path: root/docs/narr/urldispatch.rst

.. index::
   single: URL dispatch

.. _urldispatch_chapter:

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.  

:term:`URL dispatch` is one of two ways to perform :term:`resource
location` in :app:`Pyramid`; the other way is using :term:`traversal`.
If no route is matched using :term:`URL dispatch`, :app:`Pyramid` falls
back to :term:`traversal` to handle the :term:`request`.

It is the responsibility of the :term:`resource location` subsystem
(i.e., :term:`URL dispatch` or :term:`traversal`) to find the resource
object that is the :term:`context` of the :term:`request`. Once the
:term:`context` is determined, :term:`view lookup` is then responsible
for finding and invoking a :term:`view callable`.  A view callable is a
specific bit of code, defined in your application, that receives the
:term:`request` and returns a :term:`response` object.

Where appropriate, we will describe how view lookup interacts with
:term:`resource location`.  The :ref:`view_config_chapter` chapter describes
the details of :term:`view lookup`.

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` using a :term:`context`
resource generated by the route match.

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

Technically, URL dispatch is a :term:`resource location` mechanism (it finds
a context object).  But ironically, using URL dispatch (instead of
:term:`traversal`) allows you to avoid thinking about your application in
terms of "resources" entirely, because it allows you to directly map a
:term:`view callable` to a route.

Route Configuration
-------------------

:term:`Route configuration` is the act of adding a new :term:`route` to an
application.  A route has a *pattern*, representing 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``),
and a *route name*, which is used by developers within a :app:`Pyramid`
application to uniquely identify a particular route when generating a URL.
It also optionally has a ``factory``, a set of :term:`route predicate`
parameters, and a set of :term:`view` parameters.

.. index::
   single: add_route

Configuring a Route via The ``add_route`` Configurator Method
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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

.. ignore-next-block
.. code-block:: python

   # "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}', view=myview)

.. versionchanged:: 1.0a4
    Prior to 1.0a4, routes allow for a marker starting with a ``:``, for
    example ``/prefix/:one/:two``.  This style is now deprecated
    in favor or ``{}`` usage which allows for additional functionality.

.. index::
   single: route configuration; view callable

Route Configuration That Names a View Callable
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When a route configuration declaration names a ``view`` attribute, the value
of the attribute will reference a :term:`view callable`.  This view callable
will be invoked when the route matches.  A view callable, as described in
:ref:`view_chapter`, is developer-supplied code that "does stuff" as the
result of a request.  For more information about how to create view
callables, see :ref:`views_chapter`.

Here's an example route configuration that references a view callable:

.. code-block:: python
   :linenos:

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

You can also pass a :term:`dotted Python name` as the ``view`` argument
rather than an actual callable:

.. code-block:: python
   :linenos:

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

When a route configuration names a ``view`` attribute, the :term:`view
callable` named as that ``view`` attribute will always be found and invoked
when the associated route pattern matches during a request.

Route View Callable Registration and Lookup Details
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

The purpose of making it possible to specify a view callable within a route
configuration is to prevent developers from needing to deeply understand the
details of :term:`resource location` and :term:`view lookup`.  When a route
names a view callable as a ``view`` argument, and a request enters the system
which matches the pattern of the route, the 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`` attribute is attached to 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 a route configuration declaration mentions a ``view`` attribute, a
  :term:`view configuration` is registered at startup time.  This view
  configuration uses the 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 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.

In this way, we supply a shortcut to the developer.  Under the hood, the
:term:`resource location` and :term:`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.  It also means that we
can allow a developer to combine :term:`URL dispatch` and :term:`traversal`
in various exceptional cases as documented in :ref:`hybrid_chapter`.

