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Template Designer Documentation
===============================
.. highlight:: html+jinja
This document describes the syntax and semantics of the template engine and
will be most useful as reference to those creating Jinja templates. As the
template engine is very flexible the configuration from the application might
be slightly different from here in terms of delimiters and behavior of
undefined values.
Synopsis
--------
A template is simply a text file. It can generate any text-based format
(HTML, XML, CSV, LaTeX, etc.). It doesn't have a specific extension,
``.html`` or ``.xml`` are just fine.
A template contains **variables** or **expressions**, which get replaced with
values when the template is evaluated, and tags, which control the logic of
the template. The template syntax is heavily inspired by Django and Python.
Below is a minimal template that illustrates a few basics. We will cover
the details later in that document::
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<head>
<title>My Webpage</title>
</head>
<body>
<ul id="navigation">
{% for item in navigation %}
<li><a href="{{ item.href }}">{{ item.caption }}</a></li>
{% endfor %}
</ul>
<h1>My Webpage</h1>
{{ a_variable }}
</body>
</html>
This covers the default settings. The application developer might have
changed the syntax from ``{% foo %}`` to ``<% foo %>`` or something similar.
There are two kinds of delimiers. ``{% ... %}`` and ``{{ ... }}``. The first
one is used to execute statements such as for-loops or assign values, the
latter prints the result of the expression to the template.
.. _variables:
Variables
---------
The application passes variables to the templates you can mess around in the
template. Variables may have attributes or elements on them you can access
too. How a variable looks like, heavily depends on the application providing
those.
You can use a dot (``.``) to access attributes of a variable, alternative the
so-called "subscribe" syntax (``[]``) can be used. The following lines do
the same::
{{ foo.bar }}
{{ foo['bar'] }}
It's important to know that the curly braces are *not* part of the variable
but the print statement. If you access variables inside tags don't put the
braces around.
If a variable or attribute does not exist you will get back an undefined
value. What you can do with that kind of value depends on the application
configuration, the default behavior is that it evaluates to an empty string
if printed and that you can iterate over it, but every other operation fails.
.. _filters:
Filters
-------
Variables can by modified by **filters**. Filters are separated from the
variable by a pipe symbol (``|``) and may have optional arguments in
parentheses. Multiple filters can be chained. The output of one filter is
applied to the next.
``{{ name|striptags|title }}`` for example will remove all HTML Tags from the
`name` and title-cases it. Filters that accept arguments have parentheses
around the arguments, like a function call. This example will join a list
by spaces: ``{{ list|join(', ') }}``.
The :ref:`builtin-filters` below describes all the builtin filters.
.. _tests:
Tests
-----
Beside filters there are also so called "tests" available. Tests can be used
to test a variable against a common expression. To test a variable or
expression you add `is` plus the name of the test after the variable. For
example to find out if a variable is defined you can do ``name is defined``
which will then return true or false depending on if `name` is defined.
Tests can accept arguments too. If the test only takes one argument you can
leave out the parentheses to group them. For example the following two
expressions do the same::
{% if loop.index is divisibleby 3 %}
{% if loop.index is divisibleby(3) %}
The :ref:`builtin-tests` below describes all the builtin tests.
Comments
--------
To comment-out part of a line in a template, use the comment syntax which is
by default set to ``{# ... #}``. This is useful to comment out parts of the
template for debugging or to add information for other template designers or
yourself::
{# note: disabled template because we no longer user this
{% for user in users %}
...
{% endfor %}
#}
.. _template-inheritance:
Template Inheritance
--------------------
The most powerful part of Jinja is template inheritance. Template inheritance
allows you to build a base "skeleton" template that contains all the common
elements of your site and defines **blocks** that child templates can override.
Sounds complicated but is very basic. It's easiest to understand it by starting
with an example.
Base Template
~~~~~~~~~~~~~
This template, which we'll call ``base.html``, defines a simple HTML skeleton
document that you might use for a simple two-column page. It's the job of
"child" templates to fill the empty blocks with content::
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
{% block head %}
<link rel="stylesheet" href="style.css" />
<title>{% block title %}{% endblock %} - My Webpage</title>
{% endblock %}
</head>
<body>
<div id="content">{% block content %}{% endblock %}</div>
<div id="footer">
{% block footer %}
&copy; Copyright 2008 by <a href="http://domain.invalid/">you</a>.
{% endblock %}
</div>
</body>
In this example, the ``{% block %}`` tags define four blocks that child templates
can fill in. All the `block` tag does is to tell the template engine that a
child template may override those portions of the template.
