lib/django-0.96/docs/templates_python.txt

====================================================
The Django template language: For Python programmers
====================================================

This document explains the Django template system from a technical
perspective -- how it works and how to extend it. If you're just looking for
reference on the language syntax, see
`The Django template language: For template authors`_.

If you're looking to use the Django template system as part of another
application -- i.e., without the rest of the framework -- make sure to read
the `configuration`_ section later in this document.

.. _`The Django template language: For template authors`: ../templates/

Basics
======

A **template** is a text document, or a normal Python string, that is marked-up
using the Django template language. A template can contain **block tags** or
**variables**.

A **block tag** is a symbol within a template that does something.

This definition is deliberately vague. For example, a block tag can output
content, serve as a control structure (an "if" statement or "for" loop), grab
content from a database or enable access to other template tags.

Block tags are surrounded by ``"{%"`` and ``"%}"``.

Example template with block tags::

    {% if is_logged_in %}Thanks for logging in!{% else %}Please log in.{% endif %}

A **variable** is a symbol within a template that outputs a value.

Variable tags are surrounded by ``"{{"`` and ``"}}"``.

Example template with variables::

    My first name is {{ first_name }}. My last name is {{ last_name }}.

A **context** is a "variable name" -> "variable value" mapping that is passed
to a template.

A template **renders** a context by replacing the variable "holes" with values
from the context and executing all block tags.

Using the template system
=========================

Using the template system in Python is a two-step process:

    * First, you compile the raw template code into a ``Template`` object.
    * Then, you call the ``render()`` method of the ``Template`` object with a
      given context.

Compiling a string
------------------

The easiest way to create a ``Template`` object is by instantiating it
directly. The class lives at ``django.template.Template``. The constructor
takes one argument -- the raw template code::

    >>> from django.template import Template
    >>> t = Template("My name is {{ my_name }}.")
    >>> print t
    <django.template.Template instance>

.. admonition:: Behind the scenes

    The system only parses your raw template code once -- when you create the
    ``Template`` object. From then on, it's stored internally as a "node"
    structure for performance.

    Even the parsing itself is quite fast. Most of the parsing happens via a
    single call to a single, short, regular expression.

Rendering a context
-------------------

Once you have a compiled ``Template`` object, you can render a context -- or
multiple contexts -- with it. The ``Context`` class lives at
``django.template.Context``, and the constructor takes one (optional)
argument: a dictionary mapping variable names to variable values. Call the
``Template`` object's ``render()`` method with the context to "fill" the
template::

    >>> from django.template import Context, Template
    >>> t = Template("My name is {{ my_name }}.")

    >>> c = Context({"my_name": "Adrian"})
    >>> t.render(c)
    "My name is Adrian."

    >>> c = Context({"my_name": "Dolores"})
    >>> t.render(c)
    "My name is Dolores."

Variable names must consist of any letter (A-Z), any digit (0-9), an underscore
or a dot.

Dots have a special meaning in template rendering. A dot in a variable name
signifies **lookup**. Specifically, when the template system encounters a dot
in a variable name, it tries the following lookups, in this order:

    * Dictionary lookup. Example: ``foo["bar"]``
    * Attribute lookup. Example: ``foo.bar``
    * Method call. Example: ``foo.bar()``
    * List-index lookup. Example: ``foo[bar]``

The template system uses the first lookup type that works. It's short-circuit
logic.

Here are a few examples::

    >>> from django.template import Context, Template
    >>> t = Template("My name is {{ person.first_name }}.")
    >>> d = {"person": {"first_name": "Joe", "last_name": "Johnson"}}
    >>> t.render(Context(d))
    "My name is Joe."

    >>> class PersonClass: pass
    >>> p = PersonClass()
    >>> p.first_name = "Ron"
    >>> p.last_name = "Nasty"
    >>> t.render(Context({"person": p}))
    "My name is Ron."

    >>> class PersonClass2:
    ...     def first_name(self):
    ...         return "Samantha"
    >>> p = PersonClass2()
    >>> t.render(Context({"person": p}))
    "My name is Samantha."

    >>> t = Template("The first stooge in the list is {{ stooges.0 }}.")
    >>> c = Context({"stooges": ["Larry", "Curly", "Moe"]})
    >>> t.render(c)
    "The first stooge in the list is Larry."

Method lookups are slightly more complex than the other lookup types. Here are
some things to keep in mind:

    * If, during the method lookup, a method raises an exception, the exception
      will be propagated, unless the exception has an attribute
      ``silent_variable_failure`` whose value is ``True``. If the exception
      *does* have a ``silent_variable_failure`` attribute, the variable will
      render as an empty string. Example::

        >>> t = Template("My name is {{ person.first_name }}.")
        >>> class PersonClass3:
        ...     def first_name(self):
        ...         raise AssertionError, "foo"
        >>> p = PersonClass3()
        >>> t.render(Context({"person": p}))
        Traceback (most recent call last):
        ...
        AssertionError: foo

        >>> class SilentAssertionError(Exception):
        ...     silent_variable_failure = True
        >>> class PersonClass4:
        ...     def first_name(self):
        ...         raise SilentAssertionError
        >>> p = PersonClass4()
        >>> t.render(Context({"person": p}))
        "My name is ."