.. index::
   single: route path pattern syntax

.. _route_pattern_syntax:

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:

.. code-block:: text

   {foo}/bar/baz

and:

.. code-block:: text

   /{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 non-alphanumeric character and use this as the
``name`` matchdict value."  For example, the following pattern defines one
literal segment ("foo") and two dynamic replacement markers ("baz", and
"bar"):

.. code-block:: text

   foo/{baz}/{bar}

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

.. code-block:: text

   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:

.. code-block:: text

   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:

.. code-block:: text

   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:

.. code-block:: text
    
    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 the replacement marker ``{name}`` has a literal part of ``.``
(period) between the other replacement marker ``{ext}``.

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 path segments matched with a ``{segment}``
match will be url-unquoted and decoded from UTF-8 into Unicode within the
matchdict.  So for instance, the following pattern:

.. code-block:: text

   foo/{bar}

When matching the following URL:

.. code-block:: text

   foo/La%20Pe%C3%B1a

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

.. code-block:: text

   {'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:

.. code-block:: text

   foo/{baz}/{bar}*fizzle

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

.. code-block:: text

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

.. code-block:: text

   foo/*fizzle

When matching the following path:

.. code-block:: text

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

Will generate the following matchdict:

.. code-block:: text

   {'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:

.. code-block:: text
    
    foo/{baz}/{bar}{fizzle:.*}

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

.. code-block:: text

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

.. index::
   single: route ordering

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
:term:`traversal`, which depends on emergent behavior which happens as a
result of traversing a resource tree.

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:

.. code-block:: text

   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.

.. index::
   single: route factory

.. _route_factories:

