter are:

schema_renderers

May be used to pass the set of renderer classes that can be used to render schema output.

from rest_framework.renderers import CoreJSONRenderer
from my_custom_package import APIBlueprintRenderer

router = DefaultRouter(schema_title='Server Monitoring API', schema_renderers=[
    CoreJSONRenderer, APIBlueprintRenderer
])

schema_url

May be used to pass the root URL for the schema. This can either be used to ensure that the schema URLs include a canonical hostname and schema, or to ensure that all the schema URLs include a path prefix.

router = DefaultRouter(
    schema_title='Server Monitoring API',
    schema_url='https://www.example.org/api/'
)

If you want more flexibility over the schema output then you'll need to consider using SchemaGenerator instead.

root_renderers

May be used to pass the set of renderer classes that can be used to render the API root endpoint.

Using SchemaGenerator

The most common way to add a schema to your API is to use the SchemaGenerator class to auto-generate the Document instance, and to return that from a view.

This option gives you the flexibility of setting up the schema endpoint with whatever behavior you want. For example, you can apply different permission, throttling or authentication policies to the schema endpoint.

Here's an example of using SchemaGenerator together with a view to return the schema.

views.py:

from rest_framework.decorators import api_view, renderer_classes
from rest_framework import renderers, response, schemas


@api_view()
@renderer_classes([renderers.CoreJSONRenderer])
def schema_view(request):
    generator = schemas.SchemaGenerator(title='Bookings API')
    return response.Response(generator.get_schema())

urls.py:

urlpatterns = [
    url('/', schema_view),
    ...
]

You can also serve different schemas to different users, depending on the permissions they have available. This approach can be used to ensure that unauthenticated requests are presented with a different schema to authenticated requests, or to ensure that different parts of the API are made visible to different users depending on their role.

In order to present a schema with endpoints filtered by user permissions, you need to pass the request argument to the get_schema() method, like so:

@api_view()
@renderer_classes([renderers.CoreJSONRenderer])
def schema_view(request):
    generator = schemas.SchemaGenerator(title='Bookings API')
    return response.Response(generator.get_schema(request=request))

Explicit schema definition

An alternative to the auto-generated approach is to specify the API schema explicitly, by declaring a Document object in your codebase. Doing so is a little more work, but ensures that you have full control over the schema representation.

import coreapi
from rest_framework.decorators import api_view, renderer_classes
from rest_framework import renderers, response

schema = coreapi.Document(
    title='Bookings API',
    content={
        ...
    }
)

@api_view()
@renderer_classes([renderers.CoreJSONRenderer])
def schema_view(request):
    return response.Response(schema)

Static schema file

A final option is to write your API schema as a static file, using one of the available formats, such as Core JSON or Open API.

You could then either:

Write a schema definition as a static file, and serve the static file directly.
Write a schema definition that is loaded using Core API, and then rendered to one of many available formats, depending on the client request.

Alternate schema formats

In order to support an alternate schema format, you need to implement a custom renderer class that handles converting a Document instance into a bytestring representation.

If there is a Core API codec package that supports encoding into the format you want to use then implementing the renderer class can be done by using the codec.

Example

For example, the openapi_codec package provides support for encoding or decoding to the Open API ("Swagger") format:

from rest_framework import renderers
from openapi_codec import OpenAPICodec

class SwaggerRenderer(renderers.BaseRenderer):
    media_type = 'application/openapi+json'
    format = 'swagger'

    def render(self, data, media_type=None, renderer_context=None):
        codec = OpenAPICodec()
        return codec.dump(data)

API Reference

SchemaGenerator

A class that deals with introspecting your API views, which can be used to generate a schema.

Typically you'll instantiate SchemaGenerator with a single argument, like so:

generator = SchemaGenerator(title='Stock Prices API')

Arguments:

title - The name of the API. required
url - The root URL of the API schema. This option is not required unless the schema is included under path prefix.
patterns - A list of URLs to inspect when generating the schema. Defaults to the project's URL conf.
urlconf - A URL conf module name to use when generating the schema. Defaults to settings.ROOT_URLCONF.

get_schema()

Returns a coreapi.Document instance that represents the API schema.

@api_view
@renderer_classes([renderers.CoreJSONRenderer])
def schema_view(request):
    generator = schemas.SchemaGenerator(title='Bookings API')
    return Response(generator.get_schema())

Arguments:

request - The incoming request. Optionally used if you want to apply per-user permissions to the schema-generation.

Core API

This documentation gives a brief overview of the components within the coreapi package that are used to represent an API schema.

Note that these classes are imported from the coreapi package, rather than from the rest_framework package.

Document

Represents a container for the API schema.

`title`

A name for the API.

`url`

A canonical URL for the API.

`content`

A dictionary, containing the Link objects that the schema contains.

In order to provide more structure to the schema, the content dictionary may be nested, typically to a second level. For example:

content={
    "bookings": {
        "list": Link(...),
        "create": Link(...),
        ...
    },
    "venues": {
        "list": Link(...),
        ...
    },
    ...
}

Link

Represents an individual API endpoint.

`url`

The URL of the endpoint. May be a URI template, such as /users/{username}/.

`action`

The HTTP method associated with the endpoint. Note that URLs that support more than one HTTP method, should correspond to a single Link for each.

`fields`

A list of Field instances, describing the available parameters on the input.

`description`

A short description of the meaning and intended usage of the endpoint.

Field

Represents a single input parameter on a given API endpoint.

`name`

A descriptive name for the input.

`required`

A boolean, indicated if the client is required to included a value, or if the parameter can be omitted.

`location`

Determines how the information is encoded into the request. Should be one of the following strings:

"path"

Included in a templated URI. For example a url value of /products/{product_code}/ could be used together with a "path" field, to handle API inputs in a URL path such as /products/slim-fit-jeans/.

These fields will normally correspond with named arguments in the project URL conf.

"query"

Included as a URL query parameter. For example ?search=sale. Typically for GET requests.

These fields will normally correspond with pagination and filtering controls on a view.

"form"

Included in the request body, as a single item of a JSON object or HTML form. For example {"colour": "blue", ...}. Typically for POST, PUT and PATCH requests. Multiple "form" fields may be included on a single link.

These fields will normally correspond with serializer fields on a view.

"body"

Included as the complete request body. Typically for POST, PUT and PATCH requests. No more than one "body" field may exist on a link. May not be used together with "form" fields.

These fields will normally correspond with views that use ListSerializer to validate the request input, or with file upload views.

`encoding`

"application/json"

JSON encoded request content. Corresponds to views using JSONParser. Valid only if either one or more location="form" fields, or a single location="body" field is included on the Link.

"multipart/form-data"

Multipart encoded request content. Corresponds to views using MultiPartParser. Valid only if one or more location="form" fields is included on the Link.

"application/x-www-form-urlencoded"

URL encoded request content. Corresponds to views using FormParser. Valid only if one or more location="form" fields is included on the Link.

"application/octet-stream"

Binary upload request content. Corresponds to views using FileUploadParser. Valid only if a location="body" field is included on the Link.

`description`

A short description of the meaning and intended usage of the input field.