The API Class

Falcon’s API class is a WSGI “application” that you can host with any standard-compliant WSGI server.

import falcon

app = falcon.API()
class falcon.API(media_type='application/json', request_type=<class 'falcon.request.Request'>, response_type=<class 'falcon.response.Response'>, middleware=None, router=None, independent_middleware=True) [source]

This class is the main entry point into a Falcon-based app.

Each API instance provides a callable WSGI interface and a routing engine.

Keyword Arguments:
  • media_type (str) – Default media type to use as the value for the Content-Type header on responses (default ‘application/json’). The falcon module provides a number of constants for common media types, such as falcon.MEDIA_MSGPACK, falcon.MEDIA_YAML, falcon.MEDIA_XML, etc.
  • middleware (object or list) –

    Either a single object or a list of objects (instantiated classes) that implement the following middleware component interface:

    class ExampleComponent(object):
        def process_request(self, req, resp):
            """Process the request before routing it.
                Because Falcon routes each request based on
                req.path, a request can be effectively re-routed
                by setting that attribute to a new value from
                within process_request().
                req: Request object that will eventually be
                    routed to an on_* responder method.
                resp: Response object that will be routed to
                    the on_* responder.
        def process_resource(self, req, resp, resource, params):
            """Process the request and resource *after* routing.
                This method is only called when the request matches
                a route to a resource.
                req: Request object that will be passed to the
                    routed responder.
                resp: Response object that will be passed to the
                resource: Resource object to which the request was
                    routed. May be None if no route was found for
                    the request.
                params: A dict-like object representing any
                    additional params derived from the route's URI
                    template fields, that will be passed to the
                    resource's responder method as keyword
        def process_response(self, req, resp, resource, req_succeeded)
            """Post-processing of the response (after routing).
                req: Request object.
                resp: Response object.
                resource: Resource object to which the request was
                    routed. May be None if no route was found
                    for the request.
                req_succeeded: True if no exceptions were raised
                    while the framework processed and routed the
                    request; otherwise False.

    (See also: Middleware)

  • request_type (Request) – Request-like class to use instead of Falcon’s default class. Among other things, this feature affords inheriting from falcon.request.Request in order to override the context_type class variable. (default falcon.request.Request)
  • response_type (Response) – Response-like class to use instead of Falcon’s default class. (default falcon.response.Response)
  • router (object) – An instance of a custom router to use in lieu of the default engine. (See also: Custom Routers)
  • independent_middleware (bool) – Set to False if response middleware should not be executed independently of whether or not request middleware raises an exception (default True). When this option is set to False, a middleware component’s process_response() method will NOT be called when that same component’s process_request() (or that of a component higher up in the stack) raises an exception.

A set of behavioral options related to incoming requests. (See also: RequestOptions)


A set of behavioral options related to outgoing responses. (See also: ResponseOptions)


Configuration options for the router. If a custom router is in use, and it does not expose any configurable options, referencing this attribute will raise an instance of AttributeError.

(See also: CompiledRouterOptions)

add_error_handler(exception, handler=None) [source]

Register a handler for one or more exception types.

Error handlers may be registered for any exception type, including HTTPError or HTTPStatus. This feature provides a central location for logging and otherwise handling exceptions raised by responders, hooks, and middleware components.

A handler can raise an instance of HTTPError or HTTPStatus to communicate information about the issue to the client. Alternatively, a handler may modify resp directly.

Error handlers are matched in LIFO order. In other words, when searching for an error handler to match a raised exception, and more than one handler matches the exception type, the framework will choose the one that was most recently registered. Therefore, more general error handlers (e.g., for the standard Exception type) should be added first, to avoid masking more specific handlers for subclassed types.


By default, the framework installs two handlers, one for HTTPError and one for HTTPStatus. These can be overridden by adding a custom error handler method for the exception type in question.

  • exception (type or iterable of types) – When handling a request, whenever an error occurs that is an instance of the specified type(s), the associated handler will be called. Either a single type or an iterable of types may be specified.
  • handler (callable) –

    A function or callable object taking the form func(req, resp, ex, params).

    If not specified explicitly, the handler will default to exception.handle, where exception is the error type specified above, and handle is a static method (i.e., decorated with @staticmethod) that accepts the same params just described. For example:

    class CustomException(CustomBaseException):
        def handle(req, resp, ex, params):
            # TODO: Log the error
            # Convert to an instance of falcon.HTTPError
            raise falcon.HTTPError(falcon.HTTP_792)

    If an iterable of exception types is specified instead of a single type, the handler must be explicitly specified.

add_route(uri_template, resource, **kwargs) [source]

Associate a templatized URI path with a resource.

