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URL Dispatch

:term:`URL dispatch` provides a simple way to map URLs to :term:`view` code using a simple pattern matching language. An ordered set of patterns is checked one-by-one. If one of the patterns matches the path information associated with a request, a particular :term:`view callable` is invoked. A view callable is a specific bit of code, defined in your application, that receives the :term:`request` and returns a :term:`response` object.

High-Level Operational Overview

If route configuration is present in an application, the :app:`Pyramid` :term:`Router` checks every incoming request against an ordered set of URL matching patterns present in a route map.

If any route pattern matches the information in the :term:`request`, :app:`Pyramid` will invoke :term:`view lookup` to find a matching view.

If no route pattern in the route map matches the information in the :term:`request` provided in your application, :app:`Pyramid` will fail over to using :term:`traversal` to perform resource location and view lookup.

Route Configuration

:term:`Route configuration` is the act of adding a new :term:`route` to an application. A route has a name, which acts as an identifier to be used for URL generation. The name also allows developers to associate a view configuration with the route. A route also has a pattern, meant to match against the PATH_INFO portion of a URL (the portion following the scheme and port, e.g. /foo/bar in the URL http://localhost:8080/foo/bar). It also optionally has a factory and a set of :term:`route predicate` attributes.

Configuring a Route to Match a View

The :meth:`pyramid.config.Configurator.add_route` method adds a single :term:`route configuration` to the :term:`application registry`. Here's an example:

# "config" below is presumed to be an instance of the
# pyramid.config.Configurator class; "myview" is assumed
# to be a "view callable" function
from views import myview
config.add_route('myroute', '/prefix/{one}/{two}')
config.add_view(myview, route_name='myroute')

When a :term:`view callable` added to the configuration by way of :meth:`~pyramid.config.Configurator.add_view` becomes associated with a route via its route_name predicate, that view callable will always be found and invoked when the associated route pattern matches during a request.

More commonly, you will not use any add_view statements in your project's "setup" code, instead only using add_route statements using a :term:`scan` for to associate view callables with routes. For example, if this is a portion of your project's __init__.py:

# in your project's __init__.py (mypackage.__init__)

config.add_route('myroute', '/prefix/{one}/{two}')
config.scan('mypackage')

Note that we don't call :meth:`~pyramid.config.Configurator.add_view` in this setup code. However, the above :term:`scan` execution config.scan('mypackage') will pick up all :term:`configuration decoration`, including any objects decorated with the :class:`pyramid.view.view_config` decorator in the mypackage Python pakage. For example, if you have a views.py in your package, a scan will pick up any of its configuration decorators, so we can add one there that that references myroute as a route_name parameter:

# in your project's views.py module (mypackage.views)

from pyramid.view import view_config
from pyramid.response import Response

@view_config(route_name='myroute')
def myview(request):
    return Response('OK')

The above combination of add_route and scan is completely equivalent to using the previous combination of add_route and add_view.

Route Pattern Syntax

The syntax of the pattern matching language used by :app:`Pyramid` URL dispatch in the pattern argument is straightforward; it is close to that of the :term:`Routes` system used by :term:`Pylons`.

The pattern used in route configuration may start with a slash character. If the pattern does not start with a slash character, an implicit slash will be prepended to it at matching time. For example, the following patterns are equivalent:

{foo}/bar/baz

and:

/{foo}/bar/baz

A pattern segment (an individual item between / characters in the pattern) may either be a literal string (e.g. foo) or it may be a replacement marker (e.g. {foo}) or a certain combination of both. A replacement marker does not need to be preceded by a / character.

A replacement marker is in the format {name}, where this means "accept any characters up to the next slash character and use this as the name :term:`matchdict` value."

A replacement marker in a pattern must begin with an uppercase or lowercase ASCII letter or an underscore, and can be composed only of uppercase or lowercase ASCII letters, underscores, and numbers. For example: a, a_b, _b, and b9 are all valid replacement marker names, but 0a is not.

