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 "subscript" 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.

.. _notes-on-subscriptions:

.. admonition:: Implementation

    For convenience sake ``foo.bar`` in Jinja2 does the following things on
    the Python layer:

    -   check if there is an attribute called `bar` on `foo`.
    -   if there is not, check if there is an item ``'bar'`` in `foo`.
    -   if there is not, return an undefined object.

    ``foo['bar']`` on the other hand works mostly the same with the a small
    difference in the order:

    -   check if there is an item ``'bar'`` in `foo`.
    -   if there is not, check if there is an attribute called `bar` on `foo`.
    -   if there is not, return an undefined object.

    This is important if an object has an item or attribute with the same
    name.  Additionally there is the :func:`attr` filter that just looks up
    attributes.

.. _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 commas:  ``{{ 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 use this
        {% for user in users %}
            ...
        {% endfor %}
    #}


Whitespace Control
------------------

In the default configuration whitespace is not further modified by the
template engine, so each whitespace (spaces, tabs, newlines etc.) is returned
unchanged.  If the application configures Jinja to `trim_blocks` the first
newline after a a template tag is removed automatically (like in PHP).

But you can also strip whitespace in templates by hand.  If you put an minus
sign (``-``) to the start or end of an block (for example a for tag), a
comment or variable expression you can remove the whitespaces after or before
that block::

    {% for item in seq -%}
        {{ item }}
    {%- endfor %}
    
This will yield all elements without whitespace between them.  If `seq` was
a list of numbers from ``1`` to ``9`` the output would be ``123456789``.

If :ref:`line-statements` are enabled they strip leading whitespace
automatically up to the beginning of the line.

.. admonition:: Note

    You must not use a whitespace between the tag and the minus sign.

    **valid**::

        {%- if foo -%}...{% endif %}

    **invalid**::

        {% - if foo - %}...{% endif %}


Escaping
--------

It is sometimes desirable or even necessary to have Jinja ignore parts it
would otherwise handle as variables or blocks.  For example if the default
syntax is used and you want to use ``{{`` as raw string in the template and
not start a variable you have to use a trick.

The easiest way is to output the variable delimiter (``{{``) by using a
variable expression::

    {{ '{{' }}

For bigger sections it makes sense to mark a block `raw`.  For example to
put Jinja syntax as example into a template you can use this snippet::

    {% raw %}
        <ul>
        {% for item in seq %}
            <li>{{ item }}</li>
        {% endfor %}
        </ul>
    {% endraw %}


.. _line-statements:

Line Statements
---------------

If line statements are enabled by the application it's possible to mark a
line as a statement.  For example if the line statement prefix is configured
to ``#`` the following two examples are equivalent::

    <ul>
    # for item in seq
        <li>{{ item }}</li>
    # endfor
    </ul>

    <ul>
    {% for item in seq %}
        <li>{{ item }}</li>
    {% endfor %}
    </ul>

The line statement prefix can appear anywhere on the line as long as no text
precedes it.  For better readability statements that start a block (such as
`for`, `if`, `elif` etc.) may end with a colon::

    # for item in seq:
        ...
    # endfor


.. admonition:: Note

    Line statements can span multiple lines if there are open parentheses,
    braces or brackets::

        <ul>
        # for href, caption in [('index.html', 'Index'),
                                ('about.html', 'About')]:
            <li><a href="{{ href }}">{{ caption }}</a></li>
        # endfor
        </ul>

Since Jinja 2.2 line-based comments are available as well.  For example if
the line-comment prefix is configured to be ``##`` everything from ``##`` to
the end of the line is ignored (excluding the newline sign)::

    # for item in seq:
        <li>{{ item }}</li>     ## this comment is ignored
    # 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 awesome 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.  For details about this behavior and how to take
advantage of it, see :ref:`null-master-fallback`.

