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pyramid_handlers

Overview

pyramid_handlers is a package which allows Pyramid to largely emulate the functionality of Pylons “controllers”. Handlers are a synthesis of Pyramid url dispatch and method introspection of a view class that makes it easier to create bundles of view logic which reacts to particular route patterns.

pyramid_handlers works under Python 2.6 and 2.7. It also works under Python 3.2, but ZCML support is not available under Python 3.2.

Installation

Install using setuptools, e.g. (within a virtualenv):

$ easy_install pyramid_handlers

Setup

Once pyramid_handlers is installed, you must use the config.include mechanism to include it into your Pyramid project’s configuration. In your Pyramid project’s __init__.py:

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config = Configurator(.....)
config.include('pyramid_handlers')

At this point, it will be possible to use the pyramid_handlers.add_handler() function as a method of the configurator, ala:

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config.add_handler(....)

Handler Registration Using add_handler()

pyramid_handlers provides the special concept of a view handler. View handlers are view classes that implement a number of methods, each of which is a view callable as a convenience for URL dispatch users.

Note

View handlers are not useful when using traversal, only when using url dispatch.

Using a view handler instead of a plain function or class view callable makes it unnecessary to call pyramid.config.Configurator.add_route() (and/or pyramid.config.Configurator.add_view()) “by hand” multiple times, making it more pleasant to register a collection of views as a single class when using url dispatch. The view handler machinery also introduces the concept of an action, which is used as a view predicate to control which method of the handler is called. The method name is the default action name of a handler view callable.

The concept of a view handler is analogous to a “controller” in Pylons 1.0.

The view handler class is initialized by Pyramid in the same manner as a “plain” view class. Its __init__ is called with a request object (see class_as_view). It implements methods, each of which is a view callable. When a request enters the system which corresponds with an action related to one of its view callable methods, this method is called, and it is expected to return a response.

Here’s an example view handler class:

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from pyramid_handlers import action

from pyramid.response import Response

class Hello(object):
    def __init__(self, request):
        self.request = request

    def index(self):
        return Response('Hello world!')

    @action(renderer="mytemplate.mak")
    def bye(self):
        return {}

The pyramid_handlers.action decorator is used to fine-tune the view parameters for each potential view callable which is a method of the handler.

Handlers are added to application configuration via the pyramid_handlers.add_handler() API, which is accessible after configuration as the method pyramid.config.Configurator.add_handler. This function will scan a view handler class and automatically set up view configurations for its methods that represent “auto-exposed” view callable, or those that were decorated explicitly with the action decorator. This decorator is used to setup additional view configuration information for individual methods of the class, and can be used repeatedly for a single view method to register multiple view configurations for it.

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from myapp.handlers import Hello
config.add_handler('hello', '/hello/{action}', handler=Hello)

This example will result in a route being added for the pattern /hello/{action}, and each method of the Hello class will then be examined to see if it should be registered as a potential view callable when the /hello/{action} pattern matches. The value of {action} in the route pattern will be used to determine which view should be called, and each view in the class will be setup with a view predicate that requires a specific action name. By default, the action name for a method of a handler is the method name.

If the URL was /hello/index, the above example pattern would match, and, by default, the index method of the Hello class would be called.

Alternatively, the action can be declared specifically for a URL to be registered for a specific action name:

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from myapp.handlers import Hello
config.add_handler('hello_index', '/hello/index',
                   handler=Hello, action='index')

This will result one of the methods that are configured for the action of ‘index’ in the Hello handler class to be called. In this case the name of the method is the same as the action name: index. However, this need not be the case, as we will see below.

When calling pyramid_handlers.add_handler(), an action is required in either the route pattern or as a keyword argument, but cannot appear in both places. A handler argument must also be supplied, which can be either a asset specification or a Python reference to the handler class. Additional keyword arguments are passed directly through to pyramid.config.Configurator.add_route().

For example:

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config.add_handler('hello', '/hello/{action}',
                   handler='mypackage.handlers.MyHandler')

Multiple add_handler() calls can specify the same handler, to register specific route names for different handler/action combinations. For example:

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config.add_handler('hello_index', '/hello/index',
                   handler=Hello, action='index')
config.add_handler('bye_index', '/hello/bye',
                   handler=Hello, action='bye')

Note

Handler configuration may also be added to the system via ZCML (see Configuring a Handler via ZCML).

View Setup in the Handler Class

A handler class can have a single class level attribute called __autoexpose__ which should be a regular expression or the value None. It’s used to determine which method names will result in additional view configurations being registered.

When pyramid_handlers.add_handler() runs, every method in the handler class will be searched and a view registered if the method name matches the __autoexpose__ regular expression, or if the method was decorated with action.

Every method in the handler class that has a name meeting the __autoexpose__ regular expression will have a view registered for an action name corresponding to the method name. This functionality can be disabled by setting the __autoexpose__ attribute to None:

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from pyramid_handlers import action

class Hello(object):
    __autoexpose__ = None

    def __init__(self, request):
        self.request = request

    @action()
    def index(self):
        return Response('Hello world!')

    @action(renderer="mytemplate.mak")
    def bye(self):
        return {}

With auto-expose effectively disabled, no views will be registered for a method unless it is specifically decorated with action.

