11. List of Control Structures¶

11.1. 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>

As variables in templates retain their object properties, it is possible to iterate over containers like dict:

<dl>
{% for key, value in my_dict.iteritems() %}
    <dt>{{ key|e }}</dt>
    <dd>{{ value|e }}</dd>
{% endfor %}
</dl>

Note, however, that Python dicts are not ordered; so you might want to either pass a sorted list of tuple s – or a collections.OrderedDict – to the template, or use the dictsort filter.

Inside of a for-loop block, you can access some special variables:

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 %}

Since Jinja 2.1, an extra cycle helper exists that allows loop-unbound cycling. For more information, have a look at the List of Global Functions.

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 default block by using else:

<ul>
{% for user in users %}
    <li>{{ user.username|e }}</li>
{% else %}
    <li><em>no users found</em></li>
{% endfor %}
</ul>

Note that, in Python, else blocks are executed whenever the corresponding loop did not break. Since Jinja loops cannot break anyway, a slightly different behavior of the else keyword was chosen.

It is also possible to use loops recursively. This is useful if you are dealing with recursive data such as sitemaps or RDFa. 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>

The loop variable always refers to the closest (innermost) loop. If we have more than one level of loops, we can rebind the variable loop by writing {% set outer_loop = loop %} after the loop that we want to use recursively. Then, we can call it using {{ outer_loop(...) }}

11.2. If¶

The if statement in Jinja is comparable with the Python if statement. 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 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 an inline expression and for loop filtering.

11.3. 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 (“DRY”).

Here’s 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 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 a 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 call tag, the caller is stored in this variable as a callable macro.

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 (i.e.: accesses the special kwargs variable).

catch_varargs

This is true if the macro accepts extra positional arguments (i.e.: accesses the special varargs variable).

caller

This is true if the macro accesses the special caller variable and may be called from a call tag.

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

11.4. 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 a replacement for loops. Generally speaking, a call block works exactly like a macro without a name.

Here’s 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 %}

11.5. 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 %}

11.6. 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() %}

11.7. Block Assignments¶

Starting with Jinja 2.8, it’s possible to also use block assignments to capture the contents of a block into a variable name. This can be useful in some situations as an alternative for macros. In that case, instead of using an equals sign and a value, you just write the variable name and then everything until {% endset %} is captured.

Example:

{% set navigation %}
    <li><a href="/">Index</a>
    <li><a href="/downloads">Downloads</a>
{% endset %}

The navigation variable then contains the navigation HTML source.

11.8. Extends¶

The extends tag can be used to extend one template from another. You can have multiple extends tags in a file, but only one of them may be executed at a time.

See the section about Template Inheritance above.

11.9. Blocks¶

Blocks are used for inheritance and act as both placeholders and replacements at the same time. They are documented in detail in the Template Inheritance section.

11.10. 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 Import Context Behavior.

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 included does not exist. When combined with with or without context, it must be placed before the context visibility statement. Here are some valid examples:

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

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 %}

11.11. Import¶

Jinja2 supports putting often used code into macros. These macros can go into different templates and get imported from there. This works similarly 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 default. For more details about context behavior of imports and includes, see Import Context Behavior.

There are two ways to import templates. You can import a 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 way to access a template’s variables and macros is to import the whole template 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 specific names from a 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 or more underscores are private and cannot be imported.