Falcon routes incoming requests to resources based on a set of URI templates. If the path requested by the client matches the template for a given route, the request is then passed on to the associated resource for processing.

If no route matches the request, control then passes to a default responder that simply raises an instance of HTTPNotFound.

This method delegates to the configured router’s add_route() method. To override the default behavior, pass a custom router object to the API initializer.

(See also: Routing)

  • uri_template (str) –

    A templatized URI. Care must be taken to ensure the template does not mask any sink patterns, if any are registered.

    (See also: add_sink())

  • resource (instance) – Object which represents a REST resource. Falcon will pass GET requests to on_get(), PUT requests to on_put(), etc. If any HTTP methods are not supported by your resource, simply don’t define the corresponding request handlers, and Falcon will do the right thing.
Keyword Arguments:

suffix (str) – Optional responder name suffix for this route. If a suffix is provided, Falcon will map GET requests to on_get_{suffix}(), POST requests to on_post_{suffix}(), etc. In this way, multiple closely-related routes can be mapped to the same resource. For example, a single resource class can use suffixed responders to distinguish requests for a single item vs. a collection of those same items. Another class might use a suffixed responder to handle a shortlink route in addition to the regular route for the resource.


Any additional keyword arguments not defined above are passed through to the underlying router’s add_route() method. The default router ignores any additional keyword arguments, but custom routers may take advantage of this feature to receive additional options when setting up routes. Custom routers MUST accept such arguments using the variadic pattern (**kwargs), and ignore any keyword arguments that they don’t support.

add_sink(sink, prefix='/') [source]

Register a sink method for the API.

If no route matches a request, but the path in the requested URI matches a sink prefix, Falcon will pass control to the associated sink, regardless of the HTTP method requested.

Using sinks, you can drain and dynamically handle a large number of routes, when creating static resources and responders would be impractical. For example, you might use a sink to create a smart proxy that forwards requests to one or more backend services.

  • sink (callable) – A callable taking the form func(req, resp).
  • prefix (str) –

    A regex string, typically starting with ‘/’, which will trigger the sink if it matches the path portion of the request’s URI. Both strings and precompiled regex objects may be specified. Characters are matched starting at the beginning of the URI path.


    Named groups are converted to kwargs and passed to the sink as such.


    If the prefix overlaps a registered route template, the route will take precedence and mask the sink.

    (See also: add_route())

add_static_route(prefix, directory, downloadable=False, fallback_filename=None) [source]

Add a route to a directory of static files.

Static routes provide a way to serve files directly. This feature provides an alternative to serving files at the web server level when you don’t have that option, when authorization is required, or for testing purposes.


Serving files directly from the web server, rather than through the Python app, will always be more efficient, and therefore should be preferred in production deployments. For security reasons, the directory and the fallback_filename (if provided) should be read only for the account running the application.

Static routes are matched in LIFO order. Therefore, if the same prefix is used for two routes, the second one will override the first. This also means that more specific routes should be added after less specific ones. For example, the following sequence would result in '/foo/bar/thing.js' being mapped to the '/foo/bar' route, and '/foo/xyz/thing.js' being mapped to the '/foo' route:

api.add_static_route('/foo', foo_path)
api.add_static_route('/foo/bar', foobar_path)
  • prefix (str) –

    The path prefix to match for this route. If the path in the requested URI starts with this string, the remainder of the path will be appended to the source directory to determine the file to serve. This is done in a secure manner to prevent an attacker from requesting a file outside the specified directory.

    Note that static routes are matched in LIFO order, and are only attempted after checking dynamic routes and sinks.

  • directory (str) – The source directory from which to serve files.
  • downloadable (bool) – Set to True to include a Content-Disposition header in the response. The “filename” directive is simply set to the name of the requested file.
  • fallback_filename (str) – Fallback filename used when the requested file is not found. Can be a relative path inside the prefix folder or any valid absolute path.
set_error_serializer(serializer) [source]

Override the default serializer for instances of HTTPError.

When a responder raises an instance of HTTPError, Falcon converts it to an HTTP response automatically. The default serializer supports JSON and XML, but may be overridden by this method to use a custom serializer in order to support other media types.


If a custom media type is used and the type includes a “+json” or “+xml” suffix, the default serializer will convert the error to JSON or XML, respectively.


