Template Designer Documentation¶

This document describes the syntax and semantics of the template engine (called ‘Jinja’) and will be most useful as reference to those creating Mailing 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>

There are two kinds of delimiters. {% ... %} 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¶

You can mess around with the variables in templates provided they are passed in by the application. Variables may have attributes or elements on them you can access too. What attributes a variable has depends heavily on the application providing that variable.

You can use a dot (.) to access attributes of a variable, but alternatively the so-called “subscript” syntax ([]) can be used. The following lines do the same thing:

{{ 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 them.

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.

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 attr() filter that just looks up attributes.

Filters¶

Variables can be 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 List of Builtin Filters below describes all the builtin filters.

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 List of 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, a single trailing newline is stripped if present, and whitespace is not further modified by the template engine. Each whitespace (spaces, tabs, newlines etc.) is returned unchanged. If the application configures Jinja to trim_blocks the first newline after a template tag is removed automatically (like in PHP). The lstrip_blocks option can also be set to strip tabs and spaces from the beginning of line to the start of a block. (Nothing will be stripped if there are other characters before the start of the block.)

With both trim_blocks and lstrip_blocks enabled you can put block tags on their own lines, and the entire block line will be removed when rendered, preserving the whitespace of the contents. For example, without the trim_blocks and lstrip_blocks options, this template:

<div>
    {% if True %}
        yay
    {% endif %}
</div>

gets rendered with blank lines inside the div:

<div>

        yay

</div>

But with both trim_blocks and lstrip_blocks enabled, the lines with the template blocks are removed while preserving the whitespace of the contents:

<div>
        yay
</div>

You can manually disable the lstrip_blocks behavior by putting a plus sign (+) at the start of a block:

<div>
        {%+ if something %}yay{% endif %}
</div>

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 Line Statements are enabled they strip leading whitespace automatically up to the beginning of the line.

Jinja2 by default also removes trailing newlines. To keep the single trailing newline when it is present, configure Jinja to keep_trailing_newline.

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¶

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

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

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 configuration 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>

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 dictionaries usually are unordered so you might want to either pass it as a sorted list to the template or use the dictsort filter.

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.
loop.depth	Indicates how deep in deep in a recursive loop the rendering currently is. Starts at level 1
loop.depth0	Indicates how deep in deep in a recursive loop the rendering currently is. Starts at level 0

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 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>

Note that in Python else blocks are executed whenever the corresponding loop did not break. Since in 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. 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 levels 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(...) }}

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 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 inline expression and for 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 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 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 call tag.

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

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 %}

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 xmlattr() filter.

true / false:

true is always true and false is always false.

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.
-: Subtract 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.

Comparisons¶

==: Compares two objects for equality.
!=: Compares two objects for inequality.

>: true if the left hand side is greater than the right hand side.
>=: true if the left hand side is greater or equal to the right hand side.

<: true if the left hand side is lower than the right hand side.
<=: true if the left hand side is lower or equal to the right hand side.

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.

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 test.
|: Applies a filter.

~: 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 Variables)

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 }}

List of Builtin Filters¶

List of Builtin Tests¶

List of Global Functions¶

The following functions are available in the global scope by default:

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>

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.

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:

reset()¶: Resets the cycle to the first item.

next()¶: Goes one item a head and returns the then current item.

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 called, 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.

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 loopcontrols-extension it’s possible to use break and continue in loops. When break is reached, the loop is terminated; if continue is reached, 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¶

New in version 2.3.

If the application enables the 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 Extension¶

New in version 2.4.

If the application enables the 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.