Child Template
~~~~~~~~~~~~~~
A child template might look like this::
{% extends "base.html" %}
{% block title %}Index{% endblock %}
{% block head %}
{{ super() }}
<style type="text/css">
.important { color: #336699; }
</style>
{% endblock %}
{% block content %}
<h1>Index</h1>
<p class="important">
Welcome on my awsome homepage.
</p>
{% endblock %}
The ``{% extends %}`` tag is the key here. It tells the template engine that
this template "extends" another template. When the template system evaluates
this template, first it locates the parent. The extends tag should be the
first tag in the template. Everything before it is printed out normally and
may cause confusion.
The filename of the template depends on the template loader. For example the
:class:`FileSystemLoader` allows you to access other templates by giving the
filename. You can access templates in subdirectories with an slash::
{% extends "layout/default.html" %}
But this behavior can depend on the application embedding Jinja. Note that
since the child template doesn't define the ``footer`` block, the value from
the parent template is used instead.
You can't define multiple ``{% block %}`` tags with the same name in the
same template. This limitation exists because a block tag works in "both"
directions. That is, a block tag doesn't just provide a hole to fill - it
also defines the content that fills the hole in the *parent*. If there
were two similarly-named ``{% block %}`` tags in a template, that template's
parent wouldn't know which one of the blocks' content to use.
If you want to print a block multiple times you can however use the special
`self` variable and call the block with that name::
<title>{% block title %}{% endblock %}</title>
<h1>{{ self.title() }}</h1>
{% block body %}{% endblock %}
Unlike Python Jinja does not support multiple inheritance. So you can only have
one extends tag called per rendering.
Super Blocks
~~~~~~~~~~~~
It's possible to render the contents of the parent block by calling `super`.
This gives back the results of the parent block::
{% block sidebar %}
<h3>Table Of Contents</h3>
...
{{ super() }}
{% endblock %}
HTML Escaping
-------------
When generating HTML from templates, there's always a risk that a variable will
include characters that affect the resulting HTML. There are two approaches:
manually escaping each variable or automatically escaping everything by default.
Jinja supports both, but what is used depends on the application configuration.
The default configuaration is no automatic escaping for various reasons:
- escaping everything except of safe values will also mean that Jinja is
escaping variables known to not include HTML such as numbers which is
a huge performance hit.
- The information about the safety of a variable is very fragile. It could
happen that by coercing safe and unsafe values the return value is double
escaped HTML.
Working with Manual Escaping
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If manual escaping is enabled it's **your** responsibility to escape
variables if needed. What to escape? If you have a variable that *may*
include any of the following chars (``>``, ``<``, ``&``, or ``"``) you
**have to** escape it unless the variable contains well-formed and trusted
HTML. Escaping works by piping the variable through the ``|e`` filter:
``{{ user.username|e }}``.
Working with Automatic Escaping
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When automatic escaping is enabled everything is escaped by default except
for values explicitly marked as safe. Those can either be marked by the
application or in the template by using the `|safe` filter. The main
problem with this approach is that Python itself doesn't have the concept
of tainted values so the information if a value is safe or unsafe can get
lost. If the information is lost escaping will take place which means that
you could end up with double escaped contents.
Double escaping is easy to avoid however, just rely on the tools Jinja2
provides and don't use builtin Python constructs such as the string modulo
operator.
Functions returning template data (macros, `super`, `self.BLOCKNAME`) return
safe markup always.
String literals in templates with automatic escaping are considered unsafe
too. The reason for this is that the safe string is an extension to Python
and not every library will work properly with it.
List of Control Structures
--------------------------
A control structure refers to all those things that control the flow of a
program - conditionals (i.e. if/elif/else), for-loops, as well as things like
macros and blocks. Control structures appear inside ``{% ... %}`` blocks
in the default syntax.
