================================
Custom template tags and filters
================================

Introduction
============

Django's template system comes with a wide variety of :doc:`built-in
tags and filters </ref/templates/builtins>` designed to address the
presentation logic needs of your application. Nevertheless, you may
find yourself needing functionality that is not covered by the core
set of template primitives. You can extend the template engine by
defining custom tags and filters using Python, and then make them
available to your templates using the ``{% load %}`` tag.

Code layout
-----------

Custom template tags and filters must live inside a Django app. If they relate
to an existing app it makes sense to bundle them there; otherwise, you should
create a new app to hold them.

The app should contain a ``templatetags`` directory, at the same level as
``models.py``, ``views.py``, etc. If this doesn't already exist, create it -
don't forget the ``__init__.py`` file to ensure the directory is treated as a
Python package.

Your custom tags and filters will live in a module inside the ``templatetags``
directory. The name of the module file is the name you'll use to load the tags
later, so be careful to pick a name that won't clash with custom tags and
filters in another app.

For example, if your custom tags/filters are in a file called
``poll_extras.py``, your app layout might look like this::

    polls/
        models.py
        templatetags/
            __init__.py
            poll_extras.py
        views.py

And in your template you would use the following:

.. code-block:: html+django

    {% load poll_extras %}

The app that contains the custom tags must be in :setting:`INSTALLED_APPS` in
order for the ``{% load %}`` tag to work. This is a security feature: It allows
you to host Python code for many template libraries on a single host machine
without enabling access to all of them for every Django installation.

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.

To be a valid tag library, the module must 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:

.. code-block:: html+django

    {{ 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()

Template filters that expect strings
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you're writing a template filter that only expects a string as the first
argument, you should use the decorator ``stringfilter``. This will
convert an object to its string value before being passed to your function::

    from django.template.defaultfilters import stringfilter

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

This way, you'll be able to pass, say, an integer to this filter, and it
won't cause an ``AttributeError`` (because integers don't have ``lower()``
methods).

Registering custom filters
~~~~~~~~~~~~~~~~~~~~~~~~~~

Once 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).

You can use ``register.filter()`` as a decorator instead::

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

    @register.filter
    @stringfilter
    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.

Filters and auto-escaping
~~~~~~~~~~~~~~~~~~~~~~~~~

When writing a custom filter, give some thought to how the filter will interact
with Django's auto-escaping behavior. Note that three types of strings can be
passed around inside the template code:

    * **Raw strings** are the native Python ``str`` or ``unicode`` types. On
      output, they're escaped if auto-escaping is in effect and presented
      unchanged, otherwise.

    * **Safe strings** are strings that have been marked safe from further
      escaping at output time. Any necessary escaping has already been done.
      They're commonly used for output that contains raw HTML that is intended
      to be interpreted as-is on the client side.

      Internally, these strings are of type ``SafeString`` or ``SafeUnicode``.
      They share a common base class of ``SafeData``, so you can test
      for them using code like::

          if isinstance(value, SafeData):
              # Do something with the "safe" string.

    * **Strings marked as "needing escaping"** are *always* escaped on
      output, regardless of whether they are in an ``autoescape`` block or not.
      These strings are only escaped once, however, even if auto-escaping
      applies.

      Internally, these strings are of type ``EscapeString`` or
      ``EscapeUnicode``. Generally you don't have to worry about these; they
      exist for the implementation of the ``escape`` filter.

Template filter code falls into one of two situations:

    1. Your filter does not introduce any HTML-unsafe characters (``<``, ``>``,
       ``'``, ``"`` or ``&``) into the result that were not already present. In
       this case, you can let Django take care of all the auto-escaping
       handling for you. All you need to do is put the ``is_safe`` attribute on
       your filter function and set it to ``True``, like so::

           @register.filter
           def myfilter(value):
               return value
           myfilter.is_safe = True

       This attribute tells Django that if a "safe" string is passed into your
       filter, the result will still be "safe" and if a non-safe string is
       passed in, Django will automatically escape it, if necessary.

       You can think of this as meaning "this filter is safe -- it doesn't
       introduce any possibility of unsafe HTML."

       The reason ``is_safe`` is necessary is because there are plenty of
       normal string operations that will turn a ``SafeData`` object back into
       a normal ``str`` or ``unicode`` object and, rather than try to catch
       them all, which would be very difficult, Django repairs the damage after
       the filter has completed.

       For example, suppose you have a filter that adds the string ``xx`` to the
       end of any input. Since this introduces no dangerous HTML characters to
       the result (aside from any that were already present), you should mark
       your filter with ``is_safe``::

           @register.filter
           def add_xx(value):
               return '%sxx' % value
           add_xx.is_safe = True

       When this filter is used in a template where auto-escaping is enabled,
       Django will escape the output whenever the input is not already marked as
       "safe".

       By default, ``is_safe`` defaults to ``False``, and you can omit it from
       any filters where it isn't required.

       Be careful when deciding if your filter really does leave safe strings
       as safe. If you're *removing* characters, you might inadvertently leave
       unbalanced HTML tags or entities in the result. For example, removing a
       ``>`` from the input might turn ``<a>`` into ``<a``, which would need to
       be escaped on output to avoid causing problems. Similarly, removing a
       semicolon (``;``) can turn ``&amp;`` into ``&amp``, which is no longer a
       valid entity and thus needs further escaping. Most cases won't be nearly
       this tricky, but keep an eye out for any problems like that when
       reviewing your code.

       Marking a filter ``is_safe`` will coerce the filter's return value to
       a string.  If your filter should return a boolean or other non-string
       value, marking it ``is_safe`` will probably have unintended
       consequences (such as converting a boolean False to the string
       'False').

    2. Alternatively, your filter code can manually take care of any necessary
       escaping. This is necessary when you're introducing new HTML markup into
       the result. You want to mark the output as safe from further
       escaping so that your HTML markup isn't escaped further, so you'll need
       to handle the input yourself.

       To mark the output as a safe string, use
       :func:`django.utils.safestring.mark_safe`.

       Be careful, though. You need to do more than just mark the output as
       safe. You need to ensure it really *is* safe, and what you do depends on
       whether auto-escaping is in effect. The idea is to write filters than
       can operate in templates where auto-escaping is either on or off in
       order to make things easier for your template authors.

       In order for your filter to know the current auto-escaping state, set
       the ``needs_autoescape`` attribute to ``True`` on your function. (If you
       don't specify this attribute, it defaults to ``False``). This attribute
       tells Django that your filter function wants to be passed an extra
       keyword argument, called ``autoescape``, that is ``True`` if
       auto-escaping is in effect and ``False`` otherwise.

       For example, let's write a filter that emphasizes the first character of
       a string::

           from django.utils.html import conditional_escape
           from django.utils.safestring import mark_safe

           def initial_letter_filter(text, autoescape=None):
               first, other = text[0], text[1:]
               if autoescape:
                   esc = conditional_escape
               else:
                   esc = lambda x: x
               result = '<strong>%s</strong>%s' % (esc(first), esc(other))
               return mark_safe(result)
           initial_letter_filter.needs_autoescape = True

       The ``needs_autoescape`` attribute on the filter function and the
       ``autoescape`` keyword argument mean that our function will know whether
       automatic escaping is in effect when the filter is called. We use
       ``autoescape`` to decide whether the input data needs to be passed
       through ``django.utils.html.conditional_escape`` or not. (In the latter
       case, we just use the identity function as the "escape" function.) The
       ``conditional_escape()`` function is like ``escape()`` except it only
       escapes input that is **not** a ``SafeData`` instance. If a ``SafeData``
       instance is passed to ``conditional_escape()``, the data is returned
       unchanged.

       Finally, in the above example, we remember to mark the result as safe
       so that our HTML is inserted directly into the template without further
       escaping.

       There's no need to worry about the ``is_safe`` attribute in this case
       (although including it wouldn't hurt anything). Whenever you manually
       handle the auto-escaping issues and return a safe string, the
       ``is_safe`` attribute won't change anything either way.

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:

.. code-block:: html+django

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

.. _`strftime syntax`: http://docs.python.org/library/time.html#time.strftime

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.split()[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 contexts
without having to be parsed multiple times.

Auto-escaping considerations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The output from template tags is **not** automatically run through the
auto-escaping filters. However, there are still a couple of things you should
keep in mind when writing a template tag.

If the ``render()`` function of your template stores the result in a context
variable (rather than returning the result in a string), it should take care
to call ``mark_safe()`` if appropriate. When the variable is ultimately
rendered, it will be affected by the auto-escape setting in effect at the
time, so content that should be safe from further escaping needs to be marked
as such.

Also, if your template tag creates a new context for performing some
sub-rendering, set the auto-escape attribute to the current context's value.
The ``__init__`` method for the ``Context`` class takes a parameter called
``autoescape`` that you can use for this purpose. For example::

    def render(self, context):
        # ...
        new_context = Context({'var': obj}, autoescape=context.autoescape)
        # ... Do something with new_context ...