Note

A replacement marker could not start with an underscore until Pyramid 1.2. Previous versions required that the replacement marker start with an uppercase or lowercase letter.

A matchdict is the dictionary representing the dynamic parts extracted from a URL based on the routing pattern. It is available as request.matchdict. For example, the following pattern defines one literal segment (foo) and two replacement markers (baz, and bar):

foo/{baz}/{bar}

The above pattern will match these URLs, generating the following matchdicts:

foo/1/2        -> {'baz':u'1', 'bar':u'2'}
foo/abc/def    -> {'baz':u'abc', 'bar':u'def'}

It will not match the following patterns however:

foo/1/2/        -> No match (trailing slash)
bar/abc/def     -> First segment literal mismatch

The match for a segment replacement marker in a segment will be done only up to the first non-alphanumeric character in the segment in the pattern. So, for instance, if this route pattern was used:

foo/{name}.html

The literal path /foo/biz.html will match the above route pattern, and the match result will be {'name':u'biz'}. However, the literal path /foo/biz will not match, because it does not contain a literal .html at the end of the segment represented by {name}.html (it only contains biz, not biz.html).

To capture both segments, two replacement markers can be used:

foo/{name}.{ext}

The literal path /foo/biz.html will match the above route pattern, and the match result will be {'name': 'biz', 'ext': 'html'}. This occurs because there is a literal part of . (period) between the two replacement markers {name} and {ext}.

Replacement markers can optionally specify a regular expression which will be used to decide whether a path segment should match the marker. To specify that a replacement marker should match only a specific set of characters as defined by a regular expression, you must use a slightly extended form of replacement marker syntax. Within braces, the replacement marker name must be followed by a colon, then directly thereafter, the regular expression. The default regular expression associated with a replacement marker [^/]+ matches one or more characters which are not a slash. For example, under the hood, the replacement marker {foo} can more verbosely be spelled as {foo:[^/]+}. You can change this to be an arbitrary regular expression to match an arbitrary sequence of characters, such as {foo:\d+} to match only digits.

It is possible to use two replacement markers without any literal characters between them, for instance /{foo}{bar}. However, this would be a nonsensical pattern without specifying a custom regular expression to restrict what each marker captures.

Segments must contain at least one character in order to match a segment replacement marker. For example, for the URL /abc/:

  • /abc/{foo} will not match.
  • /{foo}/ will match.

Note that values representing matched path segments will be url-unquoted and decoded from UTF-8 into Unicode within the matchdict. So for instance, the following pattern:

foo/{bar}

When matching the following URL:

foo/La%20Pe%C3%B1a

The matchdict will look like so (the value is URL-decoded / UTF-8 decoded):

{'bar':u'La Pe\xf1a'}

If the pattern has a * in it, the name which follows it is considered a "remainder match". A remainder match must come at the end of the pattern. Unlike segment replacement markers, it does not need to be preceded by a slash. For example:

foo/{baz}/{bar}*fizzle

The above pattern will match these URLs, generating the following matchdicts:

foo/1/2/           ->
         {'baz':u'1', 'bar':u'2', 'fizzle':()}

foo/abc/def/a/b/c  ->
         {'baz':u'abc', 'bar':u'def', 'fizzle':(u'a', u'b', u'c')}

Note that when a *stararg remainder match is matched, the value put into the matchdict is turned into a tuple of path segments representing the remainder of the path. These path segments are url-unquoted and decoded from UTF-8 into Unicode. For example, for the following pattern:

foo/*fizzle

When matching the following path:

/foo/La%20Pe%C3%B1a/a/b/c

Will generate the following matchdict:

{'fizzle':(u'La Pe\xf1a', u'a', u'b', u'c')}

By default, the *stararg will parse the remainder sections into a tuple split by segment. Changing the regular expression used to match a marker can also capture the remainder of the URL, for example:

foo/{baz}/{bar}{fizzle:.*}

The above pattern will match these URLs, generating the following matchdicts:

foo/1/2/           -> {'baz':u'1', 'bar':u'2', 'fizzle':()}
foo/abc/def/a/b/c  -> {'baz':u'abc', 'bar':u'def', 'fizzle': u'a/b/c')}

This occurs because the default regular expression for a marker is [^/]+ which will match everything up to the first /, while {fizzle:.*} will result in a regular expression match of .* capturing the remainder into a single value.