The default serializer will not render any response body for HTTPError instances where the has_representation property evaluates to False (such as in the case of types that subclass falcon.http_error.NoRepresentation). However a custom serializer will be called regardless of the property value, and it may choose to override the representation logic.

The HTTPError class contains helper methods, such as to_json() and to_dict(), that can be used from within custom serializers. For example:

def my_serializer(req, resp, exception):
    representation = None

    preferred = req.client_prefers(('application/x-yaml',

    if exception.has_representation and preferred is not None:
        if preferred == 'application/json':
            representation = exception.to_json()
            representation = yaml.dump(exception.to_dict(),
        resp.body = representation
        resp.content_type = preferred

    resp.append_header('Vary', 'Accept')
Parameters: serializer (callable) – A function taking the form func(req, resp, exception), where req is the request object that was passed to the responder method, resp is the response object, and exception is an instance of falcon.HTTPError.
class falcon.RequestOptions [source]

Defines a set of configurable request options.

An instance of this class is exposed via API.req_options for configuring certain Request behaviors.


Set to False to ignore query string params that have missing or blank values (default True). For comma-separated values, this option also determines whether or not empty elements in the parsed list are retained.

Type: bool

Set to True in order to automatically consume the request stream and merge the results into the request’s query string params when the request’s content type is application/x-www-form-urlencoded (default False).

Enabling this option makes the form parameters accessible via params, get_param(), etc.


When this option is enabled, the request’s body stream will be left at EOF. The original data is not retained by the framework.


The character encoding for fields, before percent-encoding non-ASCII bytes, is assumed to be UTF-8. The special _charset_ field is ignored if present.

Falcon expects form-encoded request bodies to be encoded according to the standard W3C algorithm (see also


Set to True to split query string values on any non-percent-encoded commas (default False). When False, values containing commas are left as-is. In this mode, list items are taken only from multiples of the same parameter name within the query string (i.e. /?t=1,2,3&t=4 becomes ['1,2,3', '4']). When auto_parse_qs_csv is set to True, the query string value is also split on non-percent-encoded commas and these items are added to the final list (i.e. /?t=1,2,3&t=4 becomes ['1', '2', '3', '4']).


Set to True in order to strip the trailing slash, if present, at the end of the URL path (default False). When this option is enabled, the URL path is normalized by stripping the trailing slash character. This lets the application define a single route to a resource for a path that may or may not end in a forward slash. However, this behavior can be problematic in certain cases, such as when working with authentication schemes that employ URL-based signatures.


The default media-type to use when deserializing a response. This value is normally set to the media type provided when a falcon.API is initialized; however, if created independently, this will default to the DEFAULT_MEDIA_TYPE specified by Falcon.

Type: str

A dict-like object that allows you to configure the media-types that you would like to handle. By default, a handler is provided for the application/json media type.

Type: Handlers
class falcon.ResponseOptions [source]

Defines a set of configurable response options.

An instance of this class is exposed via API.resp_options for configuring certain Response behaviors.


Set to False in development environments to make the secure attribute for all cookies default to False. This can make testing easier by not requiring HTTPS. Note, however, that this setting can be overridden via set_cookie()’s secure kwarg.

Type: bool

The default Internet media type (RFC 2046) to use when deserializing a response. This value is normally set to the media type provided when a falcon.API is initialized; however, if created independently, this will default to the DEFAULT_MEDIA_TYPE specified by Falcon.

Type: str

A dict-like object that allows you to configure the media-types that you would like to handle. By default, a handler is provided for the application/json media type.

Type: Handlers

A mapping of dot-prefixed file extensions to Internet media types (RFC 2046). Defaults to mimetypes.types_map after calling mimetypes.init().

Type: dict
class falcon.routing.CompiledRouterOptions [source]

Defines a set of configurable router options.

An instance of this class is exposed via API.router_options for configuring certain CompiledRouter behaviors.


Represents the collection of named converters that may be referenced in URI template field expressions. Adding additional converters is simply a matter of mapping an identifier to a converter class:

api.router_options.converters['mc'] = MyConverter

The identifier can then be used to employ the converter within a URI template:

api.add_route('/{some_field:mc}', some_resource)

Converter names may only contain ASCII letters, digits, and underscores, and must start with either a letter or an underscore.


Converter instances are shared between requests. Therefore, in threaded deployments, care must be taken to implement custom converters in a thread-safe manner.

(See also: Field Converters)

© 2019 by Falcon contributors
Licensed under the Apache License, Version 2.0.