Defining Views

A view callable in a 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.

The request object has a dictionary as an attribute named matchdict. A matchdict maps the placeholders in the matching URL pattern to the substrings of the path in the request URL. For instance, if a call to pyramid.config.Configurator.add_route() has the pattern /{one}/{two}, and a user visits http://example.com/foo/bar, our pattern would be matched against /foo/bar and the matchdict would look like {'one':'foo', 'two':'bar'}.

Adding the docutils dependency

Remember in the previous chapter we added a new dependency of the bcrypt package. Again, the view code in our application will depend on a package which is not a dependency of the original "tutorial" application.

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:

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import os

from setuptools import setup, find_packages

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

requires = [
    'bcrypt',
    'docutils',
    'pyramid',
    'pyramid_jinja2',
    'pyramid_debugtoolbar',
    'pyramid_tm',
    'SQLAlchemy',
    'transaction',
    'zope.sqlalchemy',
    'waitress',
    ]

tests_require = [
    'WebTest >= 1.3.1',  # py3 compat
    'pytest',  # includes virtualenv
    'pytest-cov',
    ]

setup(name='tutorial',
      version='0.0',
      description='tutorial',
      long_description=README + '\n\n' + CHANGES,
      classifiers=[
          "Programming Language :: Python",
          "Framework :: Pyramid",
          "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,
      extras_require={
          'testing': tests_require,
      },
      install_requires=requires,
      entry_points="""\
      [paste.app_factory]
      main = tutorial:main
      [console_scripts]
      initialize_tutorial_db = tutorial.scripts.initializedb:main
      """,
      )

Only the highlighted line needs to be added.

Again, as we did in the previous chapter, the dependency now needs to be installed, so re-run the $VENV/bin/pip install -e . command.

Static assets

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 routes.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 routes.py

This is the URL Dispatch tutorial, so let's start by adding some URL patterns to our app. Later we'll attach views to handle the URLs.

The routes.py file contains 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 registered.

  1. Add a declaration which maps the pattern / (signifying the root URL) to the route named view_wiki. In the next step, we will map it to our view_wiki view callable by virtue of the @view_config decorator attached to the view_wiki view function, which in turn will be indicated by 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. Again, in the next step, we will map it to our view_page view callable by virtue of the @view_config decorator attached to the view_page view function, whin in turn will be indicated by 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. We will map it to our add_page view callable by virtue of the @view_config decorator attached to the add_page view function, which in turn will be indicated by 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. We will map it to our edit_page view callable by virtue of the @view_config decorator attached to the edit_page view function, which in turn will be indicated by route_name='edit_page'.

As a result of our edits, the routes.py file should look like the following:

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def includeme(config):
    config.add_static_view('static', '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')

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

Warning

The order of the routes is important! If you placed /{pagename}/edit_page before /add_page/{pagename}, then we would never be able to add pages. This is because the first route would always match a request to /add_page/edit_page whereas we want /add_page/.. to have priority. This isn't a huge problem in this particular app because wiki pages are always camel case, but it's important to be aware of this behavior in your own apps.

Adding view functions in views/default.py

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

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import cgi
import re
from docutils.core import publish_parts

from pyramid.httpexceptions import (
    HTTPFound,
    HTTPNotFound,
    )

from pyramid.view import view_config

from ..models import Page, User

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

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

@view_config(route_name='view_page', renderer='../templates/view.jinja2')
def view_page(request):
    pagename = request.matchdict['pagename']
    page = request.dbsession.query(Page).filter_by(name=pagename).first()
    if page is None:
        raise HTTPNotFound('No such page')

    def add_link(match):
        word = match.group(1)
        exists = request.dbsession.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, cgi.escape(word))
        else:
            add_url = request.route_url('add_page', pagename=word)
            return '<a href="%s">%s</a>' % (add_url, cgi.escape(word))

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

@view_config(route_name='edit_page', renderer='../templates/edit.jinja2')
def edit_page(request):
    pagename = request.matchdict['pagename']
    page = request.dbsession.query(Page).filter_by(name=pagename).one()
    if 'form.submitted' in request.params:
        page.data = request.params['body']
        next_url = request.route_url('view_page', pagename=page.name)
        return HTTPFound(location=next_url)
    return dict(
        pagename=page.name,
        pagedata=page.data,
        save_url=request.route_url('edit_page', pagename=page.name),
        )

