.. _qtut_json:
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========================================
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14: AJAX Development With JSON Renderers
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========================================
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Modern web apps are more than rendered HTML. Dynamic pages now use JavaScript
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to update the UI in the browser by requesting server data as JSON. Pyramid
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supports this with a *JSON renderer*.
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Background
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==========
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As we saw in :doc:`templating`, view declarations can specify a renderer.
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Output from the view is then run through the renderer, which generates and
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returns the response. We first used a Chameleon renderer, then a Jinja2
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renderer.
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Renderers aren't limited, however, to templates that generate HTML. Pyramid
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supplies a JSON renderer which takes Python data, serializes it to JSON, and
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performs some other functions such as setting the content type. In fact you can
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write your own renderer (or extend a built-in renderer) containing custom logic
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for your unique application.
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Steps
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=====
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#. First we copy the results of the ``view_classes`` step:
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.. code-block:: bash
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$ cd ..; cp -r view_classes json; cd json
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$ $VENV/bin/pip install -e .
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#. We add a new route for ``hello_json`` in ``json/tutorial/__init__.py``:
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.. literalinclude:: json/tutorial/__init__.py
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:linenos:
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#. Rather than implement a new view, we will "stack" another decorator on the
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``hello`` view in ``views.py``:
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.. literalinclude:: json/tutorial/views.py
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:linenos:
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#. We need a new functional test at the end of ``json/tutorial/tests.py``:
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.. literalinclude:: json/tutorial/tests.py
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:linenos:
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#. Run the tests:
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.. code-block:: bash
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$ $VENV/bin/py.test tutorial/tests.py -q
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.....
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5 passed in 0.47 seconds
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#. Run your Pyramid application with:
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.. code-block:: bash
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$ $VENV/bin/pserve development.ini --reload
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#. Open http://localhost:6543/howdy.json in your browser and you will see the
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resulting JSON response.
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Analysis
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========
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Earlier we changed our view functions and methods to return Python data. This
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change to a data-oriented view layer made test writing easier, decoupling the
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templating from the view logic.
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Since Pyramid has a JSON renderer as well as the templating renderers, it is an
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easy step to return JSON. In this case we kept the exact same view and arranged
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to return a JSON encoding of the view data. We did this by:
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- Adding a route to map ``/howdy.json`` to a route name.
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- Providing a ``@view_config`` that associated that route name with an existing
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view.
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- *Overriding* the view defaults in the view config that mentions the
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``hello_json`` route, so that when the route is matched, we use the JSON
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renderer rather than the ``home.pt`` template renderer that would otherwise
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be used.
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In fact, for pure AJAX-style web applications, we could re-use the existing
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route by using Pyramid's view predicates to match on the ``Accepts:`` header
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sent by modern AJAX implementations.
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Pyramid's JSON renderer uses the base Python JSON encoder, thus inheriting its
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strengths and weaknesses. For example, Python can't natively JSON encode
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DateTime objects. There are a number of solutions for this in Pyramid,
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including extending the JSON renderer with a custom renderer.
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.. seealso:: :ref:`views_which_use_a_renderer`,
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:ref:`json_renderer`, and
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:ref:`adding_and_overriding_renderers`
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