.. _qtut_more_view_classes:
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==========================
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15: More With View Classes
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==========================
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Group views into a class, sharing configuration, state, and logic.
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Background
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==========
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As part of its mission to help build more ambitious web applications, Pyramid
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provides many more features for views and view classes.
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The Pyramid documentation discusses views as a Python "callable". This callable
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can be a function, an object with a ``__call__``, or a Python class. In this
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last case, methods on the class can be decorated with ``@view_config`` to
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register the class methods with the :term:`configurator` as a view.
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At first, our views were simple, free-standing functions. Many times your views
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are related: different ways to look at or work on the same data, or a REST API
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that handles multiple operations. Grouping these together as a :ref:`view class
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<class_as_view>` makes sense:
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- Group views.
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- Centralize some repetitive defaults.
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- Share some state and helpers.
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Pyramid views have :ref:`view predicates <view_configuration_parameters>` that
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determine which view is matched to a request, based on factors such as the
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request method, the form parameters, and so on. These predicates provide many
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axes of flexibility.
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The following shows a simple example with four operations: view a home page
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which leads to a form, save a change, and press the delete button.
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Objectives
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==========
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- Group related views into a view class.
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- Centralize configuration with class-level ``@view_defaults``.
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- Dispatch one route/URL to multiple views based on request data.
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- Share states and logic between views and templates via the view class.
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Steps
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=====
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#. First we copy the results of the previous step:
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.. code-block:: bash
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$ cd ..; cp -r templating more_view_classes; cd more_view_classes
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$ $VENV/bin/pip install -e .
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#. Our route in ``more_view_classes/tutorial/__init__.py`` needs some
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replacement patterns:
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.. literalinclude:: more_view_classes/tutorial/__init__.py
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:linenos:
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#. Our ``more_view_classes/tutorial/views.py`` now has a view class with
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several views:
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.. literalinclude:: more_view_classes/tutorial/views.py
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:linenos:
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#. Our primary view needs a template at ``more_view_classes/tutorial/home.pt``:
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.. literalinclude:: more_view_classes/tutorial/home.pt
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:language: html
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#. Ditto for our other view from the previous section at
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``more_view_classes/tutorial/hello.pt``:
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.. literalinclude:: more_view_classes/tutorial/hello.pt
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:language: html
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#. We have an edit view that also needs a template at
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``more_view_classes/tutorial/edit.pt``:
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.. literalinclude:: more_view_classes/tutorial/edit.pt
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:language: html
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#. And finally the delete view's template at
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``more_view_classes/tutorial/delete.pt``:
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.. literalinclude:: more_view_classes/tutorial/delete.pt
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:language: html
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#. Our tests in ``more_view_classes/tutorial/tests.py`` fail, so let's modify
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them:
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.. literalinclude:: more_view_classes/tutorial/tests.py
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:linenos:
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#. Now run the tests:
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.. code-block:: bash
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$ $VENV/bin/py.test tutorial/tests.py -q
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..
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2 passed in 0.40 seconds
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#. Run your Pyramid application with:
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.. code-block:: bash
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$ $VENV/bin/pserve development.ini --reload
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#. Open http://localhost:6543/howdy/jane/doe in your browser. Click the
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``Save`` and ``Delete`` buttons, and watch the output in the console window.
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Analysis
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========
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As you can see, the four views are logically grouped together. Specifically:
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- We have a ``home`` view available at http://localhost:6543/ with a clickable
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link to the ``hello`` view.
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- The second view is returned when you go to ``/howdy/jane/doe``. This URL is
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mapped to the ``hello`` route that we centrally set using the optional
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``@view_defaults``.
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- The third view is returned when the form is submitted with a ``POST`` method.
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This rule is specified in the ``@view_config`` for that view.
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- The fourth view is returned when clicking on a button such as ``<input
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type="submit" name="form.delete" value="Delete"/>``.
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In this step we show, using the following information as criteria, how to
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decide which view to use:
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- Method of the HTTP request (``GET``, ``POST``, etc.)
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- Parameter information in the request (submitted form field names)
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We also centralize part of the view configuration to the class level with
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``@view_defaults``, then in one view, override that default just for that one
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view. Finally, we put this commonality between views to work in the view class
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by sharing:
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- State assigned in ``TutorialViews.__init__``
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- A computed value
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These are then available both in the view methods and in the templates (e.g.,
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``${view.view_name}`` and ``${view.full_name}``).
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As a note, we made a switch in our templates on how we generate URLs. We
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previously hardcoded the URLs, such as:
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.. code-block:: html
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<a href="/howdy/jane/doe">Howdy</a>
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In ``home.pt`` we switched to:
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.. code-block:: xml
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<a href="${request.route_url('hello', first='jane',
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last='doe')}">form</a>
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Pyramid has rich facilities to help generate URLs in a flexible, non-error
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prone fashion.
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Extra credit
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============
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#. Why could our template do ``${view.full_name}`` and not have to do
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``${view.full_name()}``?
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#. The ``edit`` and ``delete`` views are both receive ``POST`` requests. Why
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does the ``edit`` view configuration not catch the ``POST`` used by
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``delete``?
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#. We used Python ``@property`` on ``full_name``. If we reference this many
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times in a template or view code, it would re-compute this every time. Does
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Pyramid provide something that will cache the initial computation on a
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property?
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#. Can you associate more than one route with the same view?
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#. There is also a ``request.route_path`` API. How does this differ from
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``request.route_url``?
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.. seealso:: :ref:`class_as_view`, `Weird Stuff You Can Do With
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URL Dispatch <http://www.plope.com/weird_pyramid_urldispatch>`_
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