@view_config(route_name='add_page', renderer='../templates/edit.jinja2')
def add_page(request):
    pagename = request.matchdict['pagename']
    if request.dbsession.query(Page).filter_by(name=pagename).count() > 0:
        next_url = request.route_url('edit_page', pagename=pagename)
        return HTTPFound(location=next_url)
    if 'form.submitted' in request.params:
        body = request.params['body']
        page = Page(name=pagename, data=body)
        page.creator = (
            request.dbsession.query(User).filter_by(name='editor').one())
        request.dbsession.add(page)
        next_url = request.route_url('view_page', pagename=pagename)
        return HTTPFound(location=next_url)
    save_url = request.route_url('add_page', pagename=pagename)
    return dict(pagename=pagename, pagedata='', save_url=save_url)

The highlighted lines need to be added or edited.

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 alchemy scaffold. It was only an example and isn't relevant to our application. We also deleted the db_err_msg string.

Then we added four view callable functions to our views/default.py module, as mentioned in the previous step:

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

We'll describe each one briefly in the following sections.

Note

There is nothing special about the filename default.py exept that it is a Python module. A project may have many view callables throughout its codebase in arbitrarily named modules. Modules implementing view callables often have view in their name (or may live in a Python subpackage of your application package named views, as in our case), but this is only by convention, not a requirement.

The view_wiki view function

Following is the code for the view_wiki view function and its decorator:

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

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 a URL which represents the path to our "FrontPage".

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). It uses the pyramid.request.Request.route_url() API to construct a URL to the FrontPage page (i.e., http://localhost:6543/FrontPage), and uses it as the "location" of the HTTPFound response, forming an HTTP redirect.

The view_page view function

Here is the code for the view_page view function and its decorator:

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@view_config(route_name='view_page', renderer='../templates/view.jinja2')
def view_page(request):
    pagename = request.matchdict['pagename']
    page = request.dbsession.query(Page).filter_by(name=pagename).first()
    if page is None:
        raise HTTPNotFound('No such page')

    def add_link(match):
        word = match.group(1)
        exists = request.dbsession.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, cgi.escape(word))
        else:
            add_url = request.route_url('add_page', pagename=word)
            return '<a href="%s">%s</a>' % (add_url, cgi.escape(word))

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

view_page() is used to display a single page of our wiki. It renders the 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 curried function named add_link 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, add_link() 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, add_link() 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 response object) is a cue to Pyramid that it should try to use a renderer associated with the view configuration to render a response. In our case, the renderer used will be the view.jinja2 template, as indicated in the @view_config decorator that is applied to view_page().

If the page does not exist, then we need to handle that by raising a pyramid.httpexceptions.HTTPNotFound to trigger our 404 handling, defined in tutorial/views/notfound.py.

Note

Using raise versus return with the HTTP exceptions is an important distinction that can commonly mess people up. In tutorial/views/notfound.py there is an exception view registered for handling the HTTPNotFound exception. Exception views are only triggered for raised exceptions. If the HTTPNotFound is returned, then it has an internal "stock" template that it will use to render itself as a response. If you aren't seeing your exception view being executed, this is most likely the problem! See Using Special Exceptions in View Callables for more information about exception views.

The edit_page view function

Here is the code for the edit_page view function and its decorator:

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@view_config(route_name='edit_page', renderer='../templates/edit.jinja2')
def edit_page(request):
    pagename = request.matchdict['pagename']
    page = request.dbsession.query(Page).filter_by(name=pagename).one()
    if 'form.submitted' in request.params:
        page.data = request.params['body']
        next_url = request.route_url('view_page', pagename=page.name)
        return HTTPFound(location=next_url)
    return dict(
        pagename=page.name,
        pagedata=page.data,
        save_url=request.route_url('edit_page', pagename=page.name),
        )

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 which 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 that 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.

Note

Since our request.dbsession defined in the previous chapter is registered with the pyramid_tm transaction manager, any changes we make to objects managed by the that session will be committed automatically. In the event that there was an error (even later, in our template code), the changes would be aborted. This means the view itself does not need to concern itself with commit/rollback logic.

The add_page view function

Here is the code for the add_page view function and its decorator:

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@view_config(route_name='add_page', renderer='../templates/edit.jinja2')
def add_page(request):
    pagename = request.matchdict['pagename']
    if request.dbsession.query(Page).filter_by(name=pagename).count() > 0:
        next_url = request.route_url('edit_page', pagename=pagename)
        return HTTPFound(location=next_url)
    if 'form.submitted' in request.params:
        body = request.params['body']
        page = Page(name=pagename, data=body)
        page.creator = (
            request.dbsession.query(User).filter_by(name='editor').one())
        request.dbsession.add(page)
        next_url = request.route_url('view_page', pagename=pagename)
        return HTTPFound(location=next_url)
    save_url = request.route_url('add_page', pagename=pagename)
    return dict(pagename=pagename, pagedata='', save_url=save_url)

add_page() is invoked when a user clicks on a WikiWord which isn't yet represented as a page in the system. The add_link 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, for example, http://localhost:6543/add_page/SomeName, the value for 'pagename' in the matchdict will be 'SomeName'.

Next a check is performed to determine whether the Page already exists in the database. If it already exists, then the client is redirected to the edit_page view, else we continue to the next check.

If the view execution is a result of a form submission (i.e., the expression 'form.submitted' in request.params is True), we grab 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 request.dbession.add. Since we have not yet covered authentication, we don't have a logged-in user to add as the page's creator. Until we get to that point in the tutorial, we'll just assume that all pages are created by the editor user. Thus we query for that object, and set it on page.creator. Finally, we redirect the client 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.jinja2) 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. Pyramid will render the template associated with this view to a response.

Adding templates

The view_page, add_page and edit_page views that we've added reference a template. Each template is a Jinja2 template. These templates will live in the templates directory of our tutorial package. Jinja2 templates must have a .jinja2 extension to be recognized as such.

The layout.jinja2 template

Update tutorial/templates/layout.jinja2 with the following content, as indicated by the emphasized lines:

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<!DOCTYPE html>
<html lang="{{request.locale_name}}">
  <head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta name="description" content="pyramid web application">
    <meta name="author" content="Pylons Project">
    <link rel="shortcut icon" href="{{request.static_url('tutorial:static/pyramid-16x16.png')}}">

    <title>{% block subtitle %}{% endblock %}Pyramid tutorial wiki (based on TurboGears 20-Minute Wiki)</title>

    <!-- Bootstrap core CSS -->
    <link href="//oss.maxcdn.com/libs/twitter-bootstrap/3.0.3/css/bootstrap.min.css" rel="stylesheet">

    <!-- Custom styles for this scaffold -->
    <link href="{{request.static_url('tutorial:static/theme.css')}}" rel="stylesheet">

    <!-- HTML5 shim and Respond.js IE8 support of HTML5 elements and media queries -->
    <!--[if lt IE 9]>
      <script src="//oss.maxcdn.com/libs/html5shiv/3.7.0/html5shiv.js"></script>
      <script src="//oss.maxcdn.com/libs/respond.js/1.3.0/respond.min.js"></script>
    <![endif]-->
  </head>

  <body>

    <div class="starter-template">
      <div class="container">
        <div class="row">
          <div class="col-md-2">
            <img class="logo img-responsive" src="{{request.static_url('tutorial:static/pyramid.png')}}" alt="pyramid web framework">
          </div>
          <div class="col-md-10">
            <div class="content">
            {% block content %}{% endblock %}
            </div>
          </div>
        </div>
        <div class="row">
          <div class="copyright">
            Copyright &copy; Pylons Project
          </div>
        </div>
      </div>
    </div>


    <!-- Bootstrap core JavaScript
    ================================================== -->
    <!-- Placed at the end of the document so the pages load faster -->
    <script src="//oss.maxcdn.com/libs/jquery/1.10.2/jquery.min.js"></script>
    <script src="//oss.maxcdn.com/libs/twitter-bootstrap/3.0.3/js/bootstrap.min.js"></script>
  </body>
</html>

Since we're using a templating engine, we can factor common boilerplate out of our page templates into reusable components. One method for doing this is template inheritance via blocks.

