About templates

Files collected inside the templates directory are closed from direct access from public side. Such files are templates, not just static files. They can work with controllers for creation dynamic pages.

We are using syntax of Jinja2 templating engine.

Global objects

There are some standard global objects which you can use inside the templates:

request. Special object which collects specific information about current request.
controller. A controller object.
site. A site object.
datetime. A module for date/time operation.
DB. Represents an interface for access to Tabbli database.
static. Function which returns a url for static file.
url. Function which returns a url with corresponding name.
redirect_to. Initiates a redirect with status 302 to another url.
markdown. A function which converts Markdown to HTML.

About Jinja templates

A Jinja template is just a text file. In general, Jinja can generate any text-based format (HTML, XML, CSV, LaTeX, etc.).

A template contains variables and/or expressions, which get replaced with values when a template is rendered; and tags, which control the logic of the template.

Below is a minimal template that illustrates a few basics using the default Jinja configuration. We will cover the details later in this document:

{% set company=DB()
    .get_collection('companies')
    .get_record(key=key).get_dict() %}
<!DOCTYPE html>
<html lang="en">
<head>
    <title>{{ company.name.en }}</title>
</head>
<body>
    <h1>{{ company.name.en }}</h1>
    
    <h2>Services</h2>
    <ul>
    {% for item in company.properties.services %}
        <li>
            <a href="/services/{{ item.key }}/">
                {{ item.name.en }}
            </a>
        </li>
    {% endfor %}
    </ul>
</body>
</html>

There are a few kinds of delimiters. The default Jinja delimiters are configured as follows:

{% ... %} for Statements
{{ ... }} for Expressions to print to the template output
{# ... #} for Comments not included in the template output

Variables

Tabbli defines some template variables by default in the global context.

Variables may have attributes or elements on them you can access. What attributes a variable has depends heavily on the application providing that variable.

You can use a dot (.) to access attributes of a variable in addition to the [] syntax. The following lines do the same thing:

{{ foo.bar }}
{{ foo['bar'] }}

It’s important to know that the outer double-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.

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.

For example, {{ name|striptags|title }} will remove all HTML Tags from variable name and title-case the output (title(striptags(name))).

Filters that accept arguments have parentheses around the arguments, just like a function call. For example: {{listx|join(', ') }} will join a list with commas (str.join(', ', listx)).

Builtin filters

The list below describes the builtin filters. More detailed version you can read in official Jinja documentation.

abs(x, name) - Returns the absolute value of the argument.

attr(obj, name) - Get an attribute of an object. foo|attr("bar") works like foo.bar just that always an attribute is returned and items are not looked up.

batch(value, linecount, fill_with=None) - A filter that batches items. It works pretty much like slice just the other way round. It returns a list of lists with the given number of items. If you provide a second parameter this is used to fill up missing items. See this example:

<table>
{%- for row in items|batch(3, '&nbsp;') %}
  <tr>
  {%- for column in row %}
    <td>{{ column }}</td>
  {%- endfor %}
  </tr>
{%- endfor %}
</table>

capitalize(s) - Capitalize a value. The first character will be uppercase, all others lowercase.

default(value, default_value='', boolean=False) - If the value is undefined it will return the passed default value, otherwise the value of the variable:

{{ my_variable|default('my_variable is not defined') }}

escape(s) - Convert the characters &, <, >, ‘, and ” in string s to HTML-safe sequences. Use this if you need to display text that might contain such characters in HTML. Marks return value as markup string.

filesizeformat(value, binary=False) - Format the value like a "human-readable" file size (i.e. 13 kB, 4.1 MB, 102 Bytes, etc). Per default decimal prefixes are used (Mega, Giga, etc.), if the second parameter is set to True the binary prefixes are used (Mebi, Gibi).

first(seq) - Return the first item of a sequence.

float(value, default=0.0) - Convert the value into a floating point number. If the conversion doesn’t work it will return 0.0. You can override this default using the first parameter.

forceescape(value) - Enforce HTML escaping. This will probably double escape variables.

format(value, *args, **kwargs) - Apply the given values to a printf-style format string, like string % values.

{{ "%s, %s!"|format(greeting, name) }}
Hello, World!

In most cases it should be more convenient and efficient to use the % operator or str.format().

{{ "%s, %s!" % (greeting, name) }}
{{ "{}, {}!".format(greeting, name) }}

groupby(value, attribute) - Group a sequence of objects by an attribute using Python’s itertools.groupby(). The attribute can use dot notation for nested access, like "address.city". Unlike Python’s groupby, the values are sorted first so only one group is returned for each unique value.