This is not a very common situation, but it's useful if you're rendering a
template yourself. For example::

    def render(self, context):
        t = template.loader.get_template('small_fragment.html')
        return t.render(Context({'var': obj}, autoescape=context.autoescape))

If we had neglected to pass in the current ``context.autoescape`` value to our
new ``Context`` in this example, the results would have *always* been
automatically escaped, which may not be the desired behavior if the template
tag is used inside a ``{% autoescape off %}`` block.

.. _template_tag_thread_safety:

Thread-safety considerations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 1.2

Once a node is parsed, its ``render`` method may be called any number of times.
Since Django is sometimes run in multi-threaded environments, a single node may
be simultaneously rendering with different contexts in response to two separate
requests. Therefore, it's important to make sure your template tags are thread
safe.

To make sure your template tags are thread safe, you should never store state
information on the node itself. For example, Django provides a builtin ``cycle``
template tag that cycles among a list of given strings each time it's rendered::

    {% for o in some_list %}
        <tr class="{% cycle 'row1' 'row2' %}>
            ...
        </tr>
    {% endfor %}

A naive implementation of ``CycleNode`` might look something like this::

    class CycleNode(Node):
        def __init__(self, cyclevars):
            self.cycle_iter = itertools.cycle(cyclevars)
        def render(self, context):
            return self.cycle_iter.next()

But, suppose we have two templates rendering the template snippet from above at
the same time:

    1. Thread 1 performs its first loop iteration, ``CycleNode.render()``
       returns 'row1'
    2. Thread 2 performs its first loop iteration, ``CycleNode.render()``
       returns 'row2'
    3. Thread 1 performs its second loop iteration, ``CycleNode.render()``
       returns 'row1'
    4. Thread 2 performs its second loop iteration, ``CycleNode.render()``
       returns 'row2'

The CycleNode is iterating, but it's iterating globally. As far as Thread 1
and Thread 2 are concerned, it's always returning the same value. This is
obviously not what we want!

To address this problem, Django provides a ``render_context`` that's associated
with the ``context`` of the template that is currently being rendered. The
``render_context`` behaves like a Python dictionary, and should be used to store
``Node`` state between invocations of the ``render`` method.

Let's refactor our ``CycleNode`` implementation to use the ``render_context``::

    class CycleNode(Node):
        def __init__(self, cyclevars):
            self.cyclevars = cyclevars
        def render(self, context):
            if self not in context.render_context:
                context.render_context[self] = itertools.cycle(self.cyclevars)
            cycle_iter = context.render_context[self]
            return cycle_iter.next()

Note that it's perfectly safe to store global information that will not change
throughout the life of the ``Node`` as an attribute. In the case of
``CycleNode``, the ``cyclevars`` argument doesn't change after the ``Node`` is
instantiated, so we don't need to put it in the ``render_context``. But state
information that is specific to the template that is currently being rendered,
like the current iteration of the ``CycleNode``, should be stored in the
``render_context``.

.. note::
    Notice how we used ``self`` to scope the ``CycleNode`` specific information
    within the ``render_context``. There may be multiple ``CycleNodes`` in a
    given template, so we need to be careful not to clobber another node's state
    information. The easiest way to do this is to always use ``self`` as the key
    into ``render_context``. If you're keeping track of several state variables,
    make ``render_context[self]`` a dictionary.

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::

    @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 pass 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:

.. code-block:: html+django

    <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.split()[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 ``Variable()`` class in ``django.template``.

To use the ``Variable`` class, simply instantiate it with the name of the
variable to be resolved, and then call ``variable.resolve(context)``. So,
for example::

    class FormatTimeNode(template.Node):
        def __init__(self, date_to_be_formatted, format_string):
            self.date_to_be_formatted = template.Variable(date_to_be_formatted)
            self.format_string = format_string

        def render(self, context):
            try:
                actual_date = self.date_to_be_formatted.resolve(context)
                return actual_date.strftime(self.format_string)
            except template.VariableDoesNotExist:
                return ''

Variable resolution 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)

The decorator syntax also works::

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

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.

.. _howto-custom-template-tags-inclusion-tags:

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 :ref:`tutorials <creating-models>`. We'll use the tag like this:

.. code-block:: html+django

    {% show_results poll %}

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

.. code-block:: html

    <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:

.. code-block:: html+django

    <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, 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:

.. code-block:: html+django

    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:

.. code-block:: html+django

    {% 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.

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

The above examples 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:

.. code-block:: html+django

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

.. admonition:: Variable scope in context

    Any variable set in the context will only be available in the same ``block``
    of the template in which it was assigned. This behaviour is intentional;
    it provides a scope for variables so that they don't conflict with
    context in other blocks.

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:

.. code-block:: html+django

    {% 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.split()[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:

.. code-block:: html+django

    {% 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``.
