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+====================================================
+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/