Route Declaration Ordering

Route configuration declarations are evaluated in a specific order when a request enters the system. As a result, the order of route configuration declarations is very important. The order that routes declarations are evaluated is the order in which they are added to the application at startup time. (This is unlike a different way of mapping URLs to code that :app:`Pyramid` provides, named :term:`traversal`, which does not depend on pattern ordering).

For routes added via the :mod:`~pyramid.config.Configurator.add_route` method, the order that routes are evaluated is the order in which they are added to the configuration imperatively.

For example, route configuration statements with the following patterns might be added in the following order:

members/{def}
members/abc

In such a configuration, the members/abc pattern would never be matched. This is because the match ordering will always match members/{def} first; the route configuration with members/abc will never be evaluated.

Route Configuration Arguments

Route configuration add_route statements may specify a large number of arguments. They are documented as part of the API documentation at :meth:`pyramid.config.Configurator.add_route`.

Many of these arguments are :term:`route predicate` arguments. A route predicate argument specifies that some aspect of the request must be true for the associated route to be considered a match during the route matching process. Examples of route predicate arguments are pattern, xhr, and request_method.

Other arguments are name and factory. These arguments represent neither predicates nor view configuration information.

Warning

Some arguments are view-configuration related arguments, such as view_renderer. These only have an effect when the route configuration names a view and these arguments have been deprecated as of :app:`Pyramid` 1.1.

Route Matching

The main purpose of route configuration is to match (or not match) the PATH_INFO present in the WSGI environment provided during a request against a URL path pattern. PATH_INFO represents the path portion of the URL that was requested.

The way that :app:`Pyramid` does this is very simple. When a request enters the system, for each route configuration declaration present in the system, :app:`Pyramid` checks the request's PATH_INFO against the pattern declared. This checking happens in the order that the routes were declared via :meth:`pyramid.config.Configurator.add_route`.

When a route configuration is declared, it may contain :term:`route predicate` arguments. All route predicates associated with a route declaration must be True for the route configuration to be used for a given request during a check. If any predicate in the set of :term:`route predicate` arguments provided to a route configuration returns False during a check, that route is skipped and route matching continues through the ordered set of routes.

If any route matches, the route matching process stops and the :term:`view lookup` subsystem takes over to find the most reasonable view callable for the matched route. Most often, there's only one view that will match (a view configured with a route_name argument matching the matched route). To gain a better understanding of how routes and views are associated in a real application, you can use the pviews command, as documented in :ref:`displaying_matching_views`.

If no route matches after all route patterns are exhausted, :app:`Pyramid` falls back to :term:`traversal` to do :term:`resource location` and :term:`view lookup`.

The Matchdict

When the URL pattern associated with a particular route configuration is matched by a request, a dictionary named matchdict is added as an attribute of the :term:`request` object. Thus, request.matchdict will contain the values that match replacement patterns in the pattern element. The keys in a matchdict will be strings. The values will be Unicode objects.

Note

If no route URL pattern matches, the matchdict object attached to the request will be None.

The Matched Route

When the URL pattern associated with a particular route configuration is matched by a request, an object named matched_route is added as an attribute of the :term:`request` object. Thus, request.matched_route will be an object implementing the :class:`~pyramid.interfaces.IRoute` interface which matched the request. The most useful attribute of the route object is name, which is the name of the route that matched.

Note

If no route URL pattern matches, the matched_route object attached to the request will be None.

Routing Examples

Let's check out some examples of how route configuration statements might be commonly declared, and what will happen if they are matched by the information present in a request.

