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Added a couple more sections to docs/templates_python.txt
git-svn-id: http://code.djangoproject.com/svn/django/trunk@625 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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Adrian Holovaty
@@ -193,6 +193,93 @@ some things to keep in mind:
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self.database_record.delete()
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sensitive_function.alters_data = True
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Playing with Context objects
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----------------------------
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Most of the time, you'll instantiate ``Context`` objects by passing in a
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fully-populated dictionary to ``Context()``. But you can add and delete items
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from a ``Context`` object once it's been instantiated, too, using standard
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dictionary syntax::
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>>> c = Context({"foo": "bar"})
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>>> c['foo']
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'bar'
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>>> del c['foo']
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>>> c['foo']
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''
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>>> c['newvariable'] = 'hello'
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>>> c['newvariable']
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'hello'
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A ``Context`` object is a stack. That is, you can ``push()`` and ``pop()`` it.
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If you ``pop()`` too much, it'll raise
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``django.core.template.ContextPopException``.
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>>> c = Context()
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>>> c['foo'] = 'first level'
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>>> c.push()
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>>> c['foo'] = 'second level'
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>>> c['foo']
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'second level'
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>>> c.pop()
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>>> c['foo']
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'first level'
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>>> c['foo'] = 'overwritten'
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>>> c['foo']
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'overwritten'
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>>> c.pop()
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Traceback (most recent call last):
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...
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django.core.template.ContextPopException
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Using a ``Context`` as a stack comes in handy in some custom template tags, as
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you'll see below.
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Subclassing Context: DjangoContext
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----------------------------------
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Django comes with a special ``Context`` class,
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``django.core.extensions.DjangoContext``, that acts slightly differently than
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the normal ``django.core.template.Context``. It takes an ``HttpRequest`` object
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as its first argument, and it automatically populates the context with a few
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variables:
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* ``user`` -- An ``auth.User`` instance representing the currently
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logged-in user (or an ``AnonymousUser`` instance, if the client isn't
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logged in).
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* ``messages`` -- A list of ``auth.Message`` objects for the currently
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logged-in user.
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* ``perms`` -- An instance of ``django.core.extensions.PermWrapper``,
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representing the permissions that the currently logged-in user has.
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Also, if your ``DEBUG`` setting is set to ``True``, every ``DjangoContext``
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instance has the following two extra variables:
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* ``debug`` -- ``True``. You can use this in templates to test whether
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you're in ``DEBUG`` mode.
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* ``sql_queries`` -- A list of ``{'sql': ..., 'time': ...}`` dictionaries,
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representing every SQL query that has happened so far during the request.
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The list is in order by query.
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Feel free to subclass ``Context`` yourself if you find yourself wanting to give
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each template something "automatically." For instance, if you want to give
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every template automatic access to the current time, use something like this::
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from django.core.template import Context
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import datetime
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class TimeContext(template.Context):
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def __init__(self, *args, **kwargs):
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Context.__init__(self, *args, **kwargs)
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self['current_time'] = datetime.datetime.now()
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This technique has two caveats:
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* You'll have to remember to use ``TimeContext`` instead of ``Context`` in
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your template-loading code.
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* You'll have to be careful not to set the variable ``current_time`` within
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your templates. If you do, you'll override the other one.
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Loading templates
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-----------------
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@@ -271,9 +358,277 @@ To load a template that's within a subdirectory, just use a slash, like so::
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Extending the template system
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=============================
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Although the Django template language comes with several default tags and
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filters, you might want to write your own. It's easy to do.
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First, create a ``templatetags`` package in the appropriate Django app's
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package. It should be on the same level as ``models``, ``views``, etc. For
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example::
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polls/
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models/
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templatetags/
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views/
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Add two files to the ``templatetags`` package: an ``__init__.py`` file and a
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file that will contain your custom tag/filter definitions. The name of the
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latter file is the name you'll use to load the tags later. For example, if your
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custom tags/filters are in a file called ``poll_extras.py``, you'd do the
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following in a template::
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{% load poll_extras %}
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The ``{% load %}`` tag looks at your ``INSTALLED_APPS`` setting and only allows
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the loading of template libraries within installed Django apps. This is a
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security feature: It allows you to host Python code for many template libraries
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on a single computer without enabling access to all of them for every Django
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installation.