Action Decorators in a Handler

The action decorator registers view configuration information on the handler method, which is used by add_handler() to setup the view configuration.

All keyword arguments are recorded, and passed to add_view(). Any valid keyword arguments for add_view() can thus be used with the action decorator to further restrict when the view will be called.

One important difference is that a handler method can respond to an action name that is different from the method name by passing in a name argument.

Example:

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from pyramid_handlers import action

class Hello(object):
    def __init__(self, request):
        self.request = request

    @action(name='index', renderer='created.mak', request_method='POST')
    def create(self):
        return {}

    @action(renderer="view_all.mak", request_method='GET')
    def index(self):
        return {}

This will register two views that require the action to be index, with the additional view predicate requiring a specific request method.

It can be useful to decorate a single method multiple times with action. Each action decorator will register a new view for the method. By specifying different names and renderers for each action, the same view logic can be exposed and rendered differently on multiple URLs.

Example:

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from pyramid_handlers import action

class Hello(object):
    def __init__(self, request):
        self.request = request

    @action(name='home', renderer='home.mak')
    @action(name='about', renderer='about.mak')
    def show_template(self):
        # prep some template vars
        return {}

# in the config
config.add_handler('hello', '/hello/{action}', handler=Hello)

With this configuration, the url /hello/home will find a view configuration that results in calling the show_template method, then rendering the template with home.mak, and the url /hello/about will call the same method and render the about.mak template.

Handler __action_decorator__ Attribute

Note

In a Pylons 1.0 controller, it was possible to override the __call__() method, which allowed a developer to “wrap” the entire action invocation, with a try/except or any other arbitrary code. In Pyramid, this can be emulated with the use of an __action_decorator__ classmethod on your handler class.

If a handler class has an __action_decorator__ attribute, then the value of the class attribute will be passed in as the decorator argument every time a handler action is registered as a view callable. This means that, like anything passed to add_view() as the decorator argument, __action_decorator__ must be a callable accepting a single argument. This argument will itself be a callable accepting (context, request) arguments, and __action_decorator__ must return a replacement callable with the same call signature.

Note that, since handler actions are registered as views against the handler class and not a handler instance, any __action_decorator__ attribute must not be a regular instance method. Defining an __action_decorator__ instance method on a handler class will result in a ConfigurationError. Instead, __action_decorator__ can be any other type of callable: a staticmethod, classmethod, function, or some sort of callable instance.

The below example uses an __action_decorator__ which is a staticmethod of the handler class. It wraps every view callable implied by the handler in a decorator function which calls the original view callable, but catches a special exception and returns a response.

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from pyramid_handlers import action
from pyramid.response import Response

class MySpecialException(Exception):
    pass

class MyHandler(object):
    def __init__(self, request):
        self.request = request

    @staticmethod
    def __action_decorator__(view):
        def decorated_view(context, request):
            try:
                return view(context, request)
            except MySpecialException:
                return Response('Something bad happened', status=500)
        return decorated_view

    @action(renderer='index.html')
    def index(self):
        raise MySpecialException

When the index method of the above example handler is invoked, it will raise MySpecialException. As a result, the action decorator will cath this exception and turn it into a response.

Configuration Knobs

If your handler action methods that have characters in them (such as underscores) that you don’t find appropriate in a URL, such as a_method_with_underscores:

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# in a module named mypackage.handlers

from pyramid_handlers import action

class AHandler(object):
    def __init__(self, request):
        self.request = request

    @action(renderer='some/renderer.pt')
    def a_method_with_underscores(self):
        return {}

And there is some regular transform you can perform against all action method registrations (such as converting the underscores to dashes), you can define a “method name transformer”:

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# in the same module named mypackage.handlers

def transformer(method_name):
    return method_name.replace('_', '-')

You can then use the method name transformer in your Pyramid settings via the .ini` file:

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[app:myapp]
...
pyramid_handlers.method_name_xformer = mypackage.handlers.transformer

Or directly in your main() function:

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# in a module named mypackage.handlers

from mypackage.handlers import transformer

def main(global_conf, *settings):
    settings['pyramid_handlers.method_name_xformer'] = transformer
    config = Configurator(settings=settings)
    # .. rest of configuration ...

Once you’ve set up a method name transformer, any {action} substitution in the pattern associated with a handler will be matched against the transformed method name value instead of the untransformed method name value:

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# in a module named mypackage.handlers

from mypackage.handlers import transformer
from mypackage.handlers import AHandler

def main(global_conf, *settings):
    settings['pyramid_handlers.method_name_xformer'] = transformer
    config = Configurator(settings=settings)
    config.add_handler('ahandler', '/ahandler/{action}', handler=AHandler)
    # .. rest of configuration ...

Now, when /ahandler/a-method-with-underscores is visited, it will invoke the AHandler.a_method_with_underscores method. Note that /ahandler/a_method_with_underscores will however no longer work to invoke the method.

Reporting Bugs / Development Versions

Visit http://github.com/Pylons/pyramid_handlers to download development or tagged versions.

Visit http://github.com/Pylons/pyramid_handlers/issues to report bugs.