For
~~~
Loop over each item in a sequence. For example, to display a list of users
provided in a variable called `users`::
<h1>Members</h1>
<ul>
{% for user in users %}
<li>{{ user.username|e }}</li>
{% endfor %}
</ul>
Inside of a for loop block you can access some special variables:
+-----------------------+---------------------------------------------------+
| Variable | Description |
+=======================+===================================================+
| `loop.index` | The current iteration of the loop. (1 indexed) |
+-----------------------+---------------------------------------------------+
| `loop.index0` | The current iteration of the loop. (0 indexed) |
+-----------------------+---------------------------------------------------+
| `loop.revindex` | The number of iterations from the end of the loop |
| | (1 indexed) |
+-----------------------+---------------------------------------------------+
| `loop.revindex0` | The number of iterations from the end of the loop |
| | (0 indexed) |
+-----------------------+---------------------------------------------------+
| `loop.first` | True if first iteration. |
+-----------------------+---------------------------------------------------+
| `loop.last` | True if last iteration. |
+-----------------------+---------------------------------------------------+
| `loop.length` | The number of items in the sequence. |
+-----------------------+---------------------------------------------------+
| `loop.cycle` | A helper function to cycle between a list of |
| | sequences. See the explanation below. |
+-----------------------+---------------------------------------------------+
Within a for-loop, it's possible to cycle among a list of strings/variables
each time through the loop by using the special `loop.cycle` helper::
{% for row in rows %}
<li class="{{ loop.cycle('odd', 'even') }}">{{ row }}</li>
{% endfor %}
.. _loop-filtering:
Unlike in Python it's not possible to `break` or `continue` in a loop. You
can however filter the sequence during iteration which allows you to skip
items. The following example skips all the users which are hidden::
{% for user in users if not user.hidden %}
<li>{{ user.username|e }}</li>
{% endfor %}
The advantage is that the special `loop` variable will count correctly thus
not counting the users not iterated over.
If no iteration took place because the sequence was empty or the filtering
removed all the items from the sequence you can render a replacement block
by using `else`::
<ul>
{% for user in users %}
<li>{{ user.username|e }}</li>
{% else %}
<li><em>no users found</em></li>
{% endif %}
</ul>
If
~~
The `if` statement in Jinja is comparable with the if statements of Python.
In the simplest form you can use it to test if a variable is defined, not
empty or not false::
{% if users %}
<ul>
{% for user in users %}
<li>{{ user.username|e }}</li>
{% endfor %}
</ul>
{% endif %}
For multiple branches `elif` and `else` can be used like in Python. You can
use more complex :ref:`expressions` there too::
{% if kenny.sick %}
Kenny is sick.
{% elif kenny.dead %}
You killed Kenny! You bastard!!!
{% else %}
Kenny looks okay --- so far
{% endif %}
If can also be used as :ref:`inline expression <if-expression>` and for
:ref:`loop filtering <loop-filtering>`.
Macros
~~~~~~
Macros are comparable with functions in regular programming languages. They
are useful to put often used idioms into reusable functions to not repeat
yourself.
Macros can be defined in helper templates which then are :ref:`imported
<import>` or directly in the template where they are used. There is one big
difference between those two possibilities. A macro that is defined in the
template where it's also used has access to the context passed to the template.
A macro defined in another template and then imported can only access variables
defined there or in the global context.
Here a small example of a macro that renders a form element::
{% macro input(name, value='', type='text', size=20) -%}
<input type="{{ type }}" name="{{ name }}" value="{{
value|e }}" size="{{ size }}">
{%- endmacro %}
The macro can then be called like a function in the namespace::
<p>{{ input('username') }}</p>
<p>{{ input('password', type='password') }}</p>
If the macro was defined in a different template you have to
:ref:`import <import>` it first.
Inside macros you have access to three special variables:
`varargs`
If more positional arguments are passed to the macro than accepted by the
macro they end up in the special `varargs` variable as list of values.
`kwargs`
Like `varargs` but for keyword arguments. All unconsumed keyword
arguments are stored in this special variable.
`caller`
If the macro was called from a :ref:`call<call>` tag the caller is stored
in this variable as macro which can be called.
Macros also expose some of their internal details. The following attributes
are available on a macro object:
`name`
The name of the macro. ``{{ input.name }}`` will print ``input``.
`arguments`
A tuple of the names of arguments the macro accepts.
`defaults`
A tuple of default values.
`catch_kwargs`
This is `true` if the macro accepts extra keyword arguments (ie: accesses
the special `kwargs` variable).
`catch_varargs`
This is `true` if the macro accepts extra positional arguments (ie:
accesses the special `varargs` variable).
`caller`
This is `true` if the macro accesses the special `caller` variable and may
be called from a :ref:`call<call>` tag.
.. _call:
Call
~~~~
In some cases it can be useful to pass a macro to another macro. For this
purpose you can use the special `call` block. The following example shows
a macro that takes advantage of the call functionality and how it can be
used::
{% macro render_dialog(title, class='dialog') -%}
<div class="{{ class }}">
<h2>{{ title }}</h2>
<div class="contents">
{{ caller() }}
</div>
</div>
{%- endmacro %}
{% call render_dialog('Hello World') %}
This is a simple dialog rendered by using a macro and
a call block.
{% endcall %}
It's also possible to pass arguments back to the call block. This makes it
useful as replacement for loops. It is however not possible to call a
call block with another call block.
Here an example of how a call block can be used with arguments::
{% macro dump_users(users) -%}
<ul>
{%- for user in users %}
<li><p>{{ user.username|e }}</p>{{ caller(user) }}</li>
{%- endfor %}
</ul>
{%- endmacro %}
{% call(user) dump_users(list_of_user) %}
<dl>
<dl>Realname</dl>
<dd>{{ user.realname|e }}</dd>
<dl>Description</dl>
<dd>{{ user.description }}</dd>
</dl>
{% endcall %}
Assignments
~~~~~~~~~~~
Inside code blocks you can also assign values to variables. Assignments at
top level (outside of blocks, macros or loops) are exported from the template
like top level macros and can be imported by other templates.
Assignments are just written in code blocks like any other statement just
without explicit keyword::
{% navigation = [('index.html', 'Index'), ('about.html', 'About')] %}
Extends
~~~~~~~
The `extends` tag can be used to extend a template from another one. You
can have multiple of them in a file but only one of them may be executed
at the time. There is no support for multiple inheritance. See the section
about :ref:`template-inheritance` above.
Block
~~~~~
Blocks are used for inheritance and act as placeholders and replacements
at the same time. They are documented in detail as part of the section
about :ref:`template-inheritance`.
Include
~~~~~~~
The `include` statement is useful to include a template and return the
rendered contents of that file into the current namespace::
{% include 'header.html' %}
Body
{% include 'footer.html' %}
Included templates have access to the variables of the active context by
default. For more details about context behavior of imports and includes
see :ref:`import-visibility`.
.. _import:
Import
~~~~~~
Jinja2 supports putting often used code into macros. These macros can go into
different templates and get imported from there. This works similar to the
import statements in Python. It's important to know that imports are cached
and imported templates don't have access to the current template variables,
just the globals by defualt. For more details about context behavior of
imports and includes see :ref:`import-visibility`.
There are two ways to import templates. You can import the complete template
into a variable or request specific macros / exported variables from it.
Imagine we have a helper module that renders forms (called `forms.html`)::
{% macro input(name, value='', type='text') -%}
<input type="{{ type }}" value="{{ value|e }}" name="{{ name }}">
{%- endmacro %}
{%- macro textarea(name, value='', rows=10, cols=40) -%}
<textarea name="{{ name }}" rows="{{ rows }}" cols="{{ cols
}}">{{ value|e }}</textarea>
{%- endmacro %}
The easiest and most flexible is importing the whole module into a variable.
That way you can access the attributes::
{% import 'forms.html' as forms %}
<dl>
<dt>Username</dt>
<dd>{{ forms.input('username') }}</dd>
<dt>Password</dt>
<dd>{{ forms.input('password', type='password') }}</dd>
</dl>
<p>{{ forms.textarea('comment') }}</p>
Alternatively you can import names from the template into the current
namespace::
{% from 'forms.html' import input as input_field, textarea %}
<dl>
<dt>Username</dt>
<dd>{{ input_field('username') }}</dd>
<dt>Password</dt>
<dd>{{ input_field('password', type='password') }}</dd>
</dl>
<p>{{ textarea('comment') }}</p>
.. _import-visibility:
Import Context Behavior
-----------------------
Per default included templates are passed the current context and imported
templates not. The reason for this is that imports unlike includes are
cached as imports are often used just as a module that holds macros.
This however can be changed of course explicitly. By adding `with context`
or `without context` to the import/include directive the current context
can be passed to the template and caching is disabled automatically.
Here two examples::
{% from 'forms.html' import input with context %}
{% include 'header.html' without context %}
.. _expressions:
Expressions
-----------
Jinja allows basic expressions everywhere. These work very similar to regular
Python and even if you're not working with Python you should feel comfortable
with it.
Literals
~~~~~~~~
The simplest form of expressions are literals. Literals are representations
for Python objects such as strings and numbers. The following literals exist:
"Hello World":
Everything between two double or single quotes is a string. They are
useful whenever you need a string in the template (for example as
arguments to function calls, filters or just to extend or include a
template).
42 / 42.23:
Integers and floating point numbers are created by just writing the
number down. If a dot is present the number is a float, otherwise an
integer. Keep in mind that for Python ``42`` and ``42.0`` is something
different.
['list', 'of', 'objects']:
Everything between two brackets is a list. Lists are useful to store
sequential data in or to iterate over them. For example you can easily
create a list of links using lists and tuples with a for loop::
<ul>
{% for href, caption in [('index.html', 'Index'), ('about.html', 'About'),
('downloads.html', 'Downloads')] %}
<li><a href="{{ href }}">{{ caption }}</a></li>
{% endfor %}
</ul>
('tuple', 'of', 'values'):
Tuples are like lists, just that you can't modify them. If the tuple
only has one item you have to end it with a comma. Tuples are usually
used to represent items of two or more elements. See the example above
for more details.
