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Defining Views

Conventionally, view callable objects are defined within a views.py module in an Pyramid application. There is nothing automagically special about the filename views.py. Files implementing views 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. A project may have many views throughout its codebase in arbitrarily-named files. In this application, however, we’ll be continuing to use the views.py module, because there’s no reason to break convention.

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

However, a Pyramid view can also be defined as callable which accepts two arguments: a context and a request. In url dispatch based applications, the context resource is rarely used in the view body itself, so within code that uses URL-dispatch-only, it’s common to define views as callables that accept only a request to avoid the visual “noise” of a context argument. This application, however, uses traversal to map URLs 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, having context in the callable argument list is not “noise” to us; instead it’s actually rather important within the view code we’ll define in this application.

The single-arg (request -only) or two-arg (context and request) calling conventions will work in any Pyramid application for any view; they can be used interchangeably as necessary. We’ll be using the two-argument (context, request) view callable argument list syntax in this application.

We’re going to define several view callable functions then wire them into Pyramid using some view configuration.

The source code for this tutorial stage can be browsed via http://github.com/Pylons/pyramid/tree/1.0-branch/docs/tutorials/wiki/src/views/.

Adding View Functions

We’re going to add four view callable functions to our views.py module. One view 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 named edit_page will allow a page to be edited.

The view_wiki view function

The view_wiki function will be configured to respond as the default view callable for a Wiki resource. We’ll 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, 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 WebOb response interface). The pyramid.url.resource_url() API. pyramid.url.resource_url() constructs a URL to the FrontPage page resource (e.g. http://localhost:6543/FrontPage), and uses it as the “location” of the HTTPFound response, forming an HTTP redirect.

The view_page view function

The view_page function will be configured to respond as the default view of a Page resource. We’ll 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 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.

The add_page view function

The add_page function will be configured to respond when the context resource is a Wiki and the view name is add_page. We’ll 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 use 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 scrape 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.

The edit_page view function

The edit_page function will be configured to respond when the context is a Page resource and the view name is edit_page. We’ll 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 traverasal 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 request, 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.

Viewing the Result of 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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from docutils.core import publish_parts
import re

from pyramid.httpexceptions import HTTPFound
from pyramid.url import resource_url
from pyramid.view import view_config

from tutorial.models import Page

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

@view_config(context='tutorial.models.Wiki')
def view_wiki(context, request):
    return HTTPFound(location=resource_url(context, request, 'FrontPage'))

@view_config(context='tutorial.models.Page',
             renderer='tutorial:templates/view.pt')
def view_page(context, request):
    wiki = context.__parent__

    def check(match):
        word = match.group(1)
        if word in wiki:
            page = wiki[word]
            view_url = resource_url(page, request)
            return '<a href="%s">%s</a>' % (view_url, word)
        else:
            add_url = request.application_url + '/add_page/' + word 
            return '<a href="%s">%s</a>' % (add_url, word)

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

@view_config(name='add_page', context='tutorial.models.Wiki',
             renderer='tutorial:templates/edit.pt')
def add_page(context, request):
    name = request.subpath[0]
    if 'form.submitted' in request.params:
        body = request.params['body']
        page = Page(body)
        page.__name__ = name
        page.__parent__ = context
        context[name] = page
        return HTTPFound(location = resource_url(page, request))
    save_url = resource_url(context, request, 'add_page', name)
    page = Page('')
    page.__name__ = name
    page.__parent__ = context
    return dict(page = page, save_url = save_url)

@view_config(name='edit_page', context='tutorial.models.Page',
             renderer='tutorial:templates/edit.pt')
def edit_page(context, request):
    if 'form.submitted' in request.params:
        context.data = request.params['body']
        return HTTPFound(location = resource_url(context, request))

    return dict(page = context,
                save_url = resource_url(context, request, 'edit_page'))
    
    

Adding Templates

Most view callables we’ve added expected to be rendered via a template. The default templating systems in Pyramid are Chameleon and Mako. Chameleon is a variant of ZPT, which is an XML-based templating language. Mako is a non-XML-based templating language. Because we had to pick one, we chose Chameleon for this tutorial.

The templates we create 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

The view.pt template is used for viewing a single 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 template is used as a renderer.

The edit.pt Template

The edit.pt template is used for adding and editing a 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__ 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.

Testing the Views

We’ll modify our tests.py file, adding tests for each view function we added above. As a result, we’ll delete the ViewTests test in the file, and add four other test classes: ViewWikiTests, ViewPageTests, AddPageTests, and EditPageTests. These test the view_wiki, view_page, add_page, and edit_page views respectively.

Once we’re done with the tests.py module, it will look a lot like the below:

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import unittest

from pyramid import testing

class PageModelTests(unittest.TestCase):

    def _getTargetClass(self):
        from tutorial.models import Page
        return Page

    def _makeOne(self, data=u'some data'):
        return self._getTargetClass()(data=data)

    def test_constructor(self):
        instance = self._makeOne()
        self.assertEqual(instance.data, u'some data')
        
class WikiModelTests(unittest.TestCase):

    def _getTargetClass(self):
        from tutorial.models import Wiki
        return Wiki

    def _makeOne(self):
        return self._getTargetClass()()

    def test_it(self):
        wiki = self._makeOne()
        self.assertEqual(wiki.__parent__, None)
        self.assertEqual(wiki.__name__, None)

class AppmakerTests(unittest.TestCase):
    def _callFUT(self, zodb_root):
        from tutorial.models import appmaker
        return appmaker(zodb_root)

    def test_it(self):
        root = {}
        self._callFUT(root)
        self.assertEqual(root['app_root']['FrontPage'].data,
                         'This is the front page')

class ViewWikiTests(unittest.TestCase):
    def test_it(self):
        from tutorial.views import view_wiki
        context = testing.DummyResource()
        request = testing.DummyRequest()
        response = view_wiki(context, request)
        self.assertEqual(response.location, 'http://example.com/FrontPage')

class ViewPageTests(unittest.TestCase):
    def _callFUT(self, context, request):
        from tutorial.views import view_page
        return view_page(context, request)

    def test_it(self):
        wiki = testing.DummyResource()
        wiki['IDoExist'] = testing.DummyResource()
        context = testing.DummyResource(data='Hello CruelWorld IDoExist')
        context.__parent__ = wiki
        context.__name__ = 'thepage'
        request = testing.DummyRequest()
        info = self._callFUT(context, request)
        self.assertEqual(info['page'], context)
        self.assertEqual(
            info['content'], 
            '<div class="document">\n'
            '<p>Hello <a href="http://example.com/add_page/CruelWorld">'
            'CruelWorld</a> '
            '<a href="http://example.com/IDoExist/">'
            'IDoExist</a>'
            '</p>\n</div>\n')
        self.assertEqual(info['edit_url'],
                         'http://example.com/thepage/edit_page')
        
    
class AddPageTests(unittest.TestCase):
    def _callFUT(self, context, request):
        from tutorial.views import add_page
        return add_page(context, request)

    def test_it_notsubmitted(self):
        from pyramid.url import resource_url
        context = testing.DummyResource()
        request = testing.DummyRequest()
        request.subpath = ['AnotherPage']
        info = self._callFUT(context, request)
        self.assertEqual(info['page'].data,'')
        self.assertEqual(
            info['save_url'],
            resource_url(context, request, 'add_page', 'AnotherPage'))
        
    def test_it_submitted(self):
        context = testing.DummyResource()
        request = testing.DummyRequest({'form.submitted':True,
                                        'body':'Hello yo!'})
        request.subpath = ['AnotherPage']
        self._callFUT(context, request)
        page = context['AnotherPage']
        self.assertEqual(page.data, 'Hello yo!')
        self.assertEqual(page.__name__, 'AnotherPage')
        self.assertEqual(page.__parent__, context)

class EditPageTests(unittest.TestCase):
    def _callFUT(self, context, request):
        from tutorial.views import edit_page
        return edit_page(context, request)

    def test_it_notsubmitted(self):
        from pyramid.url import resource_url
        context = testing.DummyResource()
        request = testing.DummyRequest()
        info = self._callFUT(context, request)
        self.assertEqual(info['page'], context)
        self.assertEqual(info['save_url'],
                         resource_url(context, request, 'edit_page'))
        
    def test_it_submitted(self):
        context = testing.DummyResource()
        request = testing.DummyRequest({'form.submitted':True,
                                        'body':'Hello yo!'})
        response = self._callFUT(context, request)
        self.assertEqual(response.location, 'http://example.com/')
        self.assertEqual(context.data, 'Hello yo!')
        
        
        

Running the Tests

We can run these tests by using setup.py test in the same way we did in Running the Tests. Assuming our shell’s current working directory is the “tutorial” distribution directory:

On UNIX:

$ ../bin/python setup.py test -q

On Windows:

c:\pyramidtut\tutorial> ..\Scripts\python setup.py test -q

The expected result looks something like:

.........
----------------------------------------------------------------------
Ran 9 tests in 0.203s

OK

Viewing the Application in a Browser

Once we’ve completed our edits, 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 resource.
  • Visiting http://localhost:6543/FrontPage/ in a browser invokes the view_page view of the front page resource. This is because it’s the default view (a view without a name) for Page resources.
  • Visiting http://localhost:6543/FrontPage/edit_page in a browser invokes the edit view for the FrontPage Page resource.
  • Visiting http://localhost:6543/add_page/SomePageName in a browser invokes the add view for a Page.
  • To generate an error, visit http://localhost:6543/add_page which will generate an IndexError for the expression request.subpath[0]. You’ll see an interactive traceback facility provided by WebError.