Example 1

The simplest route declaration which configures a route match to directly result in a particular view callable being invoked:

When a route configuration with a view attribute is added to the system, and an incoming request matches the pattern of the route configuration, the :term:`view callable` named as the view attribute of the route configuration will be invoked.

In the case of the above example, when the URL of a request matches /site/{id}, the view callable at the Python dotted path name mypackage.views.site_view will be called with the request. In other words, we've associated a view callable directly with a route pattern.

When the /site/{id} route pattern matches during a request, the site_view view callable is invoked with that request as its sole argument. When this route matches, a matchdict will be generated and attached to the request as request.matchdict. If the specific URL matched is /site/1, the matchdict will be a dictionary with a single key, id; the value will be the string '1', ex.: {'id':'1'}.

The mypackage.views module referred to above might look like so:

The view has access to the matchdict directly via the request, and can access variables within it that match keys present as a result of the route pattern.

See :ref:`views_chapter`, and :ref:`view_config_chapter` for more information about views.

Example 2

Below is an example of a more complicated set of route statements you might add to your application:

The above configuration will allow :app:`Pyramid` to service URLs in these forms:

/ideas/{idea}
/users/{user}
/tags/{tag}
  • When a URL matches the pattern /ideas/{idea}, the view callable available at the dotted Python pathname mypackage.views.idea_view will be called. For the specific URL /ideas/1, the matchdict generated and attached to the :term:`request` will consist of {'idea':'1'}.
  • When a URL matches the pattern /users/{user}, the view callable available at the dotted Python pathname mypackage.views.user_view will be called. For the specific URL /users/1, the matchdict generated and attached to the :term:`request` will consist of {'user':'1'}.
  • When a URL matches the pattern /tags/{tag}, the view callable available at the dotted Python pathname mypackage.views.tag_view will be called. For the specific URL /tags/1, the matchdict generated and attached to the :term:`request` will consist of {'tag':'1'}.

In this example we've again associated each of our routes with a :term:`view callable` directly. In all cases, the request, which will have a matchdict attribute detailing the information found in the URL by the process will be passed to the view callable.

Example 3

The :term:`context` resource object passed in to a view found as the result of URL dispatch will, by default, be an instance of the object returned by the :term:`root factory` configured at startup time (the root_factory argument to the :term:`Configurator` used to configure the application).

You can override this behavior by passing in a factory argument to the :meth:`~pyramid.config.Configurator.add_route` method for a particular route. The factory should be a callable that accepts a :term:`request` and returns an instance of a class that will be the context resource used by the view.

An example of using a route with a factory:

The above route will manufacture an Idea resource as a :term:`context`, assuming that mypackage.resources.Idea resolves to a class that accepts a request in its __init__. For example:

In a more complicated application, this root factory might be a class representing a :term:`SQLAlchemy` model.

See :ref:`route_factories` for more details about how to use route factories.

Matching the Root URL

It's not entirely obvious how to use a route pattern to match the root URL ("/"). To do so, give the empty string as a pattern in a call to :meth:`~pyramid.config.Configurator.add_route`:

Or provide the literal string / as the pattern:

Generating Route URLs

Use the :meth:`pyramid.request.Request.route_url` method to generate URLs based on route patterns. For example, if you've configured a route with the name "foo" and the pattern "{a}/{b}/{c}", you might do this.

This would return something like the string http://example.com/1/2/3 (at least if the current protocol and hostname implied http://example.com). See the :meth:`~pyramid.request.Request.route_url` API documentation for more information.

Static Routes

Routes may be added with a static keyword argument. For example:

Routes added with a True static keyword argument will never be considered for matching at request time. Static routes are useful for URL generation purposes only. As a result, it is usually nonsensical to provide other non-name and non-pattern arguments to :meth:`~pyramid.config.Configurator.add_route` when static is passed as True, as none of the other arguments will ever be employed. A single exception to this rule is use of the pregenerator argument, which is not ignored when static is True.

