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).

If you're using Python 2.4 or above, 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 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.

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>

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.

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 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:

<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)

In Python 2.4, 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.

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.

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.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:

{% 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.