      Note that ``django.core.exceptions.ObjectDoesNotExist``, which is the
      base class for all Django database API ``DoesNotExist`` exceptions, has
      ``silent_variable_failure = True``. So if you're using Django templates
      with Django model objects, any ``DoesNotExist`` exception will fail
      silently.

    * A method call will only work if the method has no required arguments.
      Otherwise, the system will move to the next lookup type (list-index
      lookup).

    * Obviously, some methods have side effects, and it'd be either foolish or
      a security hole to allow the template system to access them.

      A good example is the ``delete()`` method on each Django model object.
      The template system shouldn't be allowed to do something like this::

        I will now delete this valuable data. {{ data.delete }}

      To prevent this, set a function attribute ``alters_data`` on the method.
      The template system won't execute a method if the method has
      ``alters_data=True`` set. The dynamically-generated ``delete()`` and
      ``save()`` methods on Django model objects get ``alters_data=True``
      automatically. Example::

        def sensitive_function(self):
            self.database_record.delete()
        sensitive_function.alters_data = True

How invalid variables are handled
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Generally, if a variable doesn't exist, the template system inserts the
value of the ``TEMPLATE_STRING_IF_INVALID`` setting, which is set to ``''``
(the empty string) by default.

Filters that are applied to an invalid variable will only be applied if
``TEMPLATE_STRING_IF_INVALID`` is set to ``''`` (the empty string). If
``TEMPLATE_STRING_IF_INVALID`` is set to any other value, variable
filters will be ignored.

This behavior is slightly different for the ``if``, ``for`` and ``regroup``
template tags. If an invalid variable is provided to one of these template
tags, the variable will be interpreted as ``None``. Filters are always
applied to invalid variables within these template tags.

.. admonition:: For debug purposes only!

    While ``TEMPLATE_STRING_IF_INVALID`` can be a useful debugging tool, 
    it is a bad idea to turn it on as a 'development default'. 
    
    Many templates, including those in the Admin site, rely upon the 
    silence of the template system when a non-existent variable is 
    encountered. If you assign a value other than ``''`` to
    ``TEMPLATE_STRING_IF_INVALID``, you will experience rendering 
    problems with these templates and sites.
    
    Generally, ``TEMPLATE_STRING_IF_INVALID`` should only be enabled 
    in order to debug a specific template problem, then cleared 
    once debugging is complete.
        
Playing with Context objects
----------------------------

Most of the time, you'll instantiate ``Context`` objects by passing in a
fully-populated dictionary to ``Context()``. But you can add and delete items
from a ``Context`` object once it's been instantiated, too, using standard
dictionary syntax::

    >>> c = Context({"foo": "bar"})
    >>> c['foo']
    'bar'
    >>> del c['foo']
    >>> c['foo']
    ''
    >>> c['newvariable'] = 'hello'
    >>> c['newvariable']
    'hello'

A ``Context`` object is a stack. That is, you can ``push()`` and ``pop()`` it.
If you ``pop()`` too much, it'll raise
``django.template.ContextPopException``::

    >>> c = Context()
    >>> c['foo'] = 'first level'
    >>> c.push()
    >>> c['foo'] = 'second level'
    >>> c['foo']
    'second level'
    >>> c.pop()
    >>> c['foo']
    'first level'
    >>> c['foo'] = 'overwritten'
    >>> c['foo']
    'overwritten'
    >>> c.pop()
    Traceback (most recent call last):
    ...
    django.template.ContextPopException

Using a ``Context`` as a stack comes in handy in some custom template tags, as
you'll see below.

Subclassing Context: RequestContext
-----------------------------------

Django comes with a special ``Context`` class,
``django.template.RequestContext``, that acts slightly differently than
the normal ``django.template.Context``. The first difference is that takes
an `HttpRequest object`_ as its first argument. For example::

    c = RequestContext(request, {
        'foo': 'bar',
    }

The second difference is that it automatically populates the context with a few
variables, according to your `TEMPLATE_CONTEXT_PROCESSORS setting`_.

The ``TEMPLATE_CONTEXT_PROCESSORS`` setting is a tuple of callables -- called
**context processors** -- that take a request object as their argument and
return a dictionary of items to be merged into the context. By default,
``TEMPLATE_CONTEXT_PROCESSORS`` is set to::

    ("django.core.context_processors.auth",
    "django.core.context_processors.debug",
    "django.core.context_processors.i18n")

Each processor is applied in order. That means, if one processor adds a
variable to the context and a second processor adds a variable with the same
name, the second will override the first. The default processors are explained
below.

Also, you can give ``RequestContext`` a list of additional processors, using the
optional, third positional argument, ``processors``. In this example, the
``RequestContext`` instance gets a ``ip_address`` variable::

    def ip_address_processor(request):
        return {'ip_address': request.META['REMOTE_ADDR']}

    def some_view(request):
        # ...
        return RequestContext(request, {
            'foo': 'bar',
        }, [ip_address_processor])

Note::
    If you're using Django's ``render_to_response()`` shortcut to populate a
    template with the contents of a dictionary, your template will be passed a
    ``Context`` instance by default (not a ``RequestContext``). To use a
    ``RequestContext`` in your template rendering, pass an optional third
    argument to ``render_to_response()``: a ``RequestContext``
    instance. Your code might look like this::

        def some_view(request):
            # ...
            return render_to_response('my_template.html',
                                      my_data_dictionary,
                                      context_instance=RequestContext(request))

Here's what each of the default processors does:

.. _HttpRequest object: ../request_response/#httprequest-objects
.. _TEMPLATE_CONTEXT_PROCESSORS setting: ../settings/#template-context-processors

django.core.context_processors.auth
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If ``TEMPLATE_CONTEXT_PROCESSORS`` contains this processor, every
``RequestContext`` will contain these three variables:

    * ``user`` -- An ``auth.User`` instance representing the currently
      logged-in user (or an ``AnonymousUser`` instance, if the client isn't
      logged in). See the `user authentication docs`.

    * ``messages`` -- A list of messages (as strings) for the currently
      logged-in user. Behind the scenes, this calls
      ``request.user.get_and_delete_messages()`` for every request. That method
      collects the user's messages and deletes them from the database.

      Note that messages are set with ``user.add_message()``. See the
      `message docs`_ for more.

    * ``perms`` -- An instance of
      ``django.core.context_processors.PermWrapper``, representing the
      permissions that the currently logged-in user has. See the `permissions
      docs`_.

.. _user authentication docs: ../authentication/#users
.. _message docs: ../authentication/#messages
.. _permissions docs: ../authentication/#permissions

django.core.context_processors.debug
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If ``TEMPLATE_CONTEXT_PROCESSORS`` contains this processor, every
``RequestContext`` will contain these two variables -- but only if your
``DEBUG`` setting is set to ``True`` and the request's IP address
(``request.META['REMOTE_ADDR']``) is in the ``INTERNAL_IPS`` setting:

    * ``debug`` -- ``True``. You can use this in templates to test whether
      you're in ``DEBUG`` mode.
    * ``sql_queries`` -- A list of ``{'sql': ..., 'time': ...}`` dictionaries,
      representing every SQL query that has happened so far during the request
      and how long it took. The list is in order by query.

django.core.context_processors.i18n
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If ``TEMPLATE_CONTEXT_PROCESSORS`` contains this processor, every
``RequestContext`` will contain these two variables:

    * ``LANGUAGES`` -- The value of the `LANGUAGES setting`_.
    * ``LANGUAGE_CODE`` -- ``request.LANGUAGE_CODE``, if it exists. Otherwise,
      the value of the `LANGUAGE_CODE setting`_.

See the `internationalization docs`_ for more.

.. _LANGUAGES setting: ../settings/#languages
.. _LANGUAGE_CODE setting: ../settings/#language-code
.. _internationalization docs: ../i18n/

django.core.context_processors.request
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If ``TEMPLATE_CONTEXT_PROCESSORS`` contains this processor, every
``RequestContext`` will contain a variable ``request``, which is the current
`HttpRequest object`_. Note that this processor is not enabled by default;
you'll have to activate it.

Writing your own context processors
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A context processor has a very simple interface: It's just a Python function
that takes one argument, an ``HttpRequest`` object, and returns a dictionary
that gets added to the template context. Each context processor *must* return
a dictionary.

Custom context processors can live anywhere in your code base. All Django cares
about is that your custom context processors are pointed-to by your
``TEMPLATE_CONTEXT_PROCESSORS`` setting.

Loading templates
-----------------

Generally, you'll store templates in files on your filesystem rather than using
the low-level ``Template`` API yourself. Save templates in a directory
specified as a **template directory**.

Django searches for template directories in a number of places, depending on
your template-loader settings (see "Loader types" below), but the most basic
way of specifying template directories is by using the ``TEMPLATE_DIRS``
setting.

The TEMPLATE_DIRS setting
~~~~~~~~~~~~~~~~~~~~~~~~~

Tell Django what your template directories are by using the ``TEMPLATE_DIRS``
setting in your settings file. This should be set to a list or tuple of strings
that contain full paths to your template directory(ies). Example::

    TEMPLATE_DIRS = (
        "/home/html/templates/lawrence.com",
        "/home/html/templates/default",
    )

Your templates can go anywhere you want, as long as the directories and
templates are readable by the Web server. They can have any extension you want,
such as ``.html`` or ``.txt``, or they can have no extension at all.

Note that these paths should use Unix-style forward slashes, even on Windows.

The Python API
~~~~~~~~~~~~~~

Django has two ways to load templates from files:

``django.template.loader.get_template(template_name)``
    ``get_template`` returns the compiled template (a ``Template`` object) for
    the template with the given name. If the template doesn't exist, it raises
    ``django.template.TemplateDoesNotExist``.

``django.template.loader.select_template(template_name_list)``
    ``select_template`` is just like ``get_template``, except it takes a list
    of template names. Of the list, it returns the first template that exists.

For example, if you call ``get_template('story_detail.html')`` and have the
above ``TEMPLATE_DIRS`` setting, here are the files Django will look for, in
order:

    * ``/home/html/templates/lawrence.com/story_detail.html``
    * ``/home/html/templates/default/story_detail.html``

If you call ``select_template(['story_253_detail.html', 'story_detail.html'])``,
here's what Django will look for:

    * ``/home/html/templates/lawrence.com/story_253_detail.html``
    * ``/home/html/templates/default/story_253_detail.html``
    * ``/home/html/templates/lawrence.com/story_detail.html``
    * ``/home/html/templates/default/story_detail.html``

When Django finds a template that exists, it stops looking.

.. admonition:: Tip

    You can use ``select_template()`` for super-flexible "templatability." For
    example, if you've written a news story and want some stories to have
    custom templates, use something like
    ``select_template(['story_%s_detail.html' % story.id, 'story_detail.html'])``.
    That'll allow you to use a custom template for an individual story, with a
    fallback template for stories that don't have custom templates.

Using subdirectories
~~~~~~~~~~~~~~~~~~~~

It's possible -- and preferable -- to organize templates in subdirectories of
the template directory. The convention is to make a subdirectory for each
Django app, with subdirectories within those subdirectories as needed.

Do this for your own sanity. Storing all templates in the root level of a
single directory gets messy.

To load a template that's within a subdirectory, just use a slash, like so::

    get_template('news/story_detail.html')

Using the same ``TEMPLATE_DIRS`` setting from above, this example
``get_template()`` call will attempt to load the following templates:

    * ``/home/html/templates/lawrence.com/news/story_detail.html``
    * ``/home/html/templates/default/news/story_detail.html``

Loader types
~~~~~~~~~~~~

By default, Django uses a filesystem-based template loader, but Django comes
with a few other template loaders, which know how to load templates from other
sources.

These other loaders are disabled by default, but you can activate them by
editing your ``TEMPLATE_LOADERS`` setting. ``TEMPLATE_LOADERS`` should be a
tuple of strings, where each string represents a template loader. Here are the
template loaders that come with Django:

``django.template.loaders.filesystem.load_template_source``
    Loads templates from the filesystem, according to ``TEMPLATE_DIRS``.

``django.template.loaders.app_directories.load_template_source``
    Loads templates from Django apps on the filesystem. For each app in
    ``INSTALLED_APPS``, the loader looks for a ``templates`` subdirectory. If
    the directory exists, Django looks for templates in there.

    This means you can store templates with your individual apps. This also
    makes it easy to distribute Django apps with default templates.

    For example, for this setting::

        INSTALLED_APPS = ('myproject.polls', 'myproject.music')

    ...then ``get_template('foo.html')`` will look for templates in these
    directories, in this order:

        * ``/path/to/myproject/polls/templates/foo.html``
        * ``/path/to/myproject/music/templates/foo.html``

    Note that the loader performs an optimization when it is first imported:
    It caches a list of which ``INSTALLED_APPS`` packages have a ``templates``
    subdirectory.

``django.template.loaders.eggs.load_template_source``
    Just like ``app_directories`` above, but it loads templates from Python
    eggs rather than from the filesystem.

Django uses the template loaders in order according to the ``TEMPLATE_LOADERS``
setting. It uses each loader until a loader finds a match.

Extending the template system
=============================

Although the Django template language comes with several default tags and
filters, you might want to write your own. It's easy to do.

First, create a ``templatetags`` package in the appropriate Django app's
package. It should be on the same level as ``models.py``, ``views.py``, etc. For
example::

    polls/
        models.py
        templatetags/
        views.py

Add two files to the ``templatetags`` package: an ``__init__.py`` file and a
file that will contain your custom tag/filter definitions. The name of the
latter file is the name you'll use to load the tags later. For example, if your
custom tags/filters are in a file called ``poll_extras.py``, you'd do the
following in a template::

    {% load poll_extras %}

The ``{% load %}`` tag looks at your ``INSTALLED_APPS`` setting and only allows
the loading of template libraries within installed Django apps. This is a
security feature: It allows you to host Python code for many template libraries
on a single computer without enabling access to all of them for every Django
installation.

If you write a template library that isn't tied to any particular models/views,
it's perfectly OK to have a Django app package that only contains a
``templatetags`` package.

There's no limit on how many modules you put in the ``templatetags`` package.
Just keep in mind that a ``{% load %}`` statement will load tags/filters for
the given Python module name, not the name of the app.

Once you've created that Python module, you'll just have to write a bit of
Python code, depending on whether you're writing filters or tags.

To be a valid tag library, the module contain a module-level variable named
``register`` that is a ``template.Library`` instance, in which all the tags and
filters are registered. So, near the top of your module, put the following::

    from django import template

    register = template.Library()

.. admonition:: Behind the scenes

    For a ton of examples, read the source code for Django's default filters
    and tags. They're in ``django/template/defaultfilters.py`` and
    ``django/template/defaulttags.py``, respectively.

Writing custom template filters
-------------------------------

Custom filters are just Python functions that take one or two arguments:

    * The value of the variable (input) -- not necessarily a string.
    * The value of the argument -- this can have a default value, or be left
      out altogether.

For example, in the filter ``{{ var|foo:"bar" }}``, the filter ``foo`` would be
passed the variable ``var`` and the argument ``"bar"``.

Filter functions should always return something. They shouldn't raise
exceptions. They should fail silently. In case of error, they should return
either the original input or an empty string -- whichever makes more sense.

Here's an example filter definition::

    def cut(value, arg):
        "Removes all values of arg from the given string"
        return value.replace(arg, '')

And here's an example of how that filter would be used::

    {{ somevariable|cut:"0" }}

Most filters don't take arguments. In this case, just leave the argument out of
your function. Example::

    def lower(value): # Only one argument.
        "Converts a string into all lowercase"
        return value.lower()

When you've written your filter definition, you need to register it with
your ``Library`` instance, to make it available to Django's template language::

    register.filter('cut', cut)
    register.filter('lower', lower)

The ``Library.filter()`` method takes two arguments:

    1. The name of the filter -- a string.
    2. The compilation function -- a Python function (not the name of the
       function as a string).

If you're using Python 2.4 or above, you can use ``register.filter()`` as a
decorator instead::

    @register.filter(name='cut')
    def cut(value, arg):
        return value.replace(arg, '')

    @register.filter
    def lower(value):
        return value.lower()

If you leave off the ``name`` argument, as in the second example above, Django
will use the function's name as the filter name.

Template filters which expect strings
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you are writing a template filter which only expects a string as the first
argument, you should use the included decorator ``stringfilter`` which will convert
an object to it's string value before being passed to your function::

    from django import template

    @template.stringfilter
    def lower(value):
    	return value.lower()

Writing custom template tags
----------------------------

Tags are more complex than filters, because tags can do anything.

A quick overview
~~~~~~~~~~~~~~~~

Above, this document explained that the template system works in a two-step
process: compiling and rendering. To define a custom template tag, you specify
how the compilation works and how the rendering works.

When Django compiles a template, it splits the raw template text into
''nodes''. Each node is an instance of ``django.template.Node`` and has
a ``render()`` method. A compiled template is, simply, a list of ``Node``
objects. When you call ``render()`` on a compiled template object, the template
calls ``render()`` on each ``Node`` in its node list, with the given context.
The results are all concatenated together to form the output of the template.

Thus, to define a custom template tag, you specify how the raw template tag is
converted into a ``Node`` (the compilation function), and what the node's
``render()`` method does.

Writing the compilation function
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For each template tag the template parser encounters, it calls a Python
function with the tag contents and the parser object itself. This function is
responsible for returning a ``Node`` instance based on the contents of the tag.

For example, let's write a template tag, ``{% current_time %}``, that displays
the current date/time, formatted according to a parameter given in the tag, in
`strftime syntax`_. It's a good idea to decide the tag syntax before anything
else. In our case, let's say the tag should be used like this::

    <p>The time is {% current_time "%Y-%m-%d %I:%M %p" %}.</p>

.. _`strftime syntax`: http://www.python.org/doc/current/lib/module-time.html#l2h-1941

The parser for this function should grab the parameter and create a ``Node``
object::

    from django import template
    def do_current_time(parser, token):
        try:
            # split_contents() knows not to split quoted strings.
            tag_name, format_string = token.split_contents()
        except ValueError:
            raise template.TemplateSyntaxError, "%r tag requires a single argument" % token.contents[0]
        if not (format_string[0] == format_string[-1] and format_string[0] in ('"', "'")):
            raise template.TemplateSyntaxError, "%r tag's argument should be in quotes" % tag_name
        return CurrentTimeNode(format_string[1:-1])

Notes:

    * ``parser`` is the template parser object. We don't need it in this
      example.

    * ``token.contents`` is a string of the raw contents of the tag. In our
      example, it's ``'current_time "%Y-%m-%d %I:%M %p"'``.

    * The ``token.split_contents()`` method separates the arguments on spaces
      while keeping quoted strings together. The more straightforward
      ``token.contents.split()`` wouldn't be as robust, as it would naively
      split on *all* spaces, including those within quoted strings. It's a good
      idea to always use ``token.split_contents()``.

    * This function is responsible for raising
      ``django.template.TemplateSyntaxError``, with helpful messages, for
      any syntax error.

    * The ``TemplateSyntaxError`` exceptions use the ``tag_name`` variable.
      Don't hard-code the tag's name in your error messages, because that
      couples the tag's name to your function. ``token.contents.split()[0]``
      will ''always'' be the name of your tag -- even when the tag has no
      arguments.

    * The function returns a ``CurrentTimeNode`` with everything the node needs
      to know about this tag. In this case, it just passes the argument --
      ``"%Y-%m-%d %I:%M %p"``. The leading and trailing quotes from the
      template tag are removed in ``format_string[1:-1]``.

    * The parsing is very low-level. The Django developers have experimented
      with writing small frameworks on top of this parsing system, using
      techniques such as EBNF grammars, but those experiments made the template
      engine too slow. It's low-level because that's fastest.

Writing the renderer
~~~~~~~~~~~~~~~~~~~~

The second step in writing custom tags is to define a ``Node`` subclass that
has a ``render()`` method.

Continuing the above example, we need to define ``CurrentTimeNode``::

    from django import template
    import datetime
    class CurrentTimeNode(template.Node):
        def __init__(self, format_string):
            self.format_string = format_string
        def render(self, context):
            return datetime.datetime.now().strftime(self.format_string)

Notes:

    * ``__init__()`` gets the ``format_string`` from ``do_current_time()``.
      Always pass any options/parameters/arguments to a ``Node`` via its
      ``__init__()``.

    * The ``render()`` method is where the work actually happens.

    * ``render()`` should never raise ``TemplateSyntaxError`` or any other
      exception. It should fail silently, just as template filters should.

Ultimately, this decoupling of compilation and rendering results in an
efficient template system, because a template can render multiple context
without having to be parsed multiple times.

Registering the tag
~~~~~~~~~~~~~~~~~~~

Finally, register the tag with your module's ``Library`` instance, as explained
in "Writing custom template filters" above. Example::

    register.tag('current_time', do_current_time)

The ``tag()`` method takes two arguments:

    1. The name of the template tag -- a string. If this is left out, the
       name of the compilation function will be used.
    2. The compilation function -- a Python function (not the name of the
       function as a string).

As with filter registration, it is also possible to use this as a decorator, in
Python 2.4 and above::

    @register.tag(name="current_time")
    def do_current_time(parser, token):
        # ...

    @register.tag
    def shout(parser, token):
        # ...

If you leave off the ``name`` argument, as in the second example above, Django
will use the function's name as the tag name.

Passing template variables to the tag
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Although you can pass any number of arguments to a template tag using
``token.split_contents()``, the arguments are all unpacked as
string literals. A little more work is required in order to dynamic content (a
template variable) to a template tag as an argument.

While the previous examples have formatted the current time into a string and
returned the string, suppose you wanted to pass in a ``DateTimeField`` from an
object and have the template tag format that date-time::

    <p>This post was last updated at {% format_time blog_entry.date_updated "%Y-%m-%d %I:%M %p" %}.</p>

Initially, ``token.split_contents()`` will return three values:

    1. The tag name ``format_time``.
    2. The string "blog_entry.date_updated" (without the surrounding quotes).
    3. The formatting string "%Y-%m-%d %I:%M %p". The return value from
       ``split_contents()`` will include the leading and trailing quotes for
       string literals like this.

Now your tag should begin to look like this::

    from django import template
    def do_format_time(parser, token):
        try:
            # split_contents() knows not to split quoted strings.
            tag_name, date_to_be_formatted, format_string = token.split_contents()
        except ValueError:
            raise template.TemplateSyntaxError, "%r tag requires exactly two arguments" % token.contents[0]
        if not (format_string[0] == format_string[-1] and format_string[0] in ('"', "'")):
            raise template.TemplateSyntaxError, "%r tag's argument should be in quotes" % tag_name
        return FormatTimeNode(date_to_be_formatted, format_string[1:-1])

You also have to change the renderer to retrieve the actual contents of the
``date_updated`` property of the ``blog_entry`` object.  This can be
accomplished by using the ``resolve_variable()`` function in
``django.template``. You pass ``resolve_variable()`` the variable name and the
current context, available in the ``render`` method::

    from django import template
    from django.template import resolve_variable
    import datetime
    class FormatTimeNode(template.Node):
        def __init__(self, date_to_be_formatted, format_string):
            self.date_to_be_formatted = date_to_be_formatted
            self.format_string = format_string
        
        def render(self, context):
            try:
                actual_date = resolve_variable(self.date_to_be_formatted, context)
                return actual_date.strftime(self.format_string)
            except VariableDoesNotExist:
                return ''

``resolve_variable`` will try to resolve ``blog_entry.date_updated`` and then
format it accordingly.

.. note::
    The ``resolve_variable()`` function will throw a ``VariableDoesNotExist``
    exception if it cannot resolve the string passed to it in the current
    context of the page.

Shortcut for simple tags
~~~~~~~~~~~~~~~~~~~~~~~~

Many template tags take a number of arguments -- strings or a template variables
-- and return a string after doing some processing based solely on
the input argument and some external information. For example, the
``current_time`` tag we wrote above is of this variety: we give it a format
string, it returns the time as a string.

To ease the creation of the types of tags, Django provides a helper function,
``simple_tag``. This function, which is a method of
``django.template.Library``, takes a function that accepts any number of
arguments, wraps it in a ``render`` function and the other necessary bits
mentioned above and registers it with the template system.

Our earlier ``current_time`` function could thus be written like this::

    def current_time(format_string):
        return datetime.datetime.now().strftime(format_string)

    register.simple_tag(current_time)

In Python 2.4, the decorator syntax also works::

    @register.simple_tag
    def current_time(token):
        ...

A couple of things to note about the ``simple_tag`` helper function:
    * Checking for the required number of arguments, etc, has already been
      done by the time our function is called, so we don't need to do that.
    * The quotes around the argument (if any) have already been stripped away,
      so we just receive a plain string.
    * If the argument was a template variable, our function is passed the
      current value of the variable, not the variable itself.

When your template tag does not need access to the current context, writing a
function to work with the input values and using the ``simple_tag`` helper is
the easiest way to create a new tag.

Inclusion tags
~~~~~~~~~~~~~~

Another common type of template tag is the type that displays some data by
rendering *another* template. For example, Django's admin interface uses custom
template tags to display the buttons along the bottom of the "add/change" form
pages. Those buttons always look the same, but the link targets change depending
on the object being edited -- so they're a perfect case for using a small
template that is filled with details from the current object. (In the admin's
case, this is the ``submit_row`` tag.)

These sorts of tags are called `inclusion tags`.

Writing inclusion tags is probably best demonstrated by example. Let's write a
tag that outputs a list of choices for a given ``Poll`` object, such as was
created in the tutorials_. We'll use the tag like this::

    {% show_results poll %}

...and the output will be something like this::

    <ul>
      <li>First choice</li>
      <li>Second choice</li>
      <li>Third choice</li>
    </ul>

First, define the function that takes the argument and produces a dictionary of
data for the result. The important point here is we only need to return a
dictionary, not anything more complex. This will be used as a template context
for the template fragment. Example::

    def show_results(poll):
        choices = poll.choice_set.all()
        return {'choices': choices}

Next, create the template used to render the tag's output. This template is a
fixed feature of the tag: the tag writer specifies it, not the template
designer. Following our example, the template is very simple::

    <ul>
    {% for choice in choices %}
        <li> {{ choice }} </li>
    {% endfor %}
    </ul>

Now, create and register the inclusion tag by calling the ``inclusion_tag()``
method on a ``Library`` object. Following our example, if the above template is
in a file called ``results.html`` in a directory that's searched by the template
loader, we'd register the tag like this::

    # Here, register is a django.template.Library instance, as before
    register.inclusion_tag('results.html')(show_results)

As always, Python 2.4 decorator syntax works as well, so we could have
written::

    @register.inclusion_tag('results.html')
    def show_results(poll):
        ...

...when first creating the function.

Sometimes, your inclusion tags might require a large number of arguments,
making it a pain for template authors to pass in all the arguments and remember
their order. To solve this, Django provides a ``takes_context`` option for
inclusion tags. If you specify ``takes_context`` in creating a template tag,
the tag will have no required arguments, and the underlying Python function
will have one argument -- the template context as of when the tag was called.

For example, say you're writing an inclusion tag that will always be used in a
context that contains ``home_link`` and ``home_title`` variables that point
back to the main page. Here's what the Python function would look like::

    # The first argument *must* be called "context" here.
    def jump_link(context):
        return {
            'link': context['home_link'],
            'title': context['home_title'],
        }
    # Register the custom tag as an inclusion tag with takes_context=True.
    register.inclusion_tag('link.html', takes_context=True)(jump_link)

(Note that the first parameter to the function *must* be called ``context``.)

In that ``register.inclusion_tag()`` line, we specified ``takes_context=True``
and the name of the template. Here's what the template ``link.html`` might look
like::

    Jump directly to <a href="{{ link }}">{{ title }}</a>.

Then, any time you want to use that custom tag, load its library and call it
without any arguments, like so::

    {% jump_link %}

Note that when you're using ``takes_context=True``, there's no need to pass
arguments to the template tag. It automatically gets access to the context.

The ``takes_context`` parameter defaults to ``False``. When it's set to *True*,
the tag is passed the context object, as in this example. That's the only
difference between this case and the previous ``inclusion_tag`` example.

.. _tutorials: ../tutorial1/#creating-models

Setting a variable in the context
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The above example simply output a value. Generally, it's more flexible if your
template tags set template variables instead of outputting values. That way,
template authors can reuse the values that your template tags create.

To set a variable in the context, just use dictionary assignment on the context
object in the ``render()`` method. Here's an updated version of
``CurrentTimeNode`` that sets a template variable ``current_time`` instead of
outputting it::

    class CurrentTimeNode2(template.Node):
        def __init__(self, format_string):
            self.format_string = format_string
        def render(self, context):
            context['current_time'] = datetime.datetime.now().strftime(self.format_string)
            return ''

Note that ``render()`` returns the empty string. ``render()`` should always
return string output. If all the template tag does is set a variable,
``render()`` should return the empty string.

Here's how you'd use this new version of the tag::

    {% current_time "%Y-%M-%d %I:%M %p" %}<p>The time is {{ current_time }}.</p>

But, there's a problem with ``CurrentTimeNode2``: The variable name
``current_time`` is hard-coded. This means you'll need to make sure your
template doesn't use ``{{ current_time }}`` anywhere else, because the
``{% current_time %}`` will blindly overwrite that variable's value. A cleaner
solution is to make the template tag specify the name of the output variable,
like so::

    {% get_current_time "%Y-%M-%d %I:%M %p" as my_current_time %}
    <p>The current time is {{ my_current_time }}.</p>

To do that, you'll need to refactor both the compilation function and ``Node``
class, like so::

    class CurrentTimeNode3(template.Node):
        def __init__(self, format_string, var_name):
            self.format_string = format_string
            self.var_name = var_name
        def render(self, context):
            context[self.var_name] = datetime.datetime.now().strftime(self.format_string)
            return ''

    import re
    def do_current_time(parser, token):
        # This version uses a regular expression to parse tag contents.
        try:
            # Splitting by None == splitting by spaces.
            tag_name, arg = token.contents.split(None, 1)
        except ValueError:
            raise template.TemplateSyntaxError, "%r tag requires arguments" % token.contents[0]
        m = re.search(r'(.*?) as (\w+)', arg)
        if not m:
            raise template.TemplateSyntaxError, "%r tag had invalid arguments" % tag_name
        format_string, var_name = m.groups()
        if not (format_string[0] == format_string[-1] and format_string[0] in ('"', "'")):
            raise template.TemplateSyntaxError, "%r tag's argument should be in quotes" % tag_name
        return CurrentTimeNode3(format_string[1:-1], var_name)

The difference here is that ``do_current_time()`` grabs the format string and
the variable name, passing both to ``CurrentTimeNode3``.

Parsing until another block tag
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Template tags can work in tandem. For instance, the standard ``{% comment %}``
tag hides everything until ``{% endcomment %}``. To create a template tag such
as this, use ``parser.parse()`` in your compilation function.

Here's how the standard ``{% comment %}`` tag is implemented::

    def do_comment(parser, token):
        nodelist = parser.parse(('endcomment',))
        parser.delete_first_token()
        return CommentNode()

    class CommentNode(template.Node):
        def render(self, context):
            return ''

``parser.parse()`` takes a tuple of names of block tags ''to parse until''. It
returns an instance of ``django.template.NodeList``, which is a list of
all ``Node`` objects that the parser encountered ''before'' it encountered
any of the tags named in the tuple.

In ``"nodelist = parser.parse(('endcomment',))"`` in the above example,
``nodelist`` is a list of all nodes between the ``{% comment %}`` and
``{% endcomment %}``, not counting ``{% comment %}`` and ``{% endcomment %}``
themselves.

After ``parser.parse()`` is called, the parser hasn't yet "consumed" the
``{% endcomment %}`` tag, so the code needs to explicitly call
``parser.delete_first_token()``.

``CommentNode.render()`` simply returns an empty string. Anything between
``{% comment %}`` and ``{% endcomment %}`` is ignored.

Parsing until another block tag, and saving contents
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In the previous example, ``do_comment()`` discarded everything between
``{% comment %}`` and ``{% endcomment %}``. Instead of doing that, it's
possible to do something with the code between block tags.

For example, here's a custom template tag, ``{% upper %}``, that capitalizes
everything between itself and ``{% endupper %}``.

Usage::

    {% upper %}This will appear in uppercase, {{ your_name }}.{% endupper %}

As in the previous example, we'll use ``parser.parse()``. But this time, we
pass the resulting ``nodelist`` to the ``Node``::

    def do_upper(parser, token):
        nodelist = parser.parse(('endupper',))
        parser.delete_first_token()
        return UpperNode(nodelist)

    class UpperNode(template.Node):
        def __init__(self, nodelist):
            self.nodelist = nodelist
        def render(self, context):
            output = self.nodelist.render(context)
            return output.upper()

The only new concept here is the ``self.nodelist.render(context)`` in
``UpperNode.render()``.

For more examples of complex rendering, see the source code for ``{% if %}``,
``{% for %}``, ``{% ifequal %}`` and ``{% ifchanged %}``. They live in
``django/template/defaulttags.py``.

.. _configuration:

Configuring the template system in standalone mode
==================================================

.. note::

    This section is only of interest to people trying to use the template
    system as an output component in another application. If you're using the
    template system as part of a Django application, nothing here applies to
    you.

Normally, Django will load all the configuration information it needs from its
own default configuration file, combined with the settings in the module given
in the ``DJANGO_SETTINGS_MODULE`` environment variable. But if you're using the
template system independently of the rest of Django, the environment variable
approach isn't very convenient, because you probably want to configure the
template system in line with the rest of your application rather than dealing
with settings files and pointing to them via environment variables.

To solve this problem, you need to use the manual configuration option
described in the `settings file`_ documentation. Simply import the appropriate
pieces of the templating system and then, *before* you call any of the
templating functions, call ``django.conf.settings.configure()`` with any
settings you wish to specify. You might want to consider setting at least
``TEMPLATE_DIRS`` (if you're going to use template loaders),
``DEFAULT_CHARSET`` (although the default of ``utf-8`` is probably fine) and
``TEMPLATE_DEBUG``. All available settings are described in the
`settings documentation`_, and any setting starting with *TEMPLATE_*
is of obvious interest.

.. _settings file: ../settings/#using-settings-without-the-django-settings-module-environment-variable
.. _settings documentation: ../settings/