Note

the static argument to :meth:`~pyramid.config.Configurator.add_route` is new as of :app:`Pyramid` 1.1.

Redirecting to Slash-Appended Routes

For behavior like Django's APPEND_SLASH=True, use the :func:`~pyramid.view.append_slash_notfound_view` view as the :term:`Not Found view` in your application. Defining this view as the :term:`Not Found view` is a way to automatically redirect requests where the URL lacks a trailing slash, but requires one to match the proper route. When configured, along with at least one other route in your application, this view will be invoked if the value of PATH_INFO does not already end in a slash, and if the value of PATH_INFO plus a slash matches any route's pattern. In this case it does an HTTP redirect to the slash-appended PATH_INFO.

Let's use an example, because this behavior is a bit magical. If the append_slash_notfound_view is configured in your application and your route configuration looks like so:

If a request enters the application with the PATH_INFO value of /has_slash/, the second route will match. If a request enters the application with the PATH_INFO value of /has_slash, a route will be found by the slash-appending not found view. An HTTP redirect to /has_slash/ will be returned to the user's browser.

If a request enters the application with the PATH_INFO value of /no_slash, the first route will match. However, if a request enters the application with the PATH_INFO value of /no_slash/, no route will match, and the slash-appending not found view will not find a matching route with an appended slash.

Warning

You should not rely on this mechanism to redirect POST requests. The redirect of the slash-appending not found view will turn a POST request into a GET, losing any POST data in the original request.

To configure the slash-appending not found view in your application, change the application's startup configuration, adding the following stanza:

See :ref:`view_module` and :ref:`changing_the_notfound_view` for more information about the slash-appending not found view and for a more general description of how to configure a not found view.

Custom Not Found View With Slash Appended Routes

There can only be one :term:`Not Found view` in any :app:`Pyramid` application. Even if you use :func:`~pyramid.view.append_slash_notfound_view` as the Not Found view, :app:`Pyramid` still must generate a 404 Not Found response when it cannot redirect to a slash-appended URL; this not found response will be visible to site users.

If you don't care what this 404 response looks like, and only you need redirections to slash-appended route URLs, you may use the :func:`~pyramid.view.append_slash_notfound_view` object as the Not Found view as described above. However, if you wish to use a custom notfound view callable when a URL cannot be redirected to a slash-appended URL, you may wish to use an instance of the :class:`~pyramid.view.AppendSlashNotFoundViewFactory` class as the Not Found view, supplying a :term:`view callable` to be used as the custom notfound view as the first argument to its constructor. For instance:

The notfound_view supplied must adhere to the two-argument view callable calling convention of (context, request) (context will be the exception object).

Debugging Route Matching

It's useful to be able to take a peek under the hood when requests that enter your application arent matching your routes as you expect them to. To debug route matching, use the PYRAMID_DEBUG_ROUTEMATCH environment variable or the pyramid.debug_routematch configuration file setting (set either to true). Details of the route matching decision for a particular request to the :app:`Pyramid` application will be printed to the stderr of the console which you started the application from. For example:

See :ref:`environment_chapter` for more information about how, and where to set these values.

You can also use the proutes command to see a display of all the routes configured in your application; for more information, see :ref:`displaying_application_routes`.

Using a Route Prefix to Compose Applications

Note

This feature is new as of :app:`Pyramid` 1.2.

The :meth:`pyramid.config.Configurator.include` method allows configuration statements to be included from separate files. See :ref:`building_an_extensible_app` for information about this method. Using :meth:`pyramid.config.Configurator.include` allows you to build your application from small and potentially reusable components.

The :meth:`pyramid.config.Configurator.include` method accepts an argument named route_prefix which can be useful to authors of URL-dispatch-based applications. If route_prefix is supplied to the include method, it must be a string. This string represents a route prefix that will be prepended to all route patterns added by the included configuration. Any calls to :meth:`pyramid.config.Configurator.add_route` within the included callable will have their pattern prefixed with the value of route_prefix. This can be used to help mount a set of routes at a different location than the included callable's author intended while still maintaining the same route names. For example:

In the above configuration, the show_users route will have an effective route pattern of /users/show, instead of /show because the route_prefix argument will be prepended to the pattern. The route will then only match if the URL path is /users/show, and when the :meth:`pyramid.request.Request.route_url` function is called with the route name show_users, it will generate a URL with that same path.

Route prefixes are recursive, so if a callable executed via an include itself turns around and includes another callable, the second-level route prefix will be prepended with the first:

In the above configuration, the show_users route will still have an effective route pattern of /users/show. The show_times route however, will have an effective pattern of /users/timing/show_times.

Route prefixes have no impact on the requirement that the set of route names in any given Pyramid configuration must be entirely unique. If you compose your URL dispatch application out of many small subapplications using :meth:`pyramid.config.Configurator.include`, it's wise to use a dotted name for your route names, so they'll be unlikely to conflict with other packages that may be added in the future. For example:

Custom Route Predicates

Each of the predicate callables fed to the custom_predicates argument of :meth:`~pyramid.config.Configurator.add_route` must be a callable accepting two arguments. The first argument passed to a custom predicate is a dictionary conventionally named info. The second argument is the current :term:`request` object.

The info dictionary has a number of contained values: match is a dictionary: it represents the arguments matched in the URL by the route. route is an object representing the route which was matched (see :class:`pyramid.interfaces.IRoute` for the API of such a route object).

info['match'] is useful when predicates need access to the route match. For example:

The above any_of function generates a predicate which ensures that the match value named segment_name is in the set of allowable values represented by allowed. We use this any_of function to generate a predicate function named num_one_two_or_three, which ensures that the num segment is one of the values one, two, or three , and use the result as a custom predicate by feeding it inside a tuple to the custom_predicates argument to :meth:`~pyramid.config.Configurator.add_route`.

A custom route predicate may also modify the match dictionary. For instance, a predicate might do some type conversion of values:

Note that a conversion predicate is still a predicate so it must return True or False; a predicate that does only conversion, such as the one we demonstrate above should unconditionally return True.

To avoid the try/except uncertainty, the route pattern can contain regular expressions specifying requirements for that marker. For instance:

Now the try/except is no longer needed because the route will not match at all unless these markers match \d+ which requires them to be valid digits for an int type conversion.

The match dictionary passed within info to each predicate attached to a route will be the same dictionary. Therefore, when registering a custom predicate which modifies the match dict, the code registering the predicate should usually arrange for the predicate to be the last custom predicate in the custom predicate list. Otherwise, custom predicates which fire subsequent to the predicate which performs the match modification will receive the modified match dictionary.

Warning

It is a poor idea to rely on ordering of custom predicates to build a conversion pipeline, where one predicate depends on the side effect of another. For instance, it's a poor idea to register two custom predicates, one which handles conversion of a value to an int, the next which handles conversion of that integer to some custom object. Just do all that in a single custom predicate.

The route object in the info dict is an object that has two useful attributes: name and pattern. The name attribute is the route name. The pattern attribute is the route pattern. An example of using the route in a set of route predicates:

The above predicate, when added to a number of route configurations ensures that the year match argument is '2010' if and only if the route name is 'ymd', 'ym', or 'y'.

You can also caption the predicates by setting the __text__ attribute. This will help you with the pviews command (see :ref:`displaying_application_routes`) and the pyramid_debugtoolbar.

If a predicate is a class just add __text__ property in a standard manner.

If a predicate is a method you'll need to assign it after method declaration (see PEP 232)

If a predicate is a classmethod using @classmethod will not work, but you can still easily do it by wrapping it in classmethod call.

Same will work with staticmethod, just use staticmethod instead of classmethod.

See also :class:`pyramid.interfaces.IRoute` for more API documentation about route objects.

