Django2.0手册:Custom template tags and filters

Django’s template language comes with a wide variety of built-in
tags and filters
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¶

The most common place to specify custom template tags and filters is inside
a Django app. If they relate to an existing app, it makes sense to bundle them
there; otherwise, they can be added to a new app. When a Django app is added
to INSTALLED_APPS, any tags it defines in the conventional location
described below are automatically made available to load within templates.

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.

开发服务器并不会自动重启

After adding the templatetags module, you will need to restart your
server before you can use the tags or filters in templates.

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

And in your template you would use the following:

{% load poll_extras %}

The app that contains the custom tags must be in 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()

Alternatively, template tag modules can be registered through the
'libraries' argument to
DjangoTemplates. This is useful if
you want to use a different label from the template tag module name when
loading template tags. It also enables you to register tags without installing
an application.

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.

For more information on the load tag, read its documentation.

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".

Since the template language doesn’t provide exception handling, any exception
raised from a template filter will be exposed as a server error. Thus, filter
functions should avoid raising exceptions if there is a reasonable fallback
value to return. In case of input that represents a clear bug in a template,
raising an exception may still be better than silent failure which hides the
bug.

这是一个过滤器定义的例子:

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

这个例子展示了如何使用这个过滤器:

{{ 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()

注册自定义过滤器¶

django.template.Library.filter()

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. 筛选器的名称——字符串。
  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')
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.

Finally, register.filter() also accepts three keyword arguments,
is_safe, needs_autoescape, and expects_localtime. These arguments
are described in filters and auto-escaping and
filters and time zones below.

Template filters that expect strings¶

django.template.defaultfilters.stringfilter()

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 import template
from django.template.defaultfilters import stringfilter

register = template.Library()

@register.filter
@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).

筛选器和自动转义¶

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

  • Raw strings are the native Python strings. 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
    SafeText. You can test for them
    using code like:

    from django.utils.safestring import SafeText
    
    if isinstance(value, SafeText):
        # Do something with the "safe" string.
        ...
    

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 set the is_safe flag to True
    when you register your filter function, like so:

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

    This flag 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 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(is_safe=True)
    def add_xx(value):
        return '%sxx' % value
    

    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 is 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 that
    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 flag to True when you register your filter function.
    (If you don’t specify this flag, it defaults to False). This flag 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. It is recommended to set the default of the
    autoescape parameter to True, so that if you call the function
    from Python code it will have escaping enabled by default.

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

    from django import template
    from django.utils.html import conditional_escape
    from django.utils.safestring import mark_safe
    
    register = template.Library()
    
    @register.filter(needs_autoescape=True)
    def initial_letter_filter(text, autoescape=True):
        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)
    

    The needs_autoescape flag 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 flag 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 flag won’t change anything either way.

Warning

Avoiding XSS vulnerabilities when reusing built-in filters

Django’s built-in filters have autoescape=True by default in order to
get the proper autoescaping behavior and avoid a cross-site script
vulnerability.

In older versions of Django, be careful when reusing Django’s built-in
filters as autoescape defaults to None. You’ll need to pass
autoescape=True to get autoescaping.

For example, if you wanted to write a custom filter called
urlize_and_linebreaks that combined the urlize and
linebreaksbr filters, the filter would look like:

from django.template.defaultfilters import linebreaksbr, urlize

@register.filter(needs_autoescape=True)
def urlize_and_linebreaks(text, autoescape=True):
    return linebreaksbr(
        urlize(text, autoescape=autoescape),
        autoescape=autoescape
    )

接下来:

{{ comment|urlize_and_linebreaks }}

would be equivalent to:

{{ comment|urlize|linebreaksbr }}

过滤器和时区¶

If you write a custom filter that operates on datetime
objects, you’ll usually register it with the expects_localtime flag set to
True:

@register.filter(expects_localtime=True)
def businesshours(value):
    try:
        return 9 <= value.hour < 17
    except AttributeError:
        return ''

When this flag is set, if the first argument to your filter is a time zone
aware datetime, Django will convert it to the current time zone before passing
it to your filter when appropriate, according to rules for time zones
conversions in templates
.

Writing custom template tags¶

Tags are more complex than filters, because tags can do anything. Django
provides a number of shortcuts that make writing most types of tags easier.
First we’ll explore those shortcuts, then explain how to write a tag from
scratch for those cases when the shortcuts aren’t powerful enough.

Simple tags¶

django.template.Library.simple_tag()

Many template tags take a number of arguments — strings or template variables
— and return a result after doing some processing based solely on
the input arguments and some external information. For example, a
current_time tag might accept a format string and return the time as a
string formatted accordingly.

To ease the creation of these 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 current_time function could thus be written like this:

import datetime
from django import template

register = template.Library()

@register.simple_tag
def current_time(format_string):
    return datetime.datetime.now().strftime(format_string)

A few 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.

Unlike other tag utilities, simple_tag passes its output through
conditional_escape() if the template context is in
autoescape mode, to ensure correct HTML and protect you from XSS
vulnerabilities.

If additional escaping is not desired, you will need to use
mark_safe() if you are absolutely sure that your
code does not contain XSS vulnerabilities. For building small HTML snippets,
use of format_html() instead of mark_safe() is
strongly recommended.

If your template tag needs to access the current context, you can use the
takes_context argument when registering your tag:

@register.simple_tag(takes_context=True)
def current_time(context, format_string):
    timezone = context['timezone']
    return your_get_current_time_method(timezone, format_string)

Note that the first argument must be called context.

For more information on how the takes_context option works, see the section
on inclusion tags.

If you need to rename your tag, you can provide a custom name for it:

register.simple_tag(lambda x: x - 1, name='minusone')

@register.simple_tag(name='minustwo')
def some_function(value):
    return value - 2

simple_tag functions may accept any number of positional or keyword
arguments. For example:

@register.simple_tag
def my_tag(a, b, *args, **kwargs):
    warning = kwargs['warning']
    profile = kwargs['profile']
    ...
    return ...

Then in the template any number of arguments, separated by spaces, may be
passed to the template tag. Like in Python, the values for keyword arguments
are set using the equal sign (“=“) and must be provided after the
positional arguments. For example:

{% my_tag 123 "abcd" book.title warning=message|lower profile=user.profile %}

It’s possible to store the tag results in a template variable rather than
directly outputting it. This is done by using the as argument followed by
the variable name. Doing so enables you to output the content yourself where
you see fit:

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

Inclusion tags¶

django.template.Library.inclusion_tag()

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')
def show_results(poll):
    ...

Alternatively it is possible to register the inclusion tag using a
django.template.Template instance:

from django.template.loader import get_template
t = get_template('results.html')
register.inclusion_tag(t)(show_results)

…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:

@register.inclusion_tag('link.html', takes_context=True)
def jump_link(context):
    return {
        'link': context['home_link'],
        'title': context['home_title'],
    }

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.

inclusion_tag functions may accept any number of positional or keyword
arguments. For example:

@register.inclusion_tag('my_template.html')
def my_tag(a, b, *args, **kwargs):
    warning = kwargs['warning']
    profile = kwargs['profile']
    ...
    return ...

Then in the template any number of arguments, separated by spaces, may be
passed to the template tag. Like in Python, the values for keyword arguments
are set using the equal sign (“=“) and must be provided after the
positional arguments. For example:

{% my_tag 123 "abcd" book.title warning=message|lower profile=user.profile %}

Advanced custom template tags¶

Sometimes the basic features for custom template tag creation aren’t enough.
Don’t worry, Django gives you complete access to the internals required to build
a template tag from the ground up.

A quick overview¶

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 full implementation of our simple 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])

注意:

  • 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:

import datetime
from django import template

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)

注意:

  • __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 generally fail silently, particularly in a production
    environment. In some cases however, particularly if
    context.template.engine.debug is True, this method may raise an
    exception to make debugging easier. For example, several core tags raise
    django.template.TemplateSyntaxError if they receive the wrong number or
    type of arguments.

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.

自动转义的注意事项¶

The output from template tags is not automatically run through the
auto-escaping filters (with the exception of
simple_tag() as described above). 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:

from django.template import Context

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 = context.template.engine.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.

Thread-safety considerations¶

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:

import itertools
from django import template

class CycleNode(template.Node):
    def __init__(self, cyclevars):
        self.cycle_iter = itertools.cycle(cyclevars)

    def render(self, context):
        return next(self.cycle_iter)

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(template.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 next(cycle_iter)

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:

<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.

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:

import datetime
from django import template

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>

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 behavior 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:

{% 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:

import re

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 ''

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.

Finally, if you only need to have a simple syntax for your custom
context-updating template tag, consider using the
simple_tag() shortcut, which supports assigning
the tag results to a template variable.

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 a simplified {% comment %} tag might be 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 ''

Note

The actual implementation of {% comment %} is slightly
different in that it allows broken template tags to appear between
{% comment %} and {% endcomment %}. It does so by calling
parser.skip_past('endcomment') instead of parser.parse(('endcomment',))
followed by parser.delete_first_token(), thus avoiding the generation of a
node list.

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 of
{% for %} in django/template/defaulttags.py and
{% if %} in django/template/smartif.py.