Route Factories
~~~~~~~~~~~~~~~

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`.

.. code-block:: python
   :linenos:

   config.add_route('abc', '/abc', view='myproject.views.theview', 
                    factory='myproject.resources.root_factory')

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.

Supplying a different resource factory 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`.

Route Configuration Arguments
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Route configuration ``add_route`` statements may specify a large number of
arguments.

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.

Other arguments are view configuration related arguments.  These only have an
effect when the route configuration names a ``view``.

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

**Non-Predicate Arguments**

``name``
  The name of the route, e.g. ``myroute``.  This attribute is required.  It
  must be unique among all defined routes in a given application.

``factory``
  A Python object (often a function or a class) or a :term:`dotted Python
  name` to such an object that will generate a :app:`Pyramid` resource object
  as the :term:`root` when this route matches. For example,
  ``mypackage.resources.MyFactoryClass``.  If this argument is not specified,
  the traversal root factory will be used.

``traverse``
  If you would like to cause the :term:`context` resource to be something
  other than the :term:`root` resource object when this route matches, you
  can spell a traversal pattern as the ``traverse`` argument.  This traversal
  pattern will be used as the traversal path: traversal will begin at the
  root object implied by this route (either the global root, or the object
  returned by the ``factory`` associated with this route).

  The syntax of the ``traverse`` argument is the same as it is for
  ``pattern``. For example, if the ``pattern`` provided is
  ``articles/{article}/edit``, and the ``traverse`` argument provided is
  ``/{article}``, when a request comes in that causes the route to match in
  such a way that the ``article`` match value is '1' (when the request URI is
  ``/articles/1/edit``), the traversal path will be generated as ``/1``.
  This means that the root object's ``__getitem__`` will be called with the
  name ``1`` during the traversal phase.  If the ``1`` object exists, it will
  become the :term:`context` resource of the request.
  :ref:`traversal_chapter` has more information about traversal.

  If the traversal path contains segment marker names which are not present
  in the ``pattern`` argument, a runtime error will occur.  The ``traverse``
  pattern should not contain segment markers that do not exist in the
  ``pattern``.

  A similar combining of routing and traversal is available when a route is
  matched which contains a ``*traverse`` remainder marker in its pattern (see
  :ref:`using_traverse_in_a_route_pattern`).  The ``traverse`` argument
  allows you to associate route patterns with an arbitrary traversal path
  without using a a ``*traverse`` remainder marker; instead you can use other
  match information.

  Note that the ``traverse`` argument is ignored when attached to a route
  that has a ``*traverse`` remainder marker in its pattern.

**Predicate Arguments**

``pattern``
  The path of the route e.g. ``ideas/{idea}``.  This argument is required.
  See :ref:`route_path_pattern_syntax` for information about the syntax of
  route paths.  If the path doesn't match the current URL, route matching
  continues.

  .. note:: In earlier releases of this framework, this argument existed
     as ``path``.  ``path`` continues to work as an alias for
     ``pattern``.
  
``xhr``
  This value should be either ``True`` or ``False``.  If this value is
  specified and is ``True``, the :term:`request` must possess an
  ``HTTP_X_REQUESTED_WITH`` (aka ``X-Requested-With``) header for this route
  to match.  This is useful for detecting AJAX requests issued from jQuery,
  Prototype and other Javascript libraries.  If this predicate returns
  ``False``, route matching continues.

``request_method``
  A string representing an HTTP method name, e.g. ``GET``, ``POST``,
  ``HEAD``, ``DELETE``, ``PUT``.  If this argument is not specified, this
  route will match if the request has *any* request method.  If this
  predicate returns ``False``, route matching continues.

``path_info``
  This value represents a regular expression pattern that will be tested
  against the ``PATH_INFO`` WSGI environment variable.  If the regex matches,
  this predicate will return ``True``.  If this predicate returns ``False``,
  route matching continues.

``request_param``
  This value can be any string.  A view declaration with this argument
  ensures that the associated route will only match when the request has a
  key in the ``request.params`` dictionary (an HTTP ``GET`` or ``POST``
  variable) that has a name which matches the supplied value.  If the value
  supplied as the argument has a ``=`` sign in it,
  e.g. ``request_params="foo=123"``, then the key (``foo``) must both exist
  in the ``request.params`` dictionary, and the value must match the right
  hand side of the expression (``123``) for the route to "match" the current
  request.  If this predicate returns ``False``, route matching continues.

``header``
  This argument represents an HTTP header name or a header name/value pair.
  If the argument contains a ``:`` (colon), it will be considered a
  name/value pair (e.g. ``User-Agent:Mozilla/.*`` or ``Host:localhost``).  If
  the value contains a colon, the value portion should be a regular
  expression.  If the value does not contain a colon, the entire value will
  be considered to be the header name (e.g. ``If-Modified-Since``).  If the
  value evaluates to a header name only without a value, the header specified
  by the name must be present in the request for this predicate to be true.
  If the value evaluates to a header name/value pair, the header specified by
  the name must be present in the request *and* the regular expression
  specified as the value must match the header value.  Whether or not the
  value represents a header name or a header name/value pair, the case of the
  header name is not significant.  If this predicate returns ``False``, route
  matching continues.

``accept``
  This value represents a match query for one or more mimetypes in the
  ``Accept`` HTTP request header.  If this value is specified, it must be in
  one of the following forms: a mimetype match token in the form
  ``text/plain``, a wildcard mimetype match token in the form ``text/*`` or a
  match-all wildcard mimetype match token in the form ``*/*``.  If any of the
  forms matches the ``Accept`` header of the request, this predicate will be
  true.  If this predicate returns ``False``, route matching continues.

``custom_predicates``