For example, a list of Person objects with a city attribute can be rendered in groups. In this example, grouperrefers to the city value of the group.

<ul>{% for city, items in persons|groupby("city") %}
  <li>{{ city }}
    <ul>{% for person in items %}
      <li>{{ person.name.en }}
    {% endfor %}</ul>
  </li>
{% endfor %}</ul>

groupby yields namedtuples of (grouper, list), which can be used instead of the tuple unpacking above. grouper is the value of the attribute, and list is the items with that value.

<ul>{% for group in persons|groupby("city") %}
  <li>{{ group.grouper }}: {{ group.list|join(", ") }}
{% endfor %}</ul>

int(value, default=0, base=10) - Convert the value into an integer. If the conversion doesn’t work it will return 0. You can override this default using the first parameter. You can also override the default base (10) in the second parameter, which handles input with prefixes such as 0b, 0o and 0x for bases 2, 8 and 16 respectively. The base is ignored for decimal numbers and non-string values.

join(value, d='', attribute=None) - Return a string which is the concatenation of the strings in the sequence. The separator between elements is an empty string per default, you can define it with the optional parameter:

{{ [1, 2, 3]|join('|') }}
    -> 1|2|3

{{ [1, 2, 3]|join }}
    -> 123

It is also possible to join certain attributes of an object:

{{ users|join(', ', attribute='email') }}

last(seq) - Return the last item of a sequence.

Note: Does not work with generators. You may want to explicitly convert it to a list:

{{ records|selectattr('properties.gender', '==', 'female')|list|last }}

length(obj, /) - Return the number of items in a container.

countlist(value) - Convert the value into a list. If it was a string the returned list will be a list of characters.

lower(s) - Convert a value to lowercase.

map(*args, **kwargs) - Applies a filter on a sequence of objects or looks up an attribute. This is useful when dealing with lists of objects but you are really only interested in a certain value of it. The basic usage is mapping on an attribute. Imagine you have a list of users but you are only interested in a list of usernames:

Users on this page: {{ users|map(attribute='email')|join(', ') }}

max(value, case_sensitive=False, attribute=None) - Return the largest item from the sequence.

{{ [1, 2, 3]|max }}
    -> 3

min(value, case_sensitive=False, attribute=None) - Return the smallest item from the sequence.

{{ [1, 2, 3]|min }}
    -> 1

pprint(value, verbose=False) - Pretty print a variable. Useful for debugging.

random(seq) - Return a random item from the sequence.

reject(*args, **kwargs) - Filters a sequence of objects by applying a test to each object, and rejecting the objects with the test succeeding. If no test is specified, each object will be evaluated as a boolean. Example usage:

{{ numbers|reject("odd") }}

rejectattr(*args, **kwargs) - Filters a sequence of objects by applying a test to the specified attribute of each object, and rejecting the objects with the test succeeding. If no test is specified, the attribute’s value will be evaluated as a boolean.

{{ users|rejectattr("is_active") }}
{{ users|rejectattr("email", "none") }}

replace(s, old, new, count=None) - Return a copy of the value with all occurrences of a substring replaced with a new one. The first argument is the substring that should be replaced, the second is the replacement string. If the optional third argument count is given, only the first count occurrences are replaced:

{{ "Hello World"|replace("Hello", "Goodbye") }}
    -> Goodbye World

{{ "aaaaargh"|replace("a", "d'oh, ", 2) }}
    -> d'oh, d'oh, aaargh

reverse(value) - Reverse the object or return an iterator that iterates over it the other way round.

round(value, precision=0, method='common') - Round the number to a given precision. The first parameter specifies the precision (default is 0), the second the rounding method:

'common' rounds either up or down
'ceil' always rounds up
'floor' always rounds down

If you don’t specify a method 'common' is used.

{{ 42.55|round }}
    -> 43.0
{{ 42.55|round(1, 'floor') }}
    -> 42.5

Note that even if rounded to 0 precision, a float is returned. If you need a real integer, pipe it through int:

{{ 42.55|round|int }}
    -> 43

safe(value) - Mark the value as safe which means that in an environment with automatic escaping enabled this variable will not be escaped.

select(*args, **kwargs) - Filters a sequence of objects by applying a test to each object, and only selecting the objects with the test succeeding. If no test is specified, each object will be evaluated as a boolean. Example usage:

{{ numbers|select("odd") }}
{{ numbers|select("odd") }}
{{ numbers|select("divisibleby", 3) }}
{{ numbers|select("lessthan", 42) }}
{{ strings|select("equalto", "mystring") }}

selectattr(*args, **kwargs) - Filters a sequence of objects by applying a test to the specified attribute of each object, and only selecting the objects with the test succeeding. If no test is specified, the attribute’s value will be evaluated as a boolean. Example usage:

{{ users|selectattr("is_active") }}
{{ users|selectattr("email", "none") }}

slice(value, slices, fill_with=None) - Slice an iterator and return a list of lists containing those items. Useful if you want to create a div containing three ul tags that represent columns:

<div class="columnwrapper">
  {%- for column in items|slice(3) %}
    <ul class="column-{{ loop.index }}">
    {%- for item in column %}
      <li>{{ item }}</li>
    {%- endfor %}
    </ul>
  {%- endfor %}
</div>

If you pass it a second argument it’s used to fill missing values on the last iteration.

sort(value, reverse=False, case_sensitive=False, attribute=None) - Sort an iterable.

{% for city in cities|sort %}
    ...
{% endfor %}

string(object) - Make a string unicode if it isn’t already. That way a markup string is not converted back to unicode.

striptags(value) - Strip SGML/XML tags and replace adjacent whitespace by one space.

sum(iterable, attribute=None, start=0) - Returns the sum of a sequence of numbers plus the value of parameter ‘start’ (which defaults to 0). When the sequence is empty it returns start. It is also possible to sum up only certain attributes:

Total: {{ items|sum(attribute='price') }}

title(s) - Return a titlecased version of the value. I.e. words will start with uppercase letters, all remaining characters are lowercase.

tojson(value, indent=None) - Dumps a structure to JSON so that it’s safe to use in <script> tags. It accepts the same arguments and returns a JSON string. Note that this is available in templates through the |tojson filter which will also mark the result as safe. Due to how this function escapes certain characters this is safe even if used outside of <script>tags.

The following characters are escaped in strings:

This makes it safe to embed such strings in any place in HTML with the notable exception of double quoted attributes. In that case single quote your attributes or HTML escape it in addition.

The indent parameter can be used to enable pretty printing. Set it to the number of spaces that the structures should be indented with.

trim(value, chars=None) - Strip leading and trailing characters, by default whitespace.

truncate(s, length=255, killwords=False, end='...', leeway=None) - Return a truncated copy of the string. The length is specified with the first parameter which defaults to 255. If the second parameter is true the filter will cut the text at length. Otherwise it will discard the last word. If the text was in fact truncated it will append an ellipsis sign ("..."). If you want a different ellipsis sign than "..."you can specify it using the third parameter. Strings that only exceed the length by the tolerance margin given in the fourth parameter will not be truncated.

{{ "foo bar baz qux"|truncate(9) }}
    -> "foo..."
{{ "foo bar baz qux"|truncate(9, True) }}
    -> "foo ba..."
{{ "foo bar baz qux"|truncate(11) }}
    -> "foo bar baz qux"
{{ "foo bar baz qux"|truncate(11, False, '...', 0) }}
    -> "foo bar..."

unique(value, case_sensitive=False, attribute=None) - Returns a list of unique items from the given iterable.

{{ ['foo', 'bar', 'foobar', 'FooBar']|unique|list }}
    -> ['foo', 'bar', 'foobar']

upper(s) - Convert a value to uppercase.

urlencode(value) - Quote data for use in a URL path or query using UTF-8. When given a string, “/” is not quoted. HTTP servers treat “/” and “%2F” equivalently in paths. If you need quoted slashes, use the |replace("/", "%2F") filter.

urlize(value, trim_url_limit=None, nofollow=False, target=None, rel=None) - Converts URLs in plain text into clickable links. If you pass the filter an additional integer it will shorten the urls to that number. Also a third argument exists that makes the urls “nofollow”:

{{ mytext|urlize(40, true) }}
    links are shortened to 40 chars and defined with rel="nofollow"

If target is specified, the target attribute will be added to the <a> tag:

{{ mytext|urlize(40, target='_blank') }}

wordcount(s) - Count the words in that string.

wordwrap(s, width=79, break_long_words=True, wrapstring=None, break_on_hyphens=True) - Wrap a string to the given width. Existing newlines are treated as paragraphs to be wrapped separately.

xmlattr(d, autospace=True) - Create an SGML/XML attribute string based on the items in a dict. All values that are neither none nor undefined are automatically escaped:

<ul{{ {'class': 'my_list', 'missing': none,
        'id': 'list-%d'|format(variable)}|xmlattr }}>
...
</ul>

Results in something like this:

<ul class="my_list" id="list-42">
...
</ul>

As you can see it automatically prepends a space in front of the item if the filter returned something unless the second parameter is false.

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 whether name is defined in the current template context.

