Routing¶
Falcon uses resource-based routing to encourage a RESTful architectural style. Each resource is represented by a class that is responsible for handling all of the HTTP methods that the resource supports.
For each HTTP method supported by the resource, the class implements a
corresponding Python method with a name that starts with on_
and ends in the
lowercased HTTP method name (e.g., on_get()
, on_patch()
,
on_delete()
, etc.)
Note
Resources in Falcon are represented by a single class instance that is created at application startup when the routes are configured. This minimizes routing overhead and simplifies the implementation of resource classes. In the case of WSGI apps, this also means that resource classes must be implemented in a thread-safe manner (see also: Is Falcon thread-safe?).
Falcon routes incoming requests (including WebSocket handshakes) 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.
Here’s a quick example to show how all the pieces fit together:
import json
import falcon
class ImagesResource:
def on_get(self, req, resp):
doc = {
'images': [
{
'href': '/images/1eaf6ef1-7f2d-4ecc-a8d5-6e8adba7cc0e.png'
}
]
}
# Create a JSON representation of the resource; this could
# also be done automatically by assigning to resp.media
resp.text = json.dumps(doc, ensure_ascii=False)
# The following line can be omitted because 200 is the default
# status returned by the framework, but it is included here to
# illustrate how this may be overridden as needed.
resp.status = falcon.HTTP_200
app = falcon.App()
images = ImagesResource()
app.add_route('/images', images)
import json
import falcon
import falcon.asgi
class ImagesResource:
async def on_get(self, req, resp):
doc = {
'images': [
{
'href': '/images/1eaf6ef1-7f2d-4ecc-a8d5-6e8adba7cc0e.png'
}
]
}
# Create a JSON representation of the resource; this could
# also be done automatically by assigning to resp.media
resp.text = json.dumps(doc, ensure_ascii=False)
# The following line can be omitted because 200 is the default
# status returned by the framework, but it is included here to
# illustrate how this may be overridden as needed.
resp.status = falcon.HTTP_200
app = falcon.asgi.App()
images = ImagesResource()
app.add_route('/images', images)
If no route matches the request, control then passes to a default responder that
simply raises an instance of HTTPRouteNotFound
. By default, this
error will be rendered as a 404 response for a regular HTTP request, and a 403
response with a 3404 close code for a WebSocket handshake. This
behavior can be modified by adding a custom error handler (see also
this FAQ topic).
On the other hand, if a route is matched but the resource does not implement a
responder for the requested HTTP method, the framework invokes a default
responder that raises an instance of HTTPMethodNotAllowed
. This class
will be rendered by default as a 405 response for a regular HTTP request, and a
403 response with a 3405 close code for a WebSocket handshake.
Falcon also provides a default responder for OPTIONS requests that takes into account which methods are implemented for the target resource.
Default Behavior¶
Falcon’s default routing engine is based on a decision tree that is
first compiled into Python code, and then evaluated by the runtime.
By default, the decision tree is compiled only when the router handles
the first request. See CompiledRouter
for more details.
The falcon.App.add_route()
and falcon.asgi.App.add_route()
methods
are used to associate a URI template with a resource. Falcon then maps incoming
requests to resources based on these templates.
Falcon’s default router uses Python classes to represent resources. In practice, these classes act as controllers in your application. They convert an incoming request into one or more internal actions, and then compose a response back to the client based on the results of those actions. (See also: Tutorial: Creating Resources)
┌────────────┐
request → │ │
│ Resource │ ↻ Orchestrate the requested action
│ Controller │ ↻ Compose the result
response ← │ │
└────────────┘
Each resource class defines various “responder” methods, one for each
HTTP method the resource allows. Responder names start with on_
and
are named according to which HTTP method they handle, as in on_get()
,
on_post()
, on_put()
, etc.
Note
If your resource does not support a particular
HTTP method, simply omit the corresponding responder and
Falcon will use a default responder that raises
an instance of HTTPMethodNotAllowed
when that
method is requested. Normally this results in sending a
405 response back to the client.
Responders must always define at least two arguments to receive
Request
and Response
objects, respectively:
def on_post(self, req, resp):
pass
For ASGI apps, the responder must be a coroutine function:
async def on_post(self, req, resp):
pass
The Request
object represents the incoming HTTP
request. It exposes properties and methods for examining headers,
query string parameters, and other metadata associated with
the request. A file-like stream object is also provided for reading
any data that was included in the body of the request.
The Response
object represents the application’s
HTTP response to the above request. It provides properties
and methods for setting status, header and body data. The
Response
object also exposes a dict-like
context
property for passing arbitrary
data to hooks and middleware methods.
Note
Rather than directly manipulate the Response
object, a responder may raise an instance of either
HTTPError
or HTTPStatus
. Falcon will
convert these exceptions to appropriate HTTP responses.
Alternatively, you can handle them yourself via
add_error_handler()
.
In addition to the standard req and resp parameters, if the route’s template contains field expressions, any responder that desires to receive requests for that route must accept arguments named after the respective field names defined in the template.
A field expression consists of a bracketed field name. For example, given the following template:
/user/{name}
A PUT request to '/user/kgriffs'
would cause the framework to invoke
the on_put()
responder method on the route’s resource class, passing
'kgriffs'
via an additional name argument defined by the responder:
# Template fields correspond to named arguments or keyword
# arguments, following the usual req and resp args.
def on_put(self, req, resp, name):
pass
# Template fields correspond to named arguments or keyword
# arguments, following the usual req and resp args.
async def on_put(self, req, resp, name):
pass
Because field names correspond to argument names in responder methods, they must be valid Python identifiers.
Individual path segments may contain one or more field expressions, and fields need not span the entire path segment. For example:
/repos/{org}/{repo}/compare/{usr0}:{branch0}...{usr1}:{branch1}
/serviceRoot/People('{name}')
(See also the Falcon tutorial for additional examples and a walkthrough of setting up routes within the context of a sample application.)
Field Converters¶
Falcon’s default router supports the use of field converters to
transform a URI template field value. Field converters may also perform
simple input validation. For example, the following URI template uses
the int converter to convert the value of tid to a Python int
,
but only if it has exactly eight digits:
/teams/{tid:int(8)}
If the value is malformed and can not be converted, Falcon will reject the request with a 404 response to the client.
Converters are instantiated with the argument specification given in the field expression. These specifications follow the standard Python syntax for passing arguments. For example, the comments in the following code show how a converter would be instantiated given different argument specifications in the URI template:
# IntConverter()
app.add_route(
'/a/{some_field:int}',
some_resource
)
# IntConverter(8)
app.add_route(
'/b/{some_field:int(8)}',
some_resource
)
# IntConverter(8, min=10000000)
app.add_route(
'/c/{some_field:int(8, min=10000000)}',
some_resource
)
Built-in Converters¶
Identifier |
Class |
Example |
---|---|---|
|
|
|
|
|
|
|
|
-
class
falcon.routing.
IntConverter
(num_digits=None, min=None, max=None)[source]¶ Converts a field value to an int.
Identifier: int
- Keyword Arguments
-
class
falcon.routing.
UUIDConverter
[source]¶ Converts a field value to a uuid.UUID.
Identifier: uuid
In order to be converted, the field value must consist of a string of 32 hexadecimal digits, as defined in RFC 4122, Section 3. Note, however, that hyphens and the URN prefix are optional.
-
class
falcon.routing.
DateTimeConverter
(format_string='%Y-%m-%dT%H:%M:%SZ')[source]¶ Converts a field value to a datetime.
Identifier: dt
- Keyword Arguments
format_string (str) – String used to parse the field value into a datetime. Any format recognized by strptime() is supported (default
'%Y-%m-%dT%H:%M:%SZ'
).
Custom Converters¶
Custom converters can be registered via the
converters
router option. A converter is
simply a class that implements the BaseConverter
interface:
Custom Routers¶
A custom routing engine may be specified when instantiating
falcon.App()
or falcon.asgi.App()
. For example:
router = MyRouter()
app = App(router=router)
Custom routers may derive from the default CompiledRouter
engine, or implement a completely different routing strategy (such as
object-based routing).
A custom router is any class that implements the following interface:
class MyRouter:
def add_route(self, uri_template, resource, **kwargs):
"""Adds a route between URI path template and resource.
Args:
uri_template (str): A URI template to use for the route
resource (object): The resource instance to associate with
the URI template.
Keyword Args:
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.
