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The Main Function

Both Pyramid and Pylons have a top-level function that returns a WSGI application. The Pyramid function is main in pyramidapp/__init__.py. The Pylons function is make_app in pylonsapp/config/middleware.py. Here’s the main function generated by Pyramid’s ‘starter’ scaffold:

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from pyramid.config import Configurator

def main(global_config, **settings):
    """ This function returns a Pyramid WSGI application.
    """
    config = Configurator(settings=settings)
    config.add_static_view('static', 'static', cache_max_age=3600)
    config.add_route('home', '/')
    config.scan()
    return config.make_wsgi_app()

Pyramid has less boilerplate code than Pylons, so the main function subsumes Pylons’ middleware.py, environment.py, and routing.py modules. Pyramid’s configuration code is just 5 lines long in the default application, while Pylons’ is 35.

Most of the function’s body deals with the Configurator (config). That isn’t the application object; it’s a helper that will instantiate the application for us. You pass in the settings as a dict to the constructor (line 6), call various methods to set up routes and such, and finally call config.make_wsgi_app() to get the application, which the main function returns. The application is an instance of pyramid.router.Router. (A Pylons application is an instance of a PylonsApp subclass.)

Dotted Python names and asset specifications

Several config methods accept either an object (e.g., a module or callable) or a string naming the object. The latter is called a dotted Python name. It’s a dot-delimited string specifying the absolute name of a module or a top-level object in a module: “module”, “package.module”, “package.subpackage.module.attribute”. Passing string names allows you to avoid importing the object merely to pass it to a method.

If the string starts with a leading dot, it’s relative to some parent package. So in this main function defined in mypyramiapp/__init__.py, the parent package is mypyramidapp. So the name ”.views” refers to mypyramidapp/views.py. (Note: in some cases it can sometimes be tricky to guess what Pyramid thinks the parent package is.)

Closely associated with this is a static asset specification, which names a non-Python file or directory inside a Python package. A colon separates the package name from the non-Python subpath: “myapp:templates/mytemplate.pt”, “myapp:static”, “myapp:assets/subdir1”. If you leave off the first part and the colon (e.g., “templates/mytemplate.pt”, it’s relative to some current package.

An alternative syntax exists, with a colon between a module and an attribute: “package.module:attribute”. This usage is discouraged; it exists for compatibility with Setuptools’ resource syntax.

Configurator methods

The Configurator has several methods to customize the application. Below are the ones most commonly used in Pylons-like applications, in order by how widely they’re used. The full list of methods is in Pyramid’s Configurator API.

add_route(...)

Register a route for URL dispatch.

add_view(...)

Register a view. Views are equivalent to Pylons’ controller actions.

scan(...)

A wrapper for registering views and certain other things. Discussed in the views chapter.

add_static_view(...)

Add a special view that publishes a directory of static files. This is somewhat akin to Pylons’ public directory, but see the static fiels chapter for caveats.

include(callable, route_prefix=None)

Allow a function to customize the configuration further. This is a wide-open interface which has become very popular in Pyramid. It has three main use cases:

  • To group related code together; e.g., to define your routes in a separate module.
  • To initialize a third-party add-on. Many add-ons provide an include function that performs all the initialization steps for you.
  • To mount a subapplication at a URL prefix. A subapplication is just any bundle of routes, views and templates that work together. You can use this to split your application into logical units. Or you can write generic subapplications that can be used in several applications, or mount a third-party subapplication.

If the add-on or subapplication has options, it will typically read them from the settings, looking for settings with a certain prefix and converting strings to their proper type. For instance, a session manager may look for keys starting with “session.” or “thesessionmanager.” as in “session.type”. Consult the add-on’s documentation to see what prefix it uses and which options it recognizes.

The callable argument should be a function, a module, or a dotted Python name. If it resolves to a module, the module should contain an includeme function which will be called. The following are equivalent:

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config.include("pyramid_beaker")

import pyramid_beaker
config.include(pyramid_beaker)

import pyramid_beaker
config.include(pyramid_beaker.includeme)

If route_prefix is specified, it should be a string that will be prepended to any URLs generated by the subconfigurator’s add_route method. Caution: the route names must be unique across the main application and all subapplications, and route_prefix does not touch the names. So you’ll want to name your routes “subapp1.route1” or “subapp1_route1” or such.

add_subscriber(subscriber, iface=None)

Insert a callback into Pyramid’s event loop to customize how it processes requests. The Renderers chapter has an example of its use.

add_renderer(name, factory)

Add a custom renderer. An example is in the Renderers chapter.

set_authentication_policy, set_authorization_policy, set_default_permission

Configure Pyramid’s built-in authorization mechanism.

Other methods sometimes used: add_notfound_view, add_exception_view, set_request_factory, add_tween, override_asset (used in theming). Add-ons can define additional config methods by calling config.add_directive.

Route arguments

config.add_route accepts a large number of keyword arguments. They are logically divided into predicate argumets and non-predicate arguments. Predicate arguments determine whether the route matches the current request. All predicates must succeed in order for the route to be chosen. Non-predicate arguments do not affect whether the route matches.

name

[Non-predicate] The first positional arg; required. This must be a unique name for the route. The name is used to identify the route when registering views or generating URLs.

pattern

[Predicate] The second positional arg; required. This is the URL path with optional “{variable}” placeholders; e.g., “/articles/{id}” or “/abc/{filename}.html”. The leading slash is optional. By default the placeholder matches all characters up to a slash, but you can specify a regex to make it match less (e.g., “{variable:d+}” for a numeric variable) or more (“{variable:.*}” to match the entire rest of the URL including slashes). The substrings matched by the placeholders will be available as request.matchdict in the view.

A wildcard syntax “*varname” matches the rest of the URL and puts it into the matchdict as a tuple of segments instead of a single string. So a pattern “/foo/{action}/*fizzle” would match a URL “/foo/edit/a/1” and produce a matchdict {'action': u'edit', 'fizzle': (u'a', u'1')}.

Two special wildcards exist, “*traverse” and “*subpath”. These are used in advanced cases to do traversal on the remainder of the URL.

XXX Should use raw string syntax for regexes with backslashes (d) ?

request_method

[Predicate] An HTTP method: “GET”, “POST”, “HEAD”, “DELETE”, “PUT”. Only requests of this type will match the route.

request_param

[Predicate] If the value doesn’t contain “=” (e.g., “q”), the request must have the specified parameter (a GET or POST variable). If it does contain “=” (e.g., “name=value”), the parameter must also have the specified value.

This is especially useful when tunnelling other HTTP methods via POST. Web browsers can’t submit a PUT or DELETE method via a form, so it’s customary to use POST and to set a parameter _method="PUT". The framework or application sees the “_method” parameter and pretends the other HTTP method was requested. In Pyramid you can do this with request_param="_method=PUT.

xhr

[Predicate] True if the request must have an “X-Requested-With” header. Some Javascript libraries (JQuery, Prototype, etc) set this header in AJAX requests to distinguish them from user-initiated browser requests.

custom_predicates

[Predicate] A sequence of callables which will be called to determine whether the route matches the request. Use this feature if none of the other predicate arguments do what you need. The request will match the route only if all callables return True. Each callable will receive two arguments, info and request. request is the current request. info is a dict containing the following:

info["match"]  =>  the match dict for the current route
info["route"].name  =>  the name of the current route
info["route"].pattern  =>  the URL pattern of the current route

You can modify the match dict to affect how the view will see it. For instance, you can look up a model object based on its ID and put the object in the match dict under another key. If the record is not found in the model, you can return False.

Other arguments available: accept, factory, header, path_info, traverse.

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