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Defining Views

A :term:`view callable` in a :app:`Pyramid` application is typically a simple Python function that accepts a single parameter named :term:`request`. A view callable is assumed to return a :term:`response` object.

The request object passed to every view that is called as the result of a route match has an attribute named matchdict that contains the elements placed into the URL by the pattern of a route statement. For instance, if a call to :meth:`pyramid.config.Configurator.add_route` in __init__.py had the pattern {one}/{two}, and the URL at http://example.com/foo/bar was invoked, matching this pattern, the matchdict dictionary attached to the request passed to the view would have a 'one' key with the value 'foo' and a 'two' key with the value 'bar'.

The source code for this tutorial stage can be browsed at http://github.com/Pylons/pyramid/tree/1.3-branch/docs/tutorials/wiki2/src/views/.

Declaring Dependencies in Our setup.py File

The view code in our application will depend on a package which is not a dependency of the original "tutorial" application. The original "tutorial" application was generated by the pcreate command; it doesn't know about our custom application requirements.

We need to add a dependency on the docutils package to our tutorial package's setup.py file by assigning this dependency to the requires parameter in setup().

Open tutorial/setup.py and edit it to look like the following:

(Only the highlighted line needs to be added.)

Running setup.py develop

Since a new software dependency was added, you will need to rerun python setup.py develop inside the root of the tutorial package to obtain and register the newly added dependency distribution.

Make sure your current working directory is the root of the project (the directory in which setup.py lives) and execute the following command.

On UNIX:

$ cd tutorial
$ ../bin/python setup.py develop

On Windows:

c:\pyramidtut> cd tutorial
c:\pyramidtut\tutorial> ..\Scripts\python setup.py develop

Success executing this command will end with a line to the console something like:

Finished processing dependencies for tutorial==0.0

Changing the views.py File

It's time for a major change. Open tutorial/tutorial/views.py and edit it to look like the following:

(The highlighted lines are the ones that need to be added or edited.)

We got rid of the my_view view function and its decorator that was added when we originally rendered the alchemy scaffold. It was only an example and isn't relevant to our application.

Then we added four :term:`view callable` functions to our views.py module:

  • view_wiki() - Displays the wiki itself. It will answer on the root URL.
  • view_page() - Displays an individual page.
  • add_page() - Allows the user to add a page.
  • edit_page() - Allows the user to edit a page.

We'll describe each one briefly and show the resulting views.py file afterward.

Note

There is nothing special about the filename views.py. A project may have many view callables throughout its codebase in arbitrarily-named files. Files implementing view callables often have view in their filenames (or may live in a Python subpackage of your application package named views), but this is only by convention.

The view_wiki view function

view_wiki() is the :term:`default view` that gets called when a request is made to the root URL of our wiki. It always redirects to a URL which represents the path to our "FrontPage".

view_wiki() returns an instance of the :class:`pyramid.httpexceptions.HTTPFound` class (instances of which implement the :class:`pyramid.interfaces.IResponse` interface like :class:`pyramid.response.Response` does).

It uses the :meth:`pyramid.request.Request.route_url` API to construct a URL to the FrontPage page (e.g. http://localhost:6543/FrontPage), which is used as the "location" of the HTTPFound response, forming an HTTP redirect.

The view_page view function

view_page() is used to display a single page of our wiki. It renders the :term:`ReStructuredText` body of a page (stored as the data attribute of a Page model object) as HTML. Then it substitutes an HTML anchor for each WikiWord reference in the rendered HTML using a compiled regular expression.

The check() function is used as the first argument to wikiwords.sub, indicating that it should be called to provide a value for each WikiWord match found in the content. If the wiki already contains a page with the matched WikiWord name, check() generates a view link to be used as the substitution value and returns it. If the wiki does not already contain a page with the matched WikiWord name, check() generates an "add" link as the substitution value and returns it.

As a result, the content variable is now a fully formed bit of HTML containing various view and add links for WikiWords based on the content of our current page object.

We then generate an edit URL (because it's easier to do here than in the template), and we return a dictionary with a number of arguments. The fact that view_page() returns a dictionary (as opposed to a :term:`response` object) is a cue to :app:`Pyramid` that it should try to use a :term:`renderer` associated with the view configuration to render a template. In our case, the template which will be rendered will be the templates/view.pt template, as indicated in the @view_config decorator that is applied to view_page().

The add_page view function

add_page() is invoked when a user clicks on a WikiWord which isn't yet represented as a page in the system. The check function within the view_page view generates URLs to this view. add_page() also acts as a handler for the form that is generated when we want to add a page object. The matchdict attribute of the request passed to the add_page() view will have the values we need to construct URLs and find model objects.

The matchdict will have a 'pagename' key that matches the name of the page we'd like to add. If our add view is invoked via, e.g. http://localhost:6543/add_page/SomeName, the value for 'pagename' in the matchdict will be 'SomeName'.

If the view execution is a result of a form submission (i.e. the expression 'form.submitted' in request.params is True), we scrape the page body from the form data, create a Page object with this page body and the name taken from matchdict['pagename'], and save it into the database using DBSession.add. We then redirect back to the view_page view for the newly created page.