**kwargs (dict): Accepts any additional keyword arguments
that were originally passed to the falcon.App.add_route()
method. These arguments MUST be accepted via the
double-star variadic pattern (**kwargs), and ignore any
unrecognized or unsupported arguments.
"""
def find(self, uri, req=None):
"""Search for a route that matches the given partial URI.
Args:
uri(str): The requested path to route.
Keyword Args:
req(Request): The Request object that will be passed to
the routed responder. The router may use `req` to
further differentiate the requested route. For
example, a header may be used to determine the
desired API version and route the request
accordingly.
Note:
The `req` keyword argument was added in version
1.2. To ensure backwards-compatibility, routers
that do not implement this argument are still
supported.
Returns:
tuple: A 4-member tuple composed of (resource, method_map,
params, uri_template), or ``None`` if no route matches
the requested path.
"""
Suffixed Responders¶
While Falcon encourages the REST architectural style, it is flexible enough to accomodate other paradigms. Consider the task of building an API for a calculator which can both add and subtract two numbers. You could implement the following:
class Add():
def on_get(self, req, resp):
resp.text = str(req.get_param_as_int('x') + req.get_param_as_int('y'))
resp.status = falcon.HTTP_200
class Subtract():
def on_get(self, req, resp):
resp.text = str(req.get_param_as_int('x') - req.get_param_as_int('y'))
resp.status = falcon.HTTP_200
add = Add()
subtract = Subtract()
app = falcon.App()
app.add_route('/add', add)
app.add_route('/subtract', subtract)
However, this approach highlights a situation in which grouping by resource may not make sense for your domain. In this context, adding and subtracting don’t seem to conceptually map to two separate resource collections. Instead of separating them based on the idea of “getting” different resources from each, we might want to group them based on the attributes of their function (i.e., take two numbers, do something to them, return the result).
With Suffixed Responders, we can do just that, rewriting the example above in a more procedural style:
class Calculator():
def on_get_add(self, req, resp):
resp.text = str(req.get_param_as_int('x') + req.get_param_as_int('y'))
resp.status = falcon.HTTP_200
def on_get_subtract(self, req, resp):
resp.text = str(req.get_param_as_int('x') - req.get_param_as_int('y'))
resp.status = falcon.HTTP_200
calc = Calculator()
app = falcon.App()
app.add_route('/add', calc, suffix='add')
app.add_route('/subtract', calc, suffix='subtract')
In the second iteration, using Suffixed Responders, we’re able to group responders based on their actions rather than the data they represent. This gives us added flexibility to accomodate situations in which a purely RESTful approach simply doesn’t fit.
Default Router¶
-
class
falcon.routing.
CompiledRouter
[source]¶ Fast URI router which compiles its routing logic to Python code.
Generally you do not need to use this router class directly, as an instance is created by default when the falcon.App class is initialized.
The router treats URI paths as a tree of URI segments and searches by checking the URI one segment at a time. Instead of interpreting the route tree for each look-up, it generates inlined, bespoke Python code to perform the search, then compiles that code. This makes the route processing quite fast.
The compilation process is delayed until the first use of the router (on the first routed request) to reduce the time it takes to start the application. This may noticeably delay the first response of the application when a large number of routes have been added. When adding the last route to the application a compile flag may be provided to force the router to compile immediately, thus avoiding any delay for the first response.
Note
When using a multi-threaded web server to host the application, it is possible that multiple requests may be routed at the same time upon startup. Therefore, the framework employs a lock to ensure that only a single compilation of the decision tree is performed.
See also
CompiledRouter.add_route()
-
add_route
(uri_template, resource, **kwargs)[source]¶ Add a route between a URI path template and a resource.
This method may be overridden to customize how a route is added.
- Parameters
- 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 toon_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.compile (bool) –
Optional flag that can be used to compile the routing logic on this call. By default,
CompiledRouter
delays compilation until the first request is routed. This may introduce a noticeable amount of latency when handling the first request, especially when the application implements a large number of routes. Setting compile toTrue
when the last route is added ensures that the first request will not be delayed in this case (defaults toFalse
).Note
Always setting this flag to
True
may slow down the addition of new routes when hundreds of them are added at once. It is advisable to only set this flag toTrue
when adding the final route.
