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Defining Views

A view callable in a url dispatch -based Pyramid application is typically a simple Python function that accepts a single parameter named request. A view callable is assumed to return a response object.

Note

A Pyramid view can also be defined as callable which accepts two arguments: a context and a request. You’ll see this two-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 url dispatch based applications, however, the context object is rarely used in the view body itself, so within this tutorial we define views as callables that accept only a request to avoid the visual “noise”. 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.

The request 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 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/1.2-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 paster create 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 install_requires parameter in the setup function.

Our resulting setup.py should look like so:

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import os
import sys

from setuptools import setup, find_packages

here = os.path.abspath(os.path.dirname(__file__))
README = open(os.path.join(here, 'README.txt')).read()
CHANGES = open(os.path.join(here, 'CHANGES.txt')).read()

requires = [
    'pyramid',
    'SQLAlchemy',
    'transaction',
    'pyramid_tm',
    'pyramid_debugtoolbar',
    'zope.sqlalchemy',
    'docutils',
    ]

if sys.version_info[:3] < (2,5,0):
    requires.append('pysqlite')

setup(name='tutorial',
      version='0.0',
      description='tutorial',
      long_description=README + '\n\n' +  CHANGES,
      classifiers=[
        "Programming Language :: Python",
        "Framework :: Pylons",
        "Topic :: Internet :: WWW/HTTP",
        "Topic :: Internet :: WWW/HTTP :: WSGI :: Application",
        ],
      author='',
      author_email='',
      url='',
      keywords='web wsgi bfg pylons pyramid',
      packages=find_packages(),
      include_package_data=True,
      zip_safe=False,
      test_suite='tutorial',
      install_requires = requires,
      entry_points = """\
      [paste.app_factory]
      main = tutorial:main
      """,
      paster_plugins=['pyramid'],
      )

Note

After these new dependencies are 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 package.

Adding View Functions

We’ll get rid of our my_view view function in our views.py file. It’s only an example and isn’t relevant to our application.

Then we’re going to add four view callable functions to our views.py module. One view callable (named view_wiki) will display the wiki itself (it will answer on the root URL), another named view_page will display an individual page, another named add_page will allow a page to be added, and a final view callable named edit_page will allow a page to be edited. 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

The view_wiki function is the default view that will be 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”.

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def view_wiki(request):
    return HTTPFound(location = request.route_url('view_page',
                                                  pagename='FrontPage'))

The view_wiki function returns an instance of the pyramid.httpexceptions.HTTPFound class (instances of which implement the pyramid.interfaces.IResponse interface like pyramid.response.Response does), It will use the pyramid.request.Request.route_url() API to construct a URL to the FrontPage page (e.g. http://localhost:6543/FrontPage), and will use it as the “location” of the HTTPFound response, forming an HTTP redirect.

The view_page view function

The view_page function will be used to show a single page of our wiki. It renders the ReStructuredText body of a page (stored as the data attribute of a Page object) as HTML. Then it substitutes an HTML anchor for each WikiWord reference in the rendered HTML using a compiled regular expression.

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def view_page(request):
    pagename = request.matchdict['pagename']
    session = DBSession()
    page = session.query(Page).filter_by(name=pagename).first()
    if page is None:
        return HTTPNotFound('No such page')

    def check(match):
        word = match.group(1)
        exists = session.query(Page).filter_by(name=word).all()
        if exists:
            view_url = request.route_url('view_page', pagename=word)
            return '<a href="%s">%s</a>' % (view_url, word)
        else:
            add_url = request.route_url('add_page', pagename=word)
            return '<a href="%s">%s</a>' % (add_url, word)

    content = publish_parts(page.data, writer_name='html')['html_body']
    content = wikiwords.sub(check, content)
    edit_url = request.route_url('edit_page', pagename=pagename)
    return dict(page=page, content=content, edit_url=edit_url)

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 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 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 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 this view returns a dictionary (as opposed to a response object) is a cue to Pyramid that it should try to use a 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 per the configuration put into effect in __init__.py.

The add_page view function

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 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.

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def add_page(request):
    name = request.matchdict['pagename']
    if 'form.submitted' in request.params:
        session = DBSession()
        body = request.params['body']
        page = Page(name, body)
        session.add(page)
        return HTTPFound(location = request.route_url('view_page',
                                                      pagename=name))
    save_url = request.route_url('add_page', pagename=name)
    page = Page('', '')
    return dict(page=page, save_url=save_url)

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 not a result of a form submission (if 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 trying to use the same template (templates/edit.pt) for the add view as well as the page edit view, so we create a dummy Page object in order to satisfy the edit form’s desire to have some page object exposed as page, and Pyramid will render the template associated with this view to a response.

If the view execution is a result of a form submission (if 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 session.add. We then redirect back to the view_page view for the newly created page.

The edit_page view function

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 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.

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def edit_page(request):
    name = request.matchdict['pagename']
    session = DBSession()
    page = session.query(Page).filter_by(name=name).one()
    if 'form.submitted' in request.params:
        page.data = request.params['body']
        session.add(page)
        return HTTPFound(location = request.route_url('view_page',
                                                      pagename=name))
    return dict(
        page=page,
        save_url = request.route_url('edit_page', pagename=name),
        )

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 object 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 parameters and sets it as the data attribute of the page object. It then redirects to the view_page view of the wiki page.