Route Factories

Although it is not a particular common need in basic applications, a "route" configuration declaration can mention a "factory". When that route matches a request, and a factory is attached to a route, the :term:`root factory` passed at startup time to the :term:`Configurator` is ignored; instead the factory associated with the route is used to generate a :term:`root` object. This object will usually be used as the :term:`context` resource of the view callable ultimately found via :term:`view lookup`.

The factory can either be a Python object or a :term:`dotted Python name` (a string) which points to such a Python object, as it is above.

In this way, each route can use a different factory, making it possible to supply a different :term:`context` resource object to the view related to each particular route.

A factory must be a callable which accepts a request and returns an arbitrary Python object. For example, the below class can be used as a factory:

A route factory is actually conceptually identical to the :term:`root factory` described at :ref:`the_resource_tree`.

Supplying a different resource factory for each route is useful when you're trying to use a :app:`Pyramid` :term:`authorization policy` to provide declarative, "context sensitive" security checks; each resource can maintain a separate :term:`ACL`, as documented in :ref:`using_security_with_urldispatch`. It is also useful when you wish to combine URL dispatch with :term:`traversal` as documented within :ref:`hybrid_chapter`.

Using :app:`Pyramid` Security With URL Dispatch

:app:`Pyramid` provides its own security framework which consults an :term:`authorization policy` before allowing any application code to be called. This framework operates in terms of an access control list, which is stored as an __acl__ attribute of a resource object. A common thing to want to do is to attach an __acl__ to the resource object dynamically for declarative security purposes. You can use the factory argument that points at a factory which attaches a custom __acl__ to an object at its creation time.

Such a factory might look like so:

If the route archives/{article} is matched, and the article number is 1, :app:`Pyramid` will generate an Article :term:`context` resource with an ACL on it that allows the editor principal the view permission. Obviously you can do more generic things than inspect the routes match dict to see if the article argument matches a particular string; our sample Article factory class is not very ambitious.

Note

See :ref:`security_chapter` for more information about :app:`Pyramid` security and ACLs.

Route View Callable Registration and Lookup Details

When a request enters the system which matches the pattern of the route, the usual result is simple: the view callable associated with the route is invoked with the request that caused the invocation.

For most usage, you needn't understand more than this; how it works is an implementation detail. In the interest of completeness, however, we'll explain how it does work in the this section. You can skip it if you're uninterested.

When a view is associated with a route configuration, :app:`Pyramid` ensures that a :term:`view configuration` is registered that will always be found when the route pattern is matched during a request. To do so:

  • A special route-specific :term:`interface` is created at startup time for each route configuration declaration.
  • When an add_view statement mentions a route name attribute, a :term:`view configuration` is registered at startup time. This view configuration uses a route-specific interface as a :term:`request` type.
  • At runtime, when a request causes any route to match, the :term:`request` object is decorated with the route-specific interface.
  • The fact that the request is decorated with a route-specific interface causes the :term:`view lookup` machinery to always use the view callable registered using that interface by the route configuration to service requests that match the route pattern.

As we can see from the above description, technically, URL dispatch doesn't actually map a URL pattern directly to a view callable. Instead, URL dispatch is a :term:`resource location` mechanism. A :app:`Pyramid` :term:`resource location` subsystem (i.e., :term:`URL dispatch` or :term:`traversal`) finds a :term:`resource` object that is the :term:`context` of a :term:`request`. Once the :term:`context` is determined, a separate subsystem named :term:`view lookup` is then responsible for finding and invoking a :term:`view callable` based on information available in the context and the request. When URL dispatch is used, the resource location and view lookup subsystems provided by :app:`Pyramid` are still being utilized, but in a way which does not require a developer to understand either of them in detail.

If no route is matched using :term:`URL dispatch`, :app:`Pyramid` falls back to :term:`traversal` to handle the :term:`request`.

References

A tutorial showing how :term:`URL dispatch` can be used to create a :app:`Pyramid` application exists in :ref:`bfg_sql_wiki_tutorial`.

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