15: More With View Classes

Group views into a class, sharing configuration, state, and logic.

Background

As part of its mission to help build more ambitious web applications, Pyramid provides many more features for views and view classes.

The Pyramid documentation discusses views as a Python "callable". This callable can be a function, an object with a __call__, or a Python class. In this last case, methods on the class can be decorated with @view_config to register the class methods with the configurator as a view.

At first, our views were simple, free-standing functions. Many times your views are related: different ways to look at or work on the same data, or a REST API that handles multiple operations. Grouping these together as a view class makes sense:

  • Group views.

  • Centralize some repetitive defaults.

  • Share some state and helpers.

Pyramid views have view predicates that determine which view is matched to a request, based on factors such as the request method, the form parameters, and so on. These predicates provide many axes of flexibility.

The following shows a simple example with four operations: view a home page which leads to a form, save a change, and press the delete button.

Objectives

  • Group related views into a view class.

  • Centralize configuration with class-level @view_defaults.

  • Dispatch one route/URL to multiple views based on request data.

  • Share states and logic between views and templates via the view class.

Steps

  1. First we copy the results of the templating step:

    cd ..; cp -r templating more_view_classes; cd more_view_classes
$VENV/bin/pip install -e .

  2. Our route in more_view_classes/tutorial/__init__.py needs some replacement patterns:

     1from pyramid.config import Configurator
 2
 3
 4def main(global_config, **settings):
 5    config = Configurator(settings=settings)
 6    config.include('pyramid_chameleon')
 7    config.add_route('home', '/')
 8    config.add_route('hello', '/howdy/{first}/{last}')
 9    config.scan('.views')
10    return config.make_wsgi_app()

  3. Our more_view_classes/tutorial/views.py now has a view class with several views:

     1from pyramid.view import (
 2    view_config,
 3    view_defaults
 4    )
 5
 6
 7@view_defaults(route_name='hello')
 8class TutorialViews:
 9    def __init__(self, request):
10        self.request = request
11        self.view_name = 'TutorialViews'
12
13    @property
14    def full_name(self):
15        first = self.request.matchdict['first']
16        last = self.request.matchdict['last']
17        return first + ' ' + last
18
19    @view_config(route_name='home', renderer='home.pt')
20    def home(self):
21        return {'page_title': 'Home View'}
22
23    # Retrieving /howdy/first/last the first time
24    @view_config(renderer='hello.pt')
25    def hello(self):
26        return {'page_title': 'Hello View'}
27
28    # Posting to /howdy/first/last via the "Edit" submit button
29    @view_config(request_method='POST', renderer='edit.pt')
30    def edit(self):
31        new_name = self.request.params['new_name']
32        return {'page_title': 'Edit View', 'new_name': new_name}
33
34    # Posting to /howdy/first/last via the "Delete" submit button
35    @view_config(request_method='POST', request_param='form.delete',
36                 renderer='delete.pt')
37    def delete(self):
38        print ('Deleted')
39        return {'page_title': 'Delete View'}

  4. Our primary view needs a template at more_view_classes/tutorial/home.pt:

    <!DOCTYPE html>
<html lang="en">
<head>
    <title>Quick Tutorial: ${view.view_name} - ${page_title}</title>
</head>
<body>
<h1>${view.view_name} - ${page_title}</h1>

<p>Go to the <a href="${request.route_url('hello', first='jane',
        last='doe')}">form</a>.</p>
</body>
</html>

  5. Ditto for our other view from the previous section at more_view_classes/tutorial/hello.pt:

    <!DOCTYPE html>
<html lang="en">
<head>
    <title>Quick Tutorial: ${view.view_name} - ${page_title}</title>
</head>
<body>
<h1>${view.view_name} - ${page_title}</h1>
<p>Welcome, ${view.full_name}</p>
<form method="POST"
      action="${request.current_route_url()}">
    <input name="new_name"/>
    <input type="submit" name="form.edit" value="Save"/>
    <input type="submit" name="form.delete" value="Delete"/>
</form>
</body>
</html>