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 %}


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 %}


Named Block End-Tags
~~~~~~~~~~~~~~~~~~~~

Jinja2 allows you to put the name of the block after the end tag for better
readability::

    {% block sidebar %}
        {% block inner_sidebar %}
            ...
        {% endblock inner_sidebar %}
    {% endblock sidebar %}

However the name after the `endblock` word must match the block name.


Block Nesting and Scope
~~~~~~~~~~~~~~~~~~~~~~~

Blocks can be nested for more complex layouts.  However per default blocks
may not access variables from outer scopes::

    {% for item in seq %}
        <li>{% block loop_item %}{{ item }}{% endblock %}</li>
    {% endfor %}

This example would output empty ``<li>`` items because `item` is unavailable
inside the block.  The reason for this is that if the block is replaced by
a child template a variable would appear that was not defined in the block or
passed to the context.

Starting with Jinja 2.2 you can explicitly specify that variables are
available in a block by setting the block to "scoped" by adding the `scoped`
modifier to a block declaration::

    {% for item in seq %}
        <li>{% block loop_item scoped %}{{ item }}{% endblock %}</li>
    {% endfor %}

When overriding a block the `scoped` modifier does not have to be provided.


Template Objects
~~~~~~~~~~~~~~~~

.. versionchanged:: 2.4

If a template object was passed to the template context you can
extend from that object as well.  Assuming the calling code passes
a layout template as `layout_template` to the environment, this
code works::

    {% extends layout_template %}

Previously the `layout_template` variable had to be a string with
the layout template's filename for this to work.


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 %}

With Jinja 2.1 an extra `cycle` helper exists that allows loop-unbound
cycling.  For more information have a look at the :ref:`builtin-globals`.

.. _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>
    {% endfor %}
    </ul>

It is also possible to use loops recursively.  This is useful if you are
dealing with recursive data such as sitemaps.  To use loops recursively you
basically have to add the `recursive` modifier to the loop definition and
call the `loop` variable with the new iterable where you want to recurse.

The following example implements a sitemap with recursive loops::

    <ul class="sitemap">
    {%- for item in sitemap recursive %}
        <li><a href="{{ item.href|e }}">{{ item.title }}</a>
        {%- if item.children -%}
            <ul class="submenu">{{ loop(item.children) }}</ul>
        {%- endif %}</li>
    {%- endfor %}
    </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.

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.

If a macro name starts with an underscore it's not exported and can't
be imported.


.. _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.  Generally speaking a call block works
exactly like an macro, just that it doesn't have a name.

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 %}


Filters
~~~~~~~

Filter sections allow you to apply regular Jinja2 filters on a block of
template data.  Just wrap the code in the special `filter` section::

    {% filter upper %}
        This text becomes uppercase
    {% endfilter %}


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 use the `set` tag and can have multiple targets::

    {% set navigation = [('index.html', 'Index'), ('about.html', 'About')] %}
    {% set key, value = call_something() %}


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.  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`.

From Jinja 2.2 onwards you can mark an include with ``ignore missing`` in
which case Jinja will ignore the statement if the template to be ignored
does not exist.  When combined with ``with`` or ``without context`` it has
to be placed *before* the context visibility statement.  Here some valid
examples::

    {% include "sidebar.html" ignore missing %}
    {% include "sidebar.html" ignore missing with context %}
    {% include "sidebar.html" ignore missing without context %}

.. versionadded:: 2.2

You can also provide a list of templates that are checked for existence
before inclusion.  The first template that exists will be included.  If
`ignore missing` is given, it will fall back to rendering nothing if
none of the templates exist, otherwise it will raise an exception.

Example::

    {% include ['page_detailed.html', 'page.html'] %}
    {% include ['special_sidebar.html', 'sidebar.html'] ignore missing %}

.. versionchanged:: 2.4
   If a template object was passed to the template context you can
   include that object using `include`.

.. _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>

Macros and variables starting with one ore more underscores are private and
cannot be imported.

.. versionchanged:: 2.4
   If a template object was passed to the template context you can
   import from that object.


.. _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 %}

.. admonition:: Note

    In Jinja 2.0 the context that was passed to the included template
    did not include variables defined in the template.  As a matter of
    fact this did not work::

        {% for box in boxes %}
            {% include "render_box.html" %}
        {% endfor %}

    The included template ``render_box.html`` is not able to access
    `box` in Jinja 2.0, but in Jinja 2.1.


.. _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.

.. admonition:: Note

    The special constants `true`, `false` and `none` are indeed lowercase.
    Because that caused confusion in the past, when writing `True` expands
    to an undefined variable that is considered false, all three of them can
    be written in title case too (`True`, `False`, and `None`).  However for
    consistency (all Jinja identifiers are lowercase) you should use the
    lowercase versions.

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 together.  Usually the objects are numbers but if both 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 the second number from the first one.  ``{{ 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.  ``{{ 11 % 7 }}`` is ``4``.

\*
    Multiply the left operand with the right one.  ``{{ 2 * 2 }}`` would
    return ``4``.  This can also be used to repeat a 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 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.

.. admonition:: Note

    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 positional 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>``.

The `else` part is optional.  If not provided the else block implicitly
evaluates into an undefined object::

    {{ '[%s]' % page.title if page.title }}


.. _builtin-filters:

List of Builtin Filters
-----------------------

.. jinjafilters::


.. _builtin-tests:

List of Builtin Tests
---------------------

.. jinjatests::

.. _builtin-globals:

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.

.. function:: dict(\**items)

    A convenient alternative to dict literals.  ``{'foo': 'bar'}`` is the same
    as ``dict(foo='bar')``.

.. class:: cycler(\*items)

    The cycler allows you to cycle among values similar to how `loop.cycle`
    works.  Unlike `loop.cycle` however you can use this cycler outside of
    loops or over multiple loops.

    This is for example very useful if you want to show a list of folders and
    files, with the folders on top, but both in the same list with alternating
    row colors.

    The following example shows how `cycler` can be used::

        {% set row_class = cycler('odd', 'even') %}
        <ul class="browser">
        {% for folder in folders %}
          <li class="folder {{ row_class.next() }}">{{ folder|e }}</li>
        {% endfor %}
        {% for filename in files %}
          <li class="file {{ row_class.next() }}">{{ filename|e }}</li>
        {% endfor %}
        </ul>

    A cycler has the following attributes and methods:

    .. method:: reset()

        Resets the cycle to the first item.

    .. method:: next()

        Goes one item a head and returns the then current item.

    .. attribute:: current

        Returns the current item.
    
    **new in Jinja 2.1**

.. class:: joiner(sep=', ')

    A tiny helper that can be use to "join" multiple sections.  A joiner is
    passed a string and will return that string every time it's calld, except
    the first time in which situation it returns an empty string.  You can
    use this to join things::

        {% set pipe = joiner("|") %}
        {% if categories %} {{ pipe() }}
            Categories: {{ categories|join(", ") }}
        {% endif %}
        {% if author %} {{ pipe() }}
            Author: {{ author() }}
        {% endif %}
        {% if can_edit %} {{ pipe() }}
            <a href="?action=edit">Edit</a>
        {% endif %}

    **new in Jinja 2.1**


Extensions
----------

The following sections cover the built-in Jinja2 extensions that may be
enabled by the application.  The application could also provide further
extensions not covered by this documentation.  In that case there should
be a separate document explaining the extensions.