  This value should be a sequence of references to custom predicate
  callables.  Use custom predicates when no set of predefined predicates does
  what you need.  Custom predicates can be combined with predefined
  predicates as necessary.  Each custom predicate callable should accept two
  arguments: ``info`` and ``request`` and should return either ``True`` or
  ``False`` after doing arbitrary evaluation of the context resource and/or
  the request.  If all callables return ``True``, the associated route will
  be considered viable for a given request.  If any custom predicate returns
  ``False``, route matching continues.  See :ref:`custom_route_predicates`
  for more information.

**View-Related Arguments**

``view``
  A Python object or a :term:`dotted Python name` to such an object that will
  be used as a view callable when this route
  matches. e.g. ``mypackage.views.my_view``.
  
``view_context``
  A class or an :term:`interface` (or a :term:`dotted Python name` to such an
  object) that the :term:`context` resource should possess for the view named
  by the route to be used.  If this attribute is not specified, the default
  (``None``) will be used.

  If the ``view`` argument is not provided, this argument has no effect.

  This attribute can also be spelled as ``for_`` or ``view_for``.

``view_permission``
  The permission name required to invoke the view associated with this route.
  e.g. ``edit``. (see :ref:`using_security_with_urldispatch` for more
  information about permissions).

  If the ``view`` attribute is not provided, this argument has no effect.

  This argument can also be spelled as ``permission``.

``view_renderer``
  This is either a single string term (e.g. ``json``) or a string implying a
  path or :term:`asset specification` (e.g. ``templates/views.pt``).  If the
  renderer value is a single term (does not contain a dot ``.``), the
  specified term will be used to look up a renderer implementation, and that
  renderer implementation will be used to construct a response from the view
  return value.  If the renderer term contains a dot (``.``), the specified
  term will be treated as a path, and the filename extension of the last
  element in the path will be used to look up the renderer implementation,
  which will be passed the full path.  The renderer implementation will be
  used to construct a response from the view return value.  See
  :ref:`views_which_use_a_renderer` for more information.

  If the ``view`` argument is not provided, this argument has no effect.

  This argument can also be spelled as ``renderer``.

``view_attr``
  The view machinery defaults to using the ``__call__`` method of the view
  callable (or the function itself, if the view callable is a function) to
  obtain a response dictionary.  The ``attr`` value allows you to vary the
  method attribute used to obtain the response.  For example, if your view
  was a class, and the class has a method named ``index`` and you wanted to
  use this method instead of the class' ``__call__`` method to return the
  response, you'd say ``attr="index"`` in the view configuration for the
  view.  This is most useful when the view definition is a class.

  If the ``view`` argument is not provided, this argument has no
  effect.

``use_global_views``
  When a request matches this route, and view lookup cannot find a view which
  has a 'route_name' predicate argument that matches the route, try to fall
  back to using a view that otherwise matches the context and request.

.. _custom_route_predicates:

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:

.. code-block:: python
   :linenos:

   def any_of(segment_name, *allowed):
       def predicate(info, request):
           if info['match'][segment_name] in allowed:
               return True
       return predicate

   num_one_two_or_three = any_of('num', 'one', 'two', 'three')

   config.add_route('num', '/{num}', 
                    custom_predicates=(num_one_two_or_three,))

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:

.. code-block:: python
   :linenos:

    def integers(*segment_names):
        def predicate(info, request):
            match = info['match']
            for segment_name in segment_names:
                try:
                    match[segment_name] = int(match[segment_name])
                except (TypeError, ValueError):
                    pass
            return True
        return predicate

    ymd_to_int = integers('year', 'month', 'day')

    config.add_route('num', '/{year}/{month}/{day}', 
                     custom_predicates=(ymd_to_int,))

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:

.. code-block:: python
   :linenos:

    def integers(*segment_names):
        def predicate(info, request):
            match = info['match']
            for segment_name in segment_names:
                match[segment_name] = int(match[segment_name])
            return True
        return predicate

    ymd_to_int = integers('year', 'month', 'day')

    config.add_route('num', '/{year:\d+}/{month:\d+}/{day:\d+}', 
                     custom_predicates=(ymd_to_int,))

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:

.. code-block:: python
   :linenos:

    def twenty_ten(info, request):
        if info['route'].name in ('ymd', 'ym', 'y'):
            return info['match']['year'] == '2010'

    config.add_route('y', '/{year}', custom_predicates=(twenty_ten,))
    config.add_route('ym', '/{year}/{month}', custom_predicates=(twenty_ten,))
    config.add_route('ymd', '/{year}/{month}/{day}', 
                     custom_predicates=(twenty_ten,))

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'.

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

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.

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 ``PATH_INFO`` against the pattern declared.

If any route matches, the route matching process stops.  The :term:`request`
is decorated with a special :term:`interface` which describes it as a "route
request", the :term:`context` resource is generated, and the context and the
resulting request are handed off to :term:`view lookup`.  During view lookup,
if any ``view`` argument was provided within the matched route configuration,
the :term:`view callable` it points to is called.

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.

If any predicate in the set of :term:`route predicate` arguments provided to
a route configuration returns ``False``, that route is skipped and route
matching continues through the ordered set of routes.

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`.

.. index::
   single: matchdict

.. _matchdict:

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``.

.. index::
   single: matched_route

.. _matched_route:

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.

.. _urldispatch_example1:

Example 1
~~~~~~~~~