  • We have defined two placeholders in the layout template where a child template can override the content. These blocks are named subtitle (line 11) and content (line 36).
  • Please refer to the Jinja2 documentation for more information about template inheritance.

The view.jinja2 template

Create tutorial/templates/view.jinja2 and add the following content:

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{% extends 'layout.jinja2' %}

{% block subtitle %}{{page.name}} - {% endblock subtitle %}

{% block content %}
<p>{{ content|safe }}</p>
<p>
<a href="{{ edit_url }}">
    Edit this page
</a>
</p>
<p>
    Viewing <strong>{{page.name}}</strong>, created by <strong>{{page.creator.name}}</strong>.
</p>
<p>You can return to the
<a href="{{request.route_url('view_page', pagename='FrontPage')}}">FrontPage</a>.
</p>
{% endblock content %}

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

  • We begin by extending the layout.jinja2 template defined above, which provides the skeleton of the page (line 1).
  • We override the subtitle block from the base layout, inserting the page name into the page's title (line 3).
  • We override the content block from the base layout to insert our markup into the body (lines 5-18).
  • We use a variable that is replaced with the content value provided by the view (line 6). content contains HTML, so the |safe filter is used to prevent escaping it (e.g., changing ">" to "&gt;").
  • We create a link that points at the "edit" URL, which when clicked invokes the edit_page view for the requested page (line 9).

The edit.jinja2 template

Create tutorial/templates/edit.jinja2 and add the following content:

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{% extends 'layout.jinja2' %}

{% block subtitle %}Edit {{pagename}} - {% endblock subtitle %}

{% block content %}
<p>
Editing <strong>{{pagename}}</strong>
</p>
<p>You can return to the
<a href="{{request.route_url('view_page', pagename='FrontPage')}}">FrontPage</a>.
</p>
<form action="{{ save_url }}" method="post">
<div class="form-group">
    <textarea class="form-control" name="body" rows="10" cols="60">{{ pagedata }}</textarea>
</div>
<div class="form-group">
    <button type="submit" name="form.submitted" value="Save" class="btn btn-default">Save</button>
</div>
</form>
{% endblock content %}

This template serves two use cases. It is used by add_page() and edit_page() for adding and editing a wiki page. It displays a page containing a form and which provides the following:

  • Again, we extend the layout.jinja2 template, which provides the skeleton of the page (line 1).
  • Override the subtitle block to affect the <title> tag in the head of the page (line 3).
  • A 10-row by 60-column textarea field named body that is filled with any existing page data when it is rendered (line 14).
  • A submit button that has the name form.submitted (line 17).
  • The form POSTs back to the save_url argument supplied by the view (line 12). The view will use the body and form.submitted values.

The 404.jinja2 template

Replace tutorial/templates/404.jinja2 with the following content:

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{% extends "layout.jinja2" %}

{% block content %}
<div class="content">
  <h1><span class="font-semi-bold">Pyramid tutorial wiki</span> <span class="smaller">(based on TurboGears 20-Minute Wiki)</span></h1>
  <p class="lead"><span class="font-semi-bold">404</span> Page Not Found</p>
</div>
{% endblock content %}

This template is linked from the notfound_view defined in tutorial/views/notfound.py as shown here:

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from pyramid.view import notfound_view_config


@notfound_view_config(renderer='../templates/404.jinja2')
def notfound_view(request):
    request.response.status = 404
    return {}

There are several important things to note about this configuration:

  • The notfound_view in the above snippet is called an exception view. For more information see Using Special Exceptions in View Callables.
  • The notfound_view sets the response status to 404. It's possible to affect the response object used by the renderer via Varying Attributes of Rendered Responses.
  • The notfound_view is registered as an exception view and will be invoked only if pyramid.httpexceptions.HTTPNotFound is raised as an exception. This means it will not be invoked for any responses returned from a view normally. For example, on line 27 of tutorial/views/default.py the exception is raised which will trigger the view.

Finally, we may delete the tutorial/templates/mytemplate.jinja2 template that was provided by the alchemy scaffold, as we have created our own templates for the wiki.

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 System Values Used During Rendering for information about other names that are available by default when a template is used as a renderer.

Viewing the application in a browser

We can finally examine our application in a browser (See Start the application). Launch a browser and visit each of the following URLs, checking that the result is as expected: