django/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`_.

.. _`The Django template language: For template authors`: http://www.djangoproject.com/documentation/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.core.template.Template``. The constructor
takes one argument -- the raw template code::

    >>> from django.core.template import Template
    >>> t = Template("My name is {{ my_name }}.")
    >>> print t
    <django.core.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.core.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.core.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.core.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 subclasses
      ``django.core.template.SilentVariableFailure``. If the exception
      subclasses ``SilentVariableFailure``, 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

        >>> from django.core.template import SilentVariableFailure
        >>> class SilentAssertionError(SilentVariableFailure): pass
        >>> class PersonClass4:
        ...     def first_name(self):
        ...         raise SilentAssertionError, "foo"
        >>> p = PersonClass4()
        >>> t.render(Context({"person": p}))
        "My name is ."

    * 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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In Django 0.91, if a variable doesn't exist, the template system fails
silently. The variable is replaced with an empty string::

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

This applies to any level of lookup::

    >>> t = Template("My name is {{ person.fname }} {{ person.lname }}.")
    >>> c = Context({"person": {"fname": "Stan"}})
    >>> t.render(c)
    "My name is Stan ."

In the Django development version, 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.

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.core.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.core.template.ContextPopException

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

Subclassing Context: DjangoContext
----------------------------------

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

    c = DjangoContext(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 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 ``DjangoContext`` a list of additional processors, using the
optional, third positional argument, ``processors``. In this example, the
``DjangoContext`` instance gets a ``ip_address`` variable::

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

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

Here's what each of the default processors does:

.. _HttpRequest object: http://www.djangoproject.com/documentation/request_response/#httprequest-objects
.. _TEMPLATE_CONTEXT_PROCESSORS setting: http://www.djangoproject.com/documentation/settings/#template-context-processors

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

If ``TEMPLATE_CONTEXT_PROCESSORS`` contains this processor, every
``DjangoContext`` 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 ``auth.Message`` objects for the currently
      logged-in user.
    * ``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: http://www.djangoproject.com/documentation/models/authentication/#users
.. _permissions docs: http://www.djangoproject.com/documentation/models/authentication/#permissions

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

If ``TEMPLATE_CONTEXT_PROCESSORS`` contains this processor, every
``DjangoContext`` 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
``DjangoContext`` 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: http://www.djangoproject.com/documentation/settings/#languages
.. _LANGUAGE_CODE setting: http://www.djangoproject.com/documentation/settings/#language-code
.. _internationalization docs: http://www.djangoproject.com/documentation/i18n/

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

**New in Django development version**

If ``TEMPLATE_CONTEXT_PROCESSORS`` contains this processor, every
``DjangoContext`` 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.

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 file with an
".html" extension in a directory specified as a **template directory**.

If you don't like the requirement that templates have an ".html" extension,
change your ``TEMPLATE_FILE_EXTENSION`` setting. It's set to ``".html"`` by
default.

Also, the .html extension doesn't mean templates can contain only HTML. They
can contain whatever textual content you want.

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.

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.core.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.core.template.TemplateDoesNotExist``.

``django.core.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")`` 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", "story_detail"])``, 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" % story.id, "story_detail"])``.
    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")

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

By default, Django uses a filesystem-based template loader, but Django comes
with a few other template loaders. They're 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 built-in template loaders:

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

``django.core.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")`` 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.core.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``, ``views.py``, etc. For
example::

    polls/
        models/
        templatetags/

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.core 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/core/template/defaultfilters.py`` and
    ``django/core/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.

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.core.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.core import template
    def do_current_time(parser, token):
        try:
            # Splitting by None == splitting by spaces.
            tag_name, format_string = token.contents.split(None, 1)
        except ValueError:
            raise template.TemplateSyntaxError, "%r tag requires an 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"'``.

    * This function is responsible for raising
      ``django.core.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.core 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.

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.core.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/core/template/defaulttags.py``.