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

.. code-block:: python
   :linenos:

    config.add_route('idea', 'site/{id}', view='mypackage.views.site_view')

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:

.. code-block:: python
   :linenos:

   from pyramid.response import Response

   def site_view(request):
       return Response(request.matchdict['id'])

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:

.. code-block:: python
   :linenos:

   config.add_route('idea', 'ideas/{idea}', view='mypackage.views.idea_view')
   config.add_route('user', 'users/{user}', view='mypackage.views.user_view')
   config.add_route('tag', 'tags/{tags}', view='mypackage.views.tag_view')

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

.. code-block:: text

   /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:

.. code-block:: python
   :linenos:

   config.add_route('idea', 'ideas/{idea}', 
                    view='myproject.views.idea_view',
                    factory='myproject.resources.Idea')

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:

.. code-block:: python
   :linenos:

   class Idea(object):
       def __init__(self, request):
           pass

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

Example 4
~~~~~~~~~

It is possible to create a route declaration without a ``view`` attribute,
but associate the route with a :term:`view callable` using a ``view``
declaration.

.. code-block:: python
   :linenos:

   config.add_route('idea', 'site/{id}')
   config.add_view(route_name='idea', view='mypackage.views.site_view')

This set of configuration parameters creates a configuration completely
equivalent to this example provided in :ref:`urldispatch_example1`:

.. code-block:: python
   :linenos:

   config.add_route('idea', 'site/{id}', view='mypackage.views.site_view')

In fact, the spelling which names a ``view`` attribute is just syntactic
sugar for the more verbose spelling which contains separate view and route
registrations.

More uses for this style of associating views with routes are explored in
:ref:`hybrid_chapter`.

.. index::
   single: matching the root URL
   single: root url (matching)

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`:

.. code-block:: python
   :linenos:

   config.add_route('root', '', view='mypackage.views.root_view')

Or provide the literal string ``/`` as the pattern:

.. code-block:: python
   :linenos:

   config.add_route('root', '/', view='mypackage.views.root_view')

.. index::
   single: generating route URLs
   single: route URLs

Generating Route URLs
---------------------

Use the :func:`pyramid.url.route_url` function 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.

.. ignore-next-block
.. code-block:: python
   :linenos:

   from pyramid.url import route_url
   url = route_url('foo', request, a='1', b='2', c='3')

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 :func:`pyramid.url.route_url` API documentation for more information.

.. index::
   single: redirecting to slash-appended routes

.. _redirecting_to_slash_appended_routes:

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:

.. code-block:: python
   :linenos:

   config.add_route('noslash', 'no_slash', view='myproject.views.no_slash')
   config.add_route('hasslash', 'has_slash/', view='myproject.views.has_slash')

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:

.. code-block:: python
   :linenos:

   config.add_view(context='pyramid.exceptions.NotFound',
                   view='pyramid.view.append_slash_notfound_view')

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:

.. code-block:: python
     :linenos:

     from pyramid.exceptions import NotFound
     from pyramid.view import AppendSlashNotFoundViewFactory

     def notfound_view(context, request):
         return HTTPNotFound('It aint there, stop trying!')

     custom_append_slash = AppendSlashNotFoundViewFactory(notfound_view)
     config.add_view(custom_append_slash, context=NotFound)

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

.. _cleaning_up_after_a_request:

Cleaning Up After a Request
---------------------------

Sometimes it's required that some cleanup be performed at the end of a
request when a database connection is involved.  When :term:`traversal` is
used, this cleanup is often done as a side effect of the traversal
:term:`root factory`.  Often the root factory will insert an object into the
WSGI environment that performs some cleanup when its ``__del__`` method is
called.  When URL dispatch is used, however, no special root factory is
required, so sometimes that option is not open to you.

Instead of putting this cleanup logic in the root factory, however, you can