Viewing the Result of all Our Edits to views.py

The result of all of our edits to views.py will leave it looking like this:

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import re

from docutils.core import publish_parts

from pyramid.httpexceptions import HTTPFound, HTTPNotFound

from tutorial.models import DBSession
from tutorial.models import Page

# regular expression used to find WikiWords
wikiwords = re.compile(r"\b([A-Z]\w+[A-Z]+\w+)")

def view_wiki(request):
    return HTTPFound(location = request.route_url('view_page',
                                                  pagename='FrontPage'))

def view_page(request):
    pagename = request.matchdict['pagename']
    session = DBSession()
    page = session.query(Page).filter_by(name=pagename).first()
    if page is None:
        return HTTPNotFound('No such page')

    def check(match):
        word = match.group(1)
        exists = session.query(Page).filter_by(name=word).all()
        if exists:
            view_url = request.route_url('view_page', pagename=word)
            return '<a href="%s">%s</a>' % (view_url, word)
        else:
            add_url = request.route_url('add_page', pagename=word)
            return '<a href="%s">%s</a>' % (add_url, word)

    content = publish_parts(page.data, writer_name='html')['html_body']
    content = wikiwords.sub(check, content)
    edit_url = request.route_url('edit_page', pagename=pagename)
    return dict(page=page, content=content, edit_url=edit_url)

def add_page(request):
    name = request.matchdict['pagename']
    if 'form.submitted' in request.params:
        session = DBSession()
        body = request.params['body']
        page = Page(name, body)
        session.add(page)
        return HTTPFound(location = request.route_url('view_page',
                                                      pagename=name))
    save_url = request.route_url('add_page', pagename=name)
    page = Page('', '')
    return dict(page=page, save_url=save_url)

def edit_page(request):
    name = request.matchdict['pagename']
    session = DBSession()
    page = session.query(Page).filter_by(name=name).one()
    if 'form.submitted' in request.params:
        page.data = request.params['body']
        session.add(page)
        return HTTPFound(location = request.route_url('view_page',
                                                      pagename=name))
    return dict(
        page=page,
        save_url = request.route_url('edit_page', pagename=name),
        )

Adding Templates

The views we’ve added all reference a template. Each template is a Chameleon ZPT template. These templates will live in the templates directory of our tutorial package.

The view.pt Template

The view.pt template is used for viewing a single wiki page. It is used by the view_page view function. It should have a div that is “structure replaced” with the content value provided by the view. It should also have a link on the rendered page that points at the “edit” URL (the URL which invokes the edit_page view for the page being viewed).

Once we’re done with the view.pt template, it will look a lot like the below:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"
      xmlns:tal="http://xml.zope.org/namespaces/tal">
<head>
  <title>${page.name} - Pyramid tutorial wiki (based on
    TurboGears 20-Minute Wiki)</title>
  <meta http-equiv="Content-Type" content="text/html;charset=UTF-8"/>
  <meta name="keywords" content="python web application" />
  <meta name="description" content="pyramid web application" />
  <link rel="shortcut icon"
        href="${request.static_url('tutorial:static/favicon.ico')}" />
  <link rel="stylesheet"
        href="${request.static_url('tutorial:static/pylons.css')}"
        type="text/css" media="screen" charset="utf-8" />
  <!--[if lte IE 6]>
  <link rel="stylesheet"
        href="${request.static_url('tutorial:static/ie6.css')}"
        type="text/css" media="screen" charset="utf-8" />
  <![endif]-->
</head>
<body>
  <div id="wrap">
    <div id="top-small">
      <div class="top-small align-center">
        <div>
          <img width="220" height="50" alt="pyramid"
        src="${request.static_url('tutorial:static/pyramid-small.png')}" />
        </div>
      </div>
    </div>
    <div id="middle">
      <div class="middle align-right">
        <div id="left" class="app-welcome align-left">
          Viewing <b><span tal:replace="page.name">Page Name
              Goes Here</span></b><br/>
          You can return to the
          <a href="${request.application_url}">FrontPage</a>.<br/>
        </div>
        <div id="right" class="app-welcome align-right"></div>
      </div>
    </div>
    <div id="bottom">
      <div class="bottom">
        <div tal:replace="structure content">
          Page text goes here.
        </div>
        <p>
          <a tal:attributes="href edit_url" href="">
            Edit this page
          </a>
        </p>
      </div>
    </div>
  </div>
  <div id="footer">
    <div class="footer"
         >&copy; Copyright 2008-2011, Agendaless Consulting.</div>
  </div>
</body>
</html>

Note

The names available for our use in a template are always those that are present in the dictionary returned by the view callable. But our templates make use of a request object that none of our tutorial views return in their dictionary. This value appears as if “by magic”. However, request is one of several names that are available “by default” in a template when a template renderer is used. See *.pt or *.txt: Chameleon Template Renderers for more information about other names that are available by default in a template when a Chameleon template is used as a renderer.