-
find
(uri, req=None)[source]¶ Search for a route that matches the given partial URI.
- Parameters
uri (str) – The requested path to route.
- Keyword Arguments
req – The
falcon.Request
orfalcon.asgi.Request
object that will be passed to the routed responder. Currently the value of this argument is ignored byCompiledRouter
. Routing is based solely on the path.- Returns
A 4-member tuple composed of (resource, method_map, params, uri_template), or
None
if no route matches the requested path.- Return type
-
map_http_methods
(resource, **kwargs)[source]¶ Map HTTP methods (e.g., GET, POST) to methods of a resource object.
This method is called from
add_route()
and may be overridden to provide a custom mapping strategy.- Parameters
resource (instance) – Object which represents a REST resource. The default maps the HTTP method
GET
toon_get()
,POST
toon_post()
, 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 toon_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.
-
Routing Utilities¶
The falcon.routing module contains the following utilities that may be used by custom routing engines.
-
falcon.routing.
map_http_methods
(resource, suffix=None)[source]¶ Map HTTP methods (e.g., GET, POST) to methods of a resource object.
- Parameters
resource – An object with responder methods, following the naming convention on_*, that correspond to each method the resource supports. For example, if a resource supports GET and POST, it should define
on_get(self, req, resp)
andon_post(self, req, resp)
.- 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 toon_post_{suffix}()
, etc.- Returns
A mapping of HTTP methods to explicitly defined resource responders.
- Return type
-
falcon.routing.
set_default_responders
(method_map, asgi=False)[source]¶ Map HTTP methods not explicitly defined on a resource to default responders.
- Parameters
method_map – A dict with HTTP methods mapped to responders explicitly defined in a resource.
asgi (bool) –
True
if using an ASGI app,False
otherwise (defaultFalse
).
-
falcon.routing.
compile_uri_template
(template)[source]¶ Compile the given URI template string into a pattern matcher.
This function can be used to construct custom routing engines that iterate through a list of possible routes, attempting to match an incoming request against each route’s compiled regular expression.
Each field is converted to a named group, so that when a match is found, the fields can be easily extracted using
re.MatchObject.groupdict()
.This function does not support the more flexible templating syntax used in the default router. Only simple paths with bracketed field expressions are recognized. For example:
/ /books /books/{isbn} /books/{isbn}/characters /books/{isbn}/characters/{name}
Also, note that if the template contains a trailing slash character, it will be stripped in order to normalize the routing logic.
-
falcon.app_helpers.
prepare_middleware
(middleware, independent_middleware=False, asgi=False)[source]¶ Check middleware interfaces and prepare the methods for request handling.
Note
This method is only applicable to WSGI apps.
- Parameters
middleware (iterable) – An iterable of middleware objects.
- Keyword Arguments
- Returns
A tuple of prepared middleware method tuples
- Return type
-
falcon.app_helpers.
prepare_middleware_ws
(middleware)[source]¶ Check middleware interfaces and prepare WebSocket methods for request handling.
Note
This method is only applicable to ASGI apps.
- Parameters
middleware (iterable) – An iterable of middleware objects.
- Returns
A two-item
(request_mw, resource_mw)
tuple, where request_mw is an ordered list ofprocess_request_ws()
methods, and resource_mw is an ordered list ofprocess_resource_ws()
methods.- Return type
Custom HTTP Methods¶
While not normally advised, some applications may need to support non-standard HTTP methods, in addition to the standard HTTP methods like GET and PUT. To support custom HTTP methods, use one of the following methods:
Ideally, if you don’t use hooks in your application, you can easily add the custom methods in your application setup by overriding the value of
falcon.constants.COMBINED_METHODS
. For example:import falcon.constants falcon.constants.COMBINED_METHODS += ['FOO', 'BAR']
Due to the nature of hooks, if you do use them, you’ll need to define the FALCON_CUSTOM_HTTP_METHODS environment variable as a comma-delimited list of custom methods. For example:
$ export FALCON_CUSTOM_HTTP_METHODS=FOO,BAR
Once you have used the appropriate method, your custom methods should be active. You then can define request methods like any other HTTP method:
# Handle the custom FOO method
def on_foo(self, req, resp):
pass
# Handle the custom FOO method
async def on_foo(self, req, resp):
pass