cause a subscriber to be fired when a new request is detected; the subscriber
can do this work.

For example, let's say you have a ``mypackage`` :app:`Pyramid` application
package that uses SQLAlchemy, and you'd like the current SQLAlchemy database
session to be removed after each request.  Put the following in the
``mypackage.__init__`` module:

.. ignore-next-block
.. code-block:: python
   :linenos:

    from mypackage.sql import DBSession

    class Cleanup:
        def __init__(self, cleaner):
            self.cleaner = cleaner
        def __del__(self):
            self.cleaner()

    def handle_teardown(event):
        environ = event.request.environ
        environ['mypackage.sqlcleaner'] = Cleanup(DBSession.remove)

Then add an event subscriber in your startup configuration:

.. code-block:: python
   :linenos:

   config.add_subscriber('mypackage.handle_teardown',
                         'pyramid.events.NewRequest')

Registering a handle_teardown subscriber will cause the DBSession to be
removed whenever the WSGI environment is destroyed (usually at the end of
every request).

.. note:: This is only an example.  In particular, it is not necessary
   to cause ``DBSession.remove`` to be called as the result of an
   event listener in an application generated from any
   :app:`Pyramid` paster template, because these all use the
   ``repoze.tm2`` middleware.  The cleanup done by
   ``DBSession.remove`` is unnecessary when ``repoze.tm2`` middleware
   is in the WSGI pipeline.

.. index::
   pair: URL dispatch; security

.. _using_security_with_urldispatch:

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:

.. code-block:: python
   :linenos:

   class Article(object):
       def __init__(self, request):
          matchdict = request.matchdict
          article = matchdict.get('article', None)
          if article == '1':
              self.__acl__ = [ (Allow, 'editor', 'view') ]

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.

.. _debug_routematch_section:

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
``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:

.. code-block:: text
   :linenos:

    [chrism@thinko pylonsbasic]$ PYRAMID_DEBUG_ROUTEMATCH=true \
                                 bin/paster serve development.ini 
    Starting server in PID 13586.
    serving on 0.0.0.0:6543 view at http://127.0.0.1:6543
    2010-12-16 14:45:19,956 no route matched for url \
                                        http://localhost:6543/wontmatch
    2010-12-16 14:45:20,010 no route matched for url \
                                http://localhost:6543/favicon.ico
    2010-12-16 14:41:52,084 route matched for url \
                                http://localhost:6543/static/logo.png; \
                                route_name: 'static/', ....

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

.. index::
   pair: routes; printing
   single: paster proutes

Displaying All Application Routes
---------------------------------

You can use the ``paster proutes`` command in a terminal window to print a
summary of routes related to your application.  Much like the ``paster
pshell`` command (see :ref:`interactive shell`), the ``paster proutes``
command accepts two arguments.  The first argument to ``proutes`` is the path
to your application's ``.ini`` file.  The second is the ``app`` section name
inside the ``.ini`` file which points to your application.

For example:

.. code-block:: text
   :linenos:

   [chrism@thinko MyProject]$ ../bin/paster proutes development.ini MyProject
   Name            Pattern                        View
   ----            -------                        ----                     
   home            /                              <function my_view>
   home2           /                              <function my_view>
   another         /another                       None                     
   static/         static/*subpath                <static_view object>
   catchall        /*subpath                      <function static_view>

``paster proutes`` generates a table.  The table has three columns: a Name
name column, a Pattern column, and a View column.  The items listed in the
Name column are route names, the items listen in the Pattern column are route
patterns, and the items listed in the View column are representations of the
view callable that will be invoked when a request matches the associated
route pattern.  The view column may show ``None`` if no associated view
callable could be found.  If no routes are configured within your
application, nothing will be printed to the console when ``paster proutes``
is executed.

References
----------

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