{'dict': 'of', 'key': 'and', 'value': 'pairs'}:
A dict in Python is a structure that combines keys and values. Keys must
be unique and always have exactly one value. Dicts are rarely used in
templates, they are useful in some rare cases such as the :func:`xmlattr`
filter.
true / false:
true is always true and false is always false. Keep in mind that those
literals are lowercase!
Math
~~~~
Jinja allows you to calculate with values. This is rarely useful in templates
but exists for completeness sake. The following operators are supported:
\+
Adds two objects with each other. Usually numbers but if both objects are
strings or lists you can concatenate them this way. This however is not
the preferred way to concatenate strings! For string concatenation have
a look at the ``~`` operator. ``{{ 1 + 1 }}`` is ``2``.
\-
Substract two numbers from each other. ``{{ 3 - 2 }}`` is ``1``.
/
Divide two numbers. The return value will be a floating point number.
``{{ 1 / 2 }}`` is ``{{ 0.5 }}``.
//
Divide two numbers and return the truncated integer result.
``{{ 20 / 7 }}`` is ``2``.
%
Calculate the remainder of an integer division between the left and right
operand. ``{{ 11 % 7 }}`` is ``4``.
\*
Multiply the left operand with the right one. ``{{ 2 * 2 }}`` would
return ``4``. This can also be used to repeat string multiple times.
``{{ '=' * 80 }}`` would print a bar of 80 equal signs.
\**
Raise the left operand to the power of the right operand. ``{{ 2**3 }}``
would return ``8``.
Logic
~~~~~
For `if` statements / `for` filtering or `if` expressions it can be useful to
combine group multiple expressions:
and
Return true if the left and the right operand is true.
or
Return true if the left or the right operand is true.
not
negate a statement (see below).
(expr)
group an expression.
Note that there is no support for any bit operations or something similar.
- special note regarding ``not``: The ``is`` and ``in`` operators support
negation using an infix notation too: ``foo is not bar`` and
``foo not in bar`` instead of ``not foo is bar`` and ``not foo in bar``.
All other expressions require a prefix notation: ``not (foo and bar).``
Other Operators
~~~~~~~~~~~~~~~
The following operators are very useful but don't fit into any of the other
two categories:
in
Perform sequence / mapping containment test. Returns true if the left
operand is contained in the right. ``{{ 1 in [1, 2, 3] }}`` would for
example return true.
is
Performs a :ref:`test <tests>`.
\|
Applies a :ref:`filter <filters>`.
~
Converts all operands into strings and concatenates them.
``{{ "Hello " ~ name ~ "!" }}`` would return (assuming `name` is
``'John'``) ``Hello John!``.
()
Call a callable: ``{{ post.render() }}``. Inside of the parentheses you
can use arguments and keyword arguments like in python:
``{{ post.render(user, full=true) }}``.
. / []
Get an attribute of an object. (See :ref:`variables`)
.. _if-expression:
If Expression
~~~~~~~~~~~~~
It is also possible to use inline `if` expressions. These are useful in some
situations. For example you can use this to extend from one template if a
variable is defined, otherwise from the default layout template::
{% extends layout_template if layout_template is defined else 'master.html' %}
The general syntax is ``<do something> if <something is true> else <do
something else>``.
.. _builtin-filters:
List of Builtin Filters
-----------------------
.. jinjafilters::
.. _builtin-tests:
List of Builtin Tests
---------------------
.. jinjatests::
List of Global Functions
------------------------
The following functions are available in the global scope by default:
.. function:: range([start,] stop[, step])
Return a list containing an arithmetic progression of integers.
range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0.
When step is given, it specifies the increment (or decrement).
For example, range(4) returns [0, 1, 2, 3]. The end point is omitted!
These are exactly the valid indices for a list of 4 elements.
This is useful to repeat a template block multiple times for example
to fill a list. Imagine you have 7 users in the list but you want to
render three empty items to enforce a height with CSS::
<ul>
{% for user in users %}
<li>{{ user.username }}</li>
{% endfor %}
{% for number in range(10 - users|count) %}
<li class="empty"><span>...</span></li>
{% endfor %}
</ul>
.. function:: lipsum(n=5, html=True, min=20, max=100)
Generates some lorem ipsum for the template. Per default five paragraphs
with HTML are generated each paragraph between 20 and 100 words. If html
is disabled regular text is returned. This is useful to generate simple
contents for layout testing.