A view callable
in a traversal
-based Pyramid
application is typically a simple Python function that accepts two parameters: context
and request
. A view callable is assumed to return a response
object.
Note
A Pyramid
view can also be defined as callable which accepts only a request
argument. You'll see this one-argument pattern used in other Pyramid
tutorials and applications. Either calling convention will work in any Pyramid
application; the calling conventions can be used interchangeably as necessary. In traversal
based applications, URLs are mapped to a context resource
, and since our resource tree
also represents our application's "domain model", we're often interested in the context because it represents the persistent storage of our application. For this reason, in this tutorial we define views as callables that accept context
in the callable argument list. If you do need the context
within a view function that only takes the request as a single argument, you can obtain it via request.context
.
We're going to define several view callable
functions, then wire them into Pyramid
using some view configuration
.
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 the setup()
function.
Open tutorial/setup.py
and edit it to look like the following:
src/views/setup.py
Only the highlighted line needs to be added.
Since a new software dependency was added, you will need to run python setup.py develop
again 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
$ $VENV/bin/python setup.py develop
On Windows:
c:\pyramidtut> cd tutorial
c:\pyramidtut\tutorial> %VENV%\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
It's time for a major change. Open tutorial/tutorial/views.py
and edit it to look like the following:
src/views/tutorial/views.py
We added some imports and created a regular expression to find "WikiWords".
We got rid of the my_view
view function and its decorator that was added when we originally rendered the zodb
scaffold. It was only an example and isn't relevant to our application.
Then we added four 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 in the following sections.
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.
Following is the code for the view_wiki
view function and its decorator:
src/views/tutorial/views.py
Note
In our code, we use an import that is relative to our package named tutorial
, meaning we can omit the name of the package in the import
and context
statements. In our narrative, however, we refer to a class and thus we use the absolute form, meaning that the name of the package is included.
view_wiki()
is the default view
that gets called when a request is made to the root URL of our wiki. It always redirects to an URL which represents the path to our "FrontPage".
We provide it with a @view_config
decorator which names the class tutorial.models.Wiki
as its context. This means that when a Wiki resource is the context and no view name
exists in the request, then this view will be used. The view configuration associated with view_wiki
does not use a renderer
because the view callable always returns a response
object rather than a dictionary. No renderer is necessary when a view returns a response object.
The view_wiki
view callable always redirects to the URL of a Page resource named "FrontPage". To do so, it returns an instance of the pyramid.httpexceptions.HTTPFound
class (instances of which implement the pyramid.interfaces.IResponse
interface, like pyramid.response.Response
does). It uses the pyramid.request.Request.route_url
API to construct an URL to the FrontPage
page resource (i.e., http://localhost:6543/FrontPage
), and uses it as the "location" of the HTTPFound
response, forming an HTTP redirect.
Here is the code for the view_page
view function and its decorator:
src/views/tutorial/views.py
The view_page
function is configured to respond as the default view of a Page resource. We provide it with a @view_config
decorator which names the class tutorial.models.Page
as its context. This means that when a Page resource is the context, and no view name
exists in the request, this view will be used. We inform Pyramid
this view will use the templates/view.pt
template file as a renderer
.
The view_page
function generates the reStructuredText
body of a page (stored as the data
attribute of the context passed to the view; the context will be a Page
resource) as HTML. Then it substitutes an HTML anchor for each WikiWord reference in the rendered HTML using a compiled regular expression.
The curried function named check
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 (our page's __parent__
) already contains a page with the matched WikiWord name, the check
function 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, the function 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 resource.
We then generate an edit URL because it's easier to do here than in the template, and we wrap up a number of arguments in a dictionary and return it.
The arguments we wrap into a dictionary include page
, content
, and edit_url
. As a result, the template associated with this view callable (via renderer=
in its configuration) will be able to use these names to perform various rendering tasks. The template associated with this view callable will be a template which lives in templates/view.pt
.
Note the contrast between this view callable and the view_wiki
view callable. In the view_wiki
view callable, we unconditionally return a response
object. In the view_page
view callable, we return a dictionary. It is always fine to return a response
object from a Pyramid
view. Returning a dictionary is allowed only when there is a renderer
associated with the view callable in the view configuration.
Here is the code for the add_page
view function and its decorator:
src/views/tutorial/views.py
The add_page
function is configured to respond when the context resource is a Wiki and the view name
is add_page
. We provide it with a @view_config
decorator which names the string add_page
as its view name
(via name=
), the class tutorial.models.Wiki
as its context, and the renderer named templates/edit.pt
. This means that when a Wiki resource is the context, and a view name
named add_page
exists as the result of traversal, this view will be used. We inform Pyramid
this view will use the templates/edit.pt
template file as a renderer
. We share the same template between add and edit views, thus edit.pt
instead of add.pt
.
The add_page
function will be 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. It also acts as a handler for the form that is generated when we want to add a page resource. The context
of the add_page
view is always a Wiki resource (not a Page resource).
The request subpath
in Pyramid
is the sequence of names that are found after the view name
in the URL segments given in the PATH_INFO
of the WSGI request as the result of traversal
. If our add view is invoked via, e.g., http://localhost:6543/add_page/SomeName
, the subpath
will be a tuple: ('SomeName',)
.
The add view takes the zeroth element of the subpath (the wiki page name), and aliases it to the name attribute in order to know the name of the page we're trying to add.
If the view rendering is not a result of a form submission (if the expression 'form.submitted' in request.params
is False
), the view 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 trying 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 resource object in order to satisfy the edit form's desire to have some page object exposed as page
, and we'll render the template to a response.
If the view rendering is a result of a form submission (if the expression 'form.submitted' in request.params
is True
), we grab the page body from the form data, create a Page object using the name in the subpath and the page body, and save it into "our context" (the Wiki) using the __setitem__
method of the context. We then redirect back to the view_page
view (the default view for a page) for the newly created page.
Here is the code for the edit_page
view function and its decorator:
src/views/tutorial/views.py
The edit_page
function is configured to respond when the context is a Page resource and the view name
is edit_page
. We provide it with a @view_config
decorator which names the string edit_page
as its view name
(via name=
), the class tutorial.models.Page
as its context, and the renderer named templates/edit.pt
. This means that when a Page resource is the context, and a view name
exists as the result of traversal named edit_page
, this view will be used. We inform Pyramid
this view will use the templates/edit.pt
template file as a renderer
.
The edit_page
function will be 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 form post view callable for the form it renders. The context
of the edit_page
view will always be a Page resource (never a Wiki resource).
If the view execution is not a result of a form submission (if the expression 'form.submitted' in request.params
is False
), the view simply renders the edit form, passing the page resource, and a save_url
which will be used as the action of the generated form.
If the view execution is a result of a form submission (if the expression 'form.submitted' in request.params
is True
), the view grabs the body
element of the request parameter and sets it as the data
attribute of the page context. It then redirects to the default view of the context (the page), which will always be the view_page
view.
The view_page
, add_page
and edit_page
views that we've added reference a template
. Each template is a Chameleon
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.
Create tutorial/tutorial/templates/view.pt
and add the following content:
src/views/tutorial/templates/view.pt
This template is used by view_page()
for displaying a single wiki page. It includes:
- A
div
element that is replaced with thecontent
value provided by the view (lines 36-38).content
contains HTML, so thestructure
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 (lines 40-42).
Create tutorial/tutorial/templates/edit.pt
and add the following content:
src/views/tutorial/templates/edit.pt
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 namedbody
that is filled with any existing page data when it is rendered (line 45). - A submit button that has the name
form.submitted
(line 48).
The form POSTs back to the save_url
argument supplied by the view (line 43). 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 renderer_system_values
for information about other names that are available by default when a template is used as a renderer.
Our templates name static assets, including CSS and images. We don't need to create these files within our package's static
directory because they were provided at the time we created the project.
As an example, the CSS file will be accessed via http://localhost:6543/static/theme.css
by virtue of the call to the 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.
We can finally examine our application in a browser (See wiki-start-the-application
). Launch a browser and visit each of the following URLs, checking that the result is as expected:
- http://localhost:6543/ invokes the
view_wiki
view. This always redirects to theview_page
view of theFrontPage
Page resource. - http://localhost:6543/FrontPage/ invokes the
view_page
view of the front page resource. This is because it's thedefault view
(a view without aname
) for Page resources. - http://localhost:6543/FrontPage/edit_page invokes the edit view for the
FrontPage
Page resource. - http://localhost:6543/add_page/SomePageName invokes the add view for a Page.
- To generate an error, visit http://localhost:6543/add_page which will generate an
IndexErrorr: tuple index out of range
error. You'll see an interactive traceback facility provided bypyramid_debugtoolbar
.