  6. We have an edit view that also needs a template at more_view_classes/tutorial/edit.pt:

    <!DOCTYPE html>
<html lang="en">
<head>
    <title>Quick Tutorial: ${view.view_name} - ${page_title}</title>
</head>
<body>
<h1>${view.view_name} - ${page_title}</h1>
<p>You submitted <code>${new_name}</code></p>
</body>
</html>

  7. And finally the delete view's template at more_view_classes/tutorial/delete.pt:

    <!DOCTYPE html>
<html lang="en">
<head>
    <title>Quick Tutorial: ${page_title}</title>
</head>
<body>
<h1>${view.view_name} - ${page_title}</h1>
</body>
</html>

  8. Our tests in more_view_classes/tutorial/tests.py fail, so let's modify them:

     1import unittest
 2
 3from pyramid import testing
 4
 5
 6class TutorialViewTests(unittest.TestCase):
 7    def setUp(self):
 8        self.config = testing.setUp()
 9
10    def tearDown(self):
11        testing.tearDown()
12
13    def test_home(self):
14        from .views import TutorialViews
15
16        request = testing.DummyRequest()
17        inst = TutorialViews(request)
18        response = inst.home()
19        self.assertEqual('Home View', response['page_title'])
20
21class TutorialFunctionalTests(unittest.TestCase):
22    def setUp(self):
23        from tutorial import main
24        app = main({})
25        from webtest import TestApp
26
27        self.testapp = TestApp(app)
28
29    def test_home(self):
30        res = self.testapp.get('/', status=200)
31        self.assertIn(b'TutorialViews - Home View', res.body)

  9. Now run the tests:

    $VENV/bin/pytest tutorial/tests.py -q
..
2 passed in 0.40 seconds

  10. Run your Pyramid application with:

    $VENV/bin/pserve development.ini --reload

  11. Open http://localhost:6543/howdy/jane/doe in your browser. Click the Save and Delete buttons, and watch the output in the console window.

Analysis

As you can see, the four views are logically grouped together. Specifically:

  • We have a home view available at http://localhost:6543/ with a clickable link to the hello view.

  • The second view is returned when you go to /howdy/jane/doe. This URL is mapped to the hello route that we centrally set using the optional @view_defaults.

  • The third view is returned when the form is submitted with a POST method. This rule is specified in the @view_config for that view.

  • The fourth view is returned when clicking on a button such as <input type="submit" name="form.delete" value="Delete"/>.

In this step we show, using the following information as criteria, how to decide which view to use:

  • Method of the HTTP request (GET, POST, etc.)

  • Parameter information in the request (submitted form field names)

We also centralize part of the view configuration to the class level with @view_defaults, then in one view, override that default just for that one view. Finally, we put this commonality between views to work in the view class by sharing:

  • State assigned in TutorialViews.__init__

  • A computed value

These are then available both in the view methods and in the templates (e.g., ${view.view_name} and ${view.full_name}).

As a note, we made a switch in our templates on how we generate URLs. We previously hardcoded the URLs, such as:

<a href="/howdy/jane/doe">Howdy</a>

In home.pt we switched to:

<a href="${request.route_url('hello', first='jane',
    last='doe')}">form</a>

Pyramid has rich facilities to help generate URLs in a flexible, non-error prone fashion.

Extra credit

  1. Why could our template do ${view.full_name} and not have to do ${view.full_name()}?

  2. The edit and delete views are both receive POST requests. Why does the edit view configuration not catch the POST used by delete?

  3. We used Python @property on full_name. If we reference this many times in a template or view code, it would re-compute this every time. Does Pyramid provide something that will cache the initial computation on a property?

  4. Can you associate more than one route with the same view?

  5. There is also a request.route_path API. How does this differ from request.route_url?

See also

Defining a View Callable as a Class, Weird Stuff You Can Do With URL Dispatch