If the view execution is not a result of a form submission (i.e. the expression 'form.submitted' in request.params is False), the view callable renders a template. To do so, it generates a "save url" which the template uses as the form post URL during rendering. We're lazy here, so we're going to use the same template (templates/edit.pt) for the add view as well as the page edit view. To do so we create a dummy Page object in order to satisfy the edit form's desire to have some page object exposed as page. :app:`Pyramid` will render the template associated with this view to a response.

The edit_page view function

edit_page() is invoked when a user clicks the "Edit this Page" button on the view form. It renders an edit form but it also acts as the handler for the form it renders. The matchdict attribute of the request passed to the edit_page view will have a 'pagename' key matching the name of the page the user wants to edit.

If the view execution is a result of a form submission (i.e. the expression 'form.submitted' in request.params is True), the view grabs the body element of the request parameters and sets it as the data attribute of the page object. It then redirects to the view_page view of the wiki page.

If the view execution is not a result of a form submission (i.e. the expression 'form.submitted' in request.params is False), the view simply renders the edit form, passing the page object and a save_url which will be used as the action of the generated form.

Adding Templates

The view_page, add_page and edit_page views that we've added reference a :term:`template`. Each template is a :term:`Chameleon` :term:`ZPT` template. These templates will live in the templates directory of our tutorial package. Chameleon templates must have a .pt extension to be recognized as such.

The view.pt Template

Create tutorial/tutorial/templates/view.pt and add the following content:

This template is used by view_page() for displaying a single wiki page. It includes:

  • A div element that is replaced with the content value provided by the view (rows 45-47). content contains HTML, so the structure keyword is used to prevent escaping it (i.e. changing ">" to ">", etc.)
  • A link that points at the "edit" URL which invokes the edit_page view for the page being viewed (rows 49-51).

The edit.pt Template

Create tutorial/tutorial/templates/edit.pt and add the following content:

This template is used by add_page() and edit_page() for adding and editing a wiki page. It displays a page containing a form that includes:

  • A 10 row by 60 column textarea field named body that is filled with any existing page data when it is rendered (rows 46-47).
  • A submit button that has the name form.submitted (row 48).

The form POSTs back to the "save_url" argument supplied by the view (row 45). The view will use the body and form.submitted values.

Note

Our templates use a request object that none of our tutorial views return in their dictionary. request is one of several names that are available "by default" in a template when a template renderer is used. See :ref:`chameleon_template_renderers` for information about other names that are available by default when a Chameleon template is used as a renderer.

Static Assets

Our templates name a single static asset named pylons.css. We don't need to create this file within our package's static directory because it was provided at the time we created the project. This file is a little too long to replicate within the body of this guide, however it is available online.

This CSS file will be accessed via e.g. http://localhost:6543/static/pylons.css by virtue of the call to add_static_view directive we've made in the __init__.py file. Any number and type of static assets can be placed in this directory (or subdirectories) and are just referred to by URL or by using the convenience method static_url e.g. request.static_url('{{package}}:static/foo.css') within templates.

Adding Routes to __init__.py

The __init__.py file contains :meth:`pyramid.config.Configurator.add_route` calls which serve to add routes to our application. First, we’ll get rid of the existing route created by the template using the name 'home'. It’s only an example and isn’t relevant to our application.

We then need to add four calls to add_route. Note that the ordering of these declarations is very important. route declarations are matched in the order they're found in the __init__.py file.

  1. Add a declaration which maps the pattern / (signifying the root URL) to the route named view_wiki. It maps to our view_wiki view callable by virtue of the @view_config attached to the view_wiki view function indicating route_name='view_wiki'.
  2. Add a declaration which maps the pattern /{pagename} to the route named view_page. This is the regular view for a page. It maps to our view_page view callable by virtue of the @view_config attached to the view_page view function indicating route_name='view_page'.
  3. Add a declaration which maps the pattern /add_page/{pagename} to the route named add_page. This is the add view for a new page. It maps to our add_page view callable by virtue of the @view_config attached to the add_page view function indicating route_name='add_page'.
  4. Add a declaration which maps the pattern /{pagename}/edit_page to the route named edit_page. This is the edit view for a page. It maps to our edit_page view callable by virtue of the @view_config attached to the edit_page view function indicating route_name='edit_page'.

As a result of our edits, the __init__.py file should look something like:

(The highlighted lines are the ones that need to be added or edited.)

Viewing the Application in a Browser

We can finally examine our application in a browser (See :ref:`wiki2-start-the-application`). Launch a browser and visit each of the following URLs, check that the result is as expected:

  • http://localhost:6543 in a browser invokes the view_wiki view. This always redirects to the view_page view of the FrontPage page object.
  • http://localhost:6543/FrontPage in a browser invokes the view_page view of the front page object.
  • http://localhost:6543/FrontPage/edit_page in a browser invokes the edit view for the front page object.
  • http://localhost:6543/add_page/SomePageName in a browser invokes the add view for a page.
  • To generate an error, visit http://localhost:6543/foobars/edit_page which will generate a NoResultFound: No row was found for one() error. You'll see an interactive traceback facility provided by :term:`pyramid_debugtoolbar`.
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