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If you write a template library that isn't tied to any particular models/views,
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it's perfectly OK to have a Django app package that only contains a
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``templatetags`` package.
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There's no limit on how many modules you put in the ``templatetags`` package.
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Just keep in mind that a ``{% load %}`` statement will load tags/filters for
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the given Python module name, not the name of the app.
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Once you've created that Python module, you'll just have to write a bit of
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Python code, depending on whether you're writing filters or tags.
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.. admonition:: Behind the scenes
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For a ton of examples, read the source code for Django's default filters
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and tags. They're in ``django/core/defaultfilters.py`` and
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``django/core/defaulttags.py``, respectively.
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Writing custom template filters
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-------------------------------
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Custom filters are just Python functions that take two arguments:
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* The value of the variable (input) -- not necessarily a string
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* The value of the argument -- always a string
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Filter functions should always return something. They shouldn't raise
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exceptions. They should fail silently. In case of error, they should return
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either the original input or the empty string -- whichever makes more sense.
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Here's an example filter definition::
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def cut(value, arg):
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"Removes all values of arg from the given string"
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return value.replace(arg, '')
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Most filters don't take arguments. For filters that don't take arguments, the
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convention is to use a single underscore as the second argument to the filter
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definition. Example::
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def lower(value, _):
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"Converts a string into all lowercase"
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return value.lower()
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When you've written your filter definition, you need to register it, to make it
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available to Django's template language.
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from django.core import template
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template.register_filter('cut', cut, True)
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template.register_filter('lower', lower, False)
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``register_filter`` takes three arguments::
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1. The name of the filter -- a string
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2. The Python function
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3. A boolean, designating whether the filter requires an argument
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The convention is to put all ``register_filter`` calls at the bottom of your
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template-library module.
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Writing custom template tags
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----------------------------
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Tags are more complex than filters, because tags can do anything.
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A quick overview
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~~~~~~~~~~~~~~~~
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Above, this document explained that the template system works in a two-step
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process: compiling and rendering. To define a custom template tag, you specify
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how the compilation works and how the rendering works.
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When Django compiles a template, it splits the raw template text into
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''nodes''. Each node is an instance of ``django.core.template.Node`` and has
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a ``render()`` method. A compiled template is, simply, a list of ``Node``
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objects. When you call ``render()`` on a compiled template object, the template
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calls ``render()`` on each ``Node`` in its node list, with the given context.
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Thus, to define a custom template tag, you specify how the raw template tag is
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converted into a ``Node`` (the compilation function), and what the node's
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``render()`` method does.
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Writing the compilation function
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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For each template tag the template parser encounters, it calls a Python
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function with the tag contents and the parser object itself. This function is
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responsible for returning a ``Node`` instance based on the contents of the tag.
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By convention, the name of each compilation function should start with ``do_``.
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For example, let's write a template tag that displays the current date/time,
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formatted according to a parameter given in the tag, in `strftime syntax`_.
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It's a good idea to decide the tag syntax before anything else. In our case,
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let's say the tag should be used like this::
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<p>The time is {% current_time "%Y-%M-%d %I:%M %p" %}.</p>
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.. _`strftime syntax`: http://www.python.org/doc/current/lib/module-time.html#l2h-1941
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The parser for this function should grab the parameter and create a ``Node``
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object::
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from django.core import template
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def do_current_time(parser, token):
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try:
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# Splitting by None == splitting by spaces.
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tag_name, format_string = token.contents.split(None, 1)
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except ValueError:
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raise template.TemplateSyntaxError, "%r tag requires an argument" % token.contents[0]
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if not (format_string[0] == format_string[-1] and format_string[0] in ('"', "'")):
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raise template.TemplateSyntaxError, "%r tag's argument should be in quotes" % tag_name
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return CurrentTimeNode(format_string[1:-1])
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Notes:
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* ``parser`` is the template parser object. We don't need it in this
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example.
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* ``token.contents`` is a string of the raw contents of the tag. In our
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example, it's ``'current_time "%Y-%M-%d %I:%M %p"'``
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* This function raises ``django.core.template.TemplateSyntaxError``, with
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helpful messages, for any syntax error.
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* The ``TemplateSyntaxError`` exceptions use the ``tag_name`` variable.
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Don't hard-code the tag's name in your error messages, because that
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couples the tag's name to your function. ``token.contents.split()[0]``
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will ''always'' be the name of your tag -- even when the tag has no
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arguments.
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* The function returns a ``CurrentTimeNode`` with everything the node needs
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to know about this tag. In this case, it just passes the argument --
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``"%Y-%M-%d %I:%M %p"``. The leading and trailing quotes from the
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template tag are removed in ``format_string[1:-1]``.
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* The parsing is very low-level. The Django developers have experimented
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with writing small frameworks on top of this parsing system, using
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techniques such as EBNF grammars, but those experiments made the template
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engine too slow. It's low-level because that's fastest.
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Writing the renderer
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~~~~~~~~~~~~~~~~~~~~
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The second step in writing custom tags is to define a ``Node`` subclass that
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has a ``render()`` method.
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Continuing the above example, we need to define ``CurrentTimeNode``::
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from django.core import template
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import datetime
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class CurrentTimeNode(template.Node):
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def __init__(self, format_string):
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self.format_string = format_string
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def render(self, context):
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return datetime.datetime.now().strftime(self.format_string)
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Notes:
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* ``__init__()`` gets the ``format_string`` from ``do_current_time()``.
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Always pass any options/parameters/arguments to a ``Node`` via its
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``__init__()``.
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* The ``render()`` method is where the work actually happens.
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Ultimately, this decoupling of compilation and rendering results in an
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efficient template system, because a template can render multiple context
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without having to be parsed multiple times.
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Registering the tag
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~~~~~~~~~~~~~~~~~~~
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Finally, use a ``register_tag`` call, as in ``register_filter`` above. Example::
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from django.core import template
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template.register_tag('cycle', do_cycle)
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``register_tag`` takes two arguments:
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* The name of the template tag -- a string
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* The compilation function -- a Python function (not the name of the
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function as a string)
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Setting a variable in the context
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The above example simply output a value. Generally, it's more flexible if your
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template tags set template variables instead of outputting values. That way,
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you allow template authors to reuse the values that your template tags create.
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To set a variable in the context, just use dictionary assignment on the context
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object in the ``render()`` method. Here's an updated version of
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``CurrentTimeNode`` that sets a template variable ``current_time`` instead of
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outputting it.
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class CurrentTimeNode2(template.Node):
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def __init__(self, format_string):
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self.format_string = format_string
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def render(self, context):
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context['current_time'] = datetime.datetime.now().strftime(self.format_string)
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return ''
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Note that ``render()`` returns the empty string. ``render()`` should always
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return string output. If all the template tag does is set a variable,
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``render()`` should return the empty string.
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Here's how you'd use this new version of the tag::
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{% current_time "%Y-%M-%d %I:%M %p" %}<p>The time is {{ current_time }}.</p>
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But, there's a naive problem with ``CurrentTimeNode2``: The variable name
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``current_time`` is hard-coded. This means you'll need to make sure your
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template doesn't use ``{{ current_time }}`` anywhere else, because the
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``{% current_time %}`` will blindly overwrite that variable's value. A cleaner
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solution is to make the template tag specify the name of the output variable,
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like so::
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{% get_current_time "%Y-%M-%d %I:%M %p" as my_current_time %}
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<p>The current time is {{ my_current_time }}.</p>
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To do that, you'll need to refactor both the compilation function and ``Node``
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class, like so::
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class CurrentTimeNode3(template.Node):
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def __init__(self, format_string, var_name):
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self.format_string = format_string
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self.var_name = var_name
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def render(self, context):
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context[self.var_name] = datetime.datetime.now().strftime(self.format_string)
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return ''
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import re
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def do_current_time(parser, token):
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# This version uses a regular expression to parse tag contents.
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try:
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# Splitting by None == splitting by spaces.
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tag_name, arg = token.contents.split(None, 1)
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except ValueError:
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raise template.TemplateSyntaxError, "%r tag requires arguments" % token.contents[0]
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m = re.search(r'(.*?) as (\w+)', arg)
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if not m:
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raise template.TemplateSyntaxError, "%r tag had invalid arguments" % tag_name
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format_string, var_name = m.groups()
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if not (format_string[0] == format_string[-1] and format_string[0] in ('"', "'")):
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raise template.TemplateSyntaxError, "%r tag's argument should be in quotes" % tag_name
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return CurrentTimeNode3(format_string[1:-1], var_name)
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The difference here is that ``do_current_time()`` grabs the format string and
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the variable name, passing both to ``CurrentTimeNode3``.
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