The edit.pt Template

The edit.pt template is used for adding and editing a wiki page. It is used by the add_page and edit_page view functions. It should display a page containing a form that POSTs back to the “save_url” argument supplied by the view. The form should have a “body” textarea field (the page data), and a submit button that has the name “form.submitted”. The textarea in the form should be filled with any existing page data when it is rendered.

Once we’re done with the edit.pt template, it will look a lot like the below:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"
      xmlns:tal="http://xml.zope.org/namespaces/tal">
<head>
  <title>${page.name} - Pyramid tutorial wiki (based on
    TurboGears 20-Minute Wiki)</title>
  <meta http-equiv="Content-Type" content="text/html;charset=UTF-8"/>
  <meta name="keywords" content="python web application" />
  <meta name="description" content="pyramid web application" />
  <link rel="shortcut icon"
        href="${request.static_url('tutorial:static/favicon.ico')}" />
  <link rel="stylesheet"
        href="${request.static_url('tutorial:static/pylons.css')}"
        type="text/css" media="screen" charset="utf-8" />
  <!--[if lte IE 6]>
  <link rel="stylesheet"
        href="${request.static_url('tutorial:static/ie6.css')}"
        type="text/css" media="screen" charset="utf-8" />
  <![endif]-->
</head>
<body>
  <div id="wrap">
    <div id="top-small">
      <div class="top-small align-center">
        <div>
          <img width="220" height="50" alt="pyramid"
        src="${request.static_url('tutorial:static/pyramid-small.png')}" />
        </div>
      </div>
    </div>
    <div id="middle">
      <div class="middle align-right">
        <div id="left" class="app-welcome align-left">
          Editing <b><span tal:replace="page.name">Page Name Goes
            Here</span></b><br/>
          You can return to the
          <a href="${request.application_url}">FrontPage</a>.<br/>
        </div>
        <div id="right" class="app-welcome align-right"></div>
      </div>
    </div>
    <div id="bottom">
      <div class="bottom">
        <form action="${save_url}" method="post">
          <textarea name="body" tal:content="page.data" rows="10"
                    cols="60"/><br/>
          <input type="submit" name="form.submitted" value="Save"/>
        </form>
      </div>
    </div>
  </div>
  <div id="footer">
    <div class="footer"
         >&copy; Copyright 2008-2011, Agendaless Consulting.</div>
  </div>
</body>
</html>

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.

Mapping Views to URLs in __init__.py

The __init__.py file contains pyramid.config.Configurator.add_view() calls which serve to map routes via url dispatch to views. 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.
  2. Add a declaration which maps the pattern /{pagename} to the route named view_page. This is the regular view for a 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.
  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.

After we’ve defined the routes for our application, we can register views to handle the processing and rendering that needs to happen when each route is requested.

  1. Add a declaration which maps the view_wiki route to the view named view_wiki in our views.py file. This is the default view for the wiki.
  2. Add a declaration which maps the view_page route to the view named view_page in our views.py file.
  3. Add a declaration which maps the add_page route to the view named add_page in our views.py file.
  4. Add a declaration which maps the edit_page route to the view named edit_page in our views.py file.

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

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from pyramid.config import Configurator
from sqlalchemy import engine_from_config

from tutorial.models import initialize_sql

def main(global_config, **settings):
    """ This function returns a WSGI application.
    """
    engine = engine_from_config(settings, 'sqlalchemy.')
    initialize_sql(engine)
    config = Configurator(settings=settings)
    config.add_static_view('static', 'tutorial:static', cache_max_age=3600)
    config.add_route('view_wiki', '/')
    config.add_route('view_page', '/{pagename}')
    config.add_route('add_page', '/add_page/{pagename}')
    config.add_route('edit_page', '/{pagename}/edit_page')
    config.add_view('tutorial.views.view_wiki', route_name='view_wiki')
    config.add_view('tutorial.views.view_page', route_name='view_page',
                    renderer='tutorial:templates/view.pt')
    config.add_view('tutorial.views.add_page', route_name='add_page',
                    renderer='tutorial:templates/edit.pt')
    config.add_view('tutorial.views.edit_page', route_name='edit_page',
                    renderer='tutorial:templates/edit.pt')
    return config.make_wsgi_app()

Viewing the Application in a Browser

We can finally examine our application in a browser. The views we’ll try are as follows:

  • Visiting http://localhost:6543 in a browser invokes the view_wiki view. This always redirects to the view_page view of the FrontPage page object.
  • Visiting http://localhost:6543/FrontPage in a browser invokes the view_page view of the front page page object.
  • Visiting http://localhost:6543/FrontPage/edit_page in a browser invokes the edit view for the front page object.
  • Visiting http://localhost:6543/add_page/SomePageName in a browser invokes the add view for a page.

Try generating an error within the body of a view by adding code to the top of it that generates an exception (e.g. raise Exception('Forced Exception')). Then visit the error-raising view in a browser. You should see an interactive exception handler in the browser which allows you to examine values in a post-mortem mode.