.. _i18n-in-templates:

i18n
~~~~

If the i18n extension is enabled it's possible to mark parts in the template
as translatable.  To mark a section as translatable you can use `trans`::

    <p>{% trans %}Hello {{ user }}!{% endtrans %}</p>

To translate a template expression --- say, using template filters or just
accessing an attribute of an object --- you need to bind the expression to a
name for use within the translation block::

    <p>{% trans user=user.username %}Hello {{ user }}!{% endtrans %}</p>

If you need to bind more than one expression inside a `trans` tag, separate
the pieces with a comma (``,``)::

    {% trans book_title=book.title, author=author.name %}
    This is {{ book_title }} by {{ author }}
    {% endtrans %}

Inside trans tags no statements are allowed, only variable tags are.

To pluralize, specify both the singular and plural forms with the `pluralize`
tag, which appears between `trans` and `endtrans`::

    {% trans count=list|length %}
    There is {{ count }} {{ name }} object.
    {% pluralize %}
    There are {{ count }} {{ name }} objects.
    {% endtrans %}

Per default the first variable in a block is used to determine the correct
singular or plural form.  If that doesn't work out you can specify the name
which should be used for pluralizing by adding it as parameter to `pluralize`::

    {% trans ..., user_count=users|length %}...
    {% pluralize user_count %}...{% endtrans %}

It's also possible to translate strings in expressions.  For that purpose
three functions exist:

_   `gettext`: translate a single string
-   `ngettext`: translate a pluralizable string
-   `_`: alias for `gettext`

For example you can print a translated string easily this way::

    {{ _('Hello World!') }}

To use placeholders you can use the `format` filter::

    {{ _('Hello %(user)s!')|format(user=user.username) }}

For multiple placeholders always use keyword arguments to `format` as other
languages may not use the words in the same order.

.. versionchanged:: 2.5

If newstyle gettext call are activated (:ref:`newstyle-gettext`), using
placeholders is a lot easier:

.. sourcecode:: html+jinja

    {{ gettext('Hello World!') }}
    {{ gettext('Hello %(name)s!', name='World') }}
    {{ ngettext('%(num)d apple', '%(num)d apples', apples|count) }}

Note that the `ngettext` function's format string automatically recieves
the count as `num` parameter additionally to the regular parameters.


Expression Statement
~~~~~~~~~~~~~~~~~~~~

If the expression-statement extension is loaded a tag called `do` is available
that works exactly like the regular variable expression (``{{ ... }}``) just
that it doesn't print anything.  This can be used to modify lists::

    {% do navigation.append('a string') %}


Loop Controls
~~~~~~~~~~~~~

If the application enables the :ref:`loopcontrols-extension` it's possible to
use `break` and `continue` in loops.  When `break` is reached, the loop is
terminated, if `continue` is eached the processing is stopped and continues
with the next iteration.

Here a loop that skips every second item::

    {% for user in users %}
        {%- if loop.index is even %}{% continue %}{% endif %}
        ...
    {% endfor %}

Likewise a look that stops processing after the 10th iteration::

    {% for user in users %}
        {%- if loop.index >= 10 %}{% break %}{% endif %}
    {%- endfor %}


With Statement
~~~~~~~~~~~~~~

.. versionadded:: 2.3

If the application enables the :ref:`with-extension` it is possible to
use the `with` keyword in templates.  This makes it possible to create
a new inner scope.  Variables set within this scope are not visible
outside of the scope.

With in a nutshell::

    {% with %}
        {% set foo = 42 %}
        {{ foo }}           foo is 42 here
    {% endwith %}
    foo is not visible here any longer

Because it is common to set variables at the beginning of the scope
you can do that within the with statement.  The following two examples
are equivalent::

    {% with foo = 42 %}
        {{ foo }}
    {% endwith %}

    {% with %}
        {% set foo = 42 %}
        {{ foo }}
    {% endwith %}

.. _autoescape-overrides:

Autoescape Extension
--------------------

.. versionadded:: 2.4

If the application enables the :ref:`autoescape-extension` one can
activate and deactivate the autoescaping from within the templates.

Example::

    {% autoescape true %}
        Autoescaping is active within this block
    {% endautoescape %}

    {% autoescape false %}
        Autoescaping is inactive within this block
    {% endautoescape %}

After the `endautoescape` the behavior is reverted to what it was before.