Tests can accept arguments, too. If the test only takes one argument, you can leave out the parentheses. For example, the following two expressions do the same thing:

{% if loop.index is divisibleby 3 %}
{% if loop.index is divisibleby(3) %}

Builtin tests

The list below describes the builtin tests. More detailed version you can read in official Jinja documentation.

boolean(value) - Return true if the object is a boolean value.

callable(obj, /) - Return whether the object is callable (i.e., some kind of function).

defined(value) - Return true if the variable is defined:

{% if variable is defined %}
    value of variable: {{ variable }}
{% else %}
    variable is not defined
{% endif %}

divisibleby(value, num) - Check if a variable is divisible by a number.

escaped(value) - Check if the value is escaped.

even(value) - Return true if the variable is even.

false(value) - Return true if the object is False.

float(value) - Return true if the object is a float.

integer(value) - Return true if the object is an integer.

iterable(value) - Check if it’s possible to iterate over an object.

lower(value) - Return true if the variable is lowercased.

mapping(value) - Return true if the object is a mapping (dict etc.).

none(value) - Return true if the variable is none.

number(value) - Return true if the variable is a number.

odd(value) - Return true if the variable is odd.

sameas(value, other) - Check if an object points to the same memory address than another object:

{% if foo.attribute is sameas false %}
    the foo attribute really is the `False` singleton
{% endif %}

sequence(value) - Return true if the variable is a sequence. Sequences are variables that are iterable.

string(value) - Return true if the object is a string.

true(value) - Return true if the object is True.

undefined(value) - Return true if the object is undefined.

upper(value) - Return true if the variable is uppercased.

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: commented-out 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
other whitespace (spaces, tabs, newlines etc.) is returned unchanged

You can strip whitespace in templates by hand. If you add a minus sign (-) to the start or end of a block (e.g. a For tag), a comment, or a variable expression, the whitespaces before or after that block will be removed:

{% 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.

Escaping

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

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

{{ '{{' }}

For bigger sections, it makes sense to mark a block raw. For example, to include example Jinja syntax in a template, you can use this snippet:

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

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>
<html lang="en">
<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>
</html>

In this example, the {% block %} tags define four blocks that child templates can fill in. All the block tag does is tell the template engine that a child template may override those placeholders in the template.

block tags can be inside other blocks such as if, but they will always be executed regardless of if the if block is actually rendered.

Child Template

A child template might look like this:

{% extends site.key+":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 to 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, it first 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 Null-Master Fallback. Also a block will always be filled in regardless of whether the surrounding condition is evaluated to be true or false.

The filename of the template should be specified like site_key:template_name , where template_name is a path to the template inside the templates directory.

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 placeholder to fill - it also defines the content that fills the placeholder 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 %

Nesting extends

In the case of multiple levels of {% extends %}, super references may be chained (as in super.super()) to skip levels in the inheritance tree.

For example:

# parent.html
body: {% block body %}Hi from parent.{% endblock %}

# child.html
{% extends site.key+":parent.html" %}
{% block body %}Hi from child. {{ super() }}{% endblock %}

# grandchild1.html
{% extends site.key+":child.html" %}
{% block body %}Hi from grandchild1.{% endblock %}

# grandchild2.html
{% extends site.key+"child.html" %}
{% block body %}Hi from grandchild2. {{ super.super() }} {% endblock %}

Rendering child.tmpl will give body: Hi from child. Hi from parent.

Rendering grandchild1.tmpl will give body: Hi from grandchild1.

Rendering grandchild2.tmpl will give body: Hi from grandchild2. Hi from parent.

Named Block End-Tags

Jinja 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.

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

If a template object was passed in 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 %}

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. What is used depends on the application configuration. The default configuration is no automatic escaping; for various reasons:

Escaping everything except for safe values will also mean that Jinja is escaping variables known to not include HTML (e.g. numbers, booleans) which can be 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 SHOULD 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. Variables and expressions can be marked as safe either in:

The context dictionary by the application with markupsafe.Markup
The template, with the |safe filter.

If a string that you marked safe is passed through other Python code that doesn’t understand that mark, it may get lost. Be aware of when your data is marked safe and how it is processed before arriving at the template.

If a value has been escaped but is not marked safe, auto-escaping will still take place and result in double-escaped characters. If you know you have data that is already safe but not marked, be sure to wrap it in Markup or use the |safe filter.

Jinja functions (macros, super, self.BLOCKNAME) always return template data that is marked as safe.

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. With the default syntax, control structures appear inside {% ...%} blocks.

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.items() %}
    <dt>{{ key|e }}</dt>
    <dd>{{ value|e }}</dd>
{% endfor %}
</dl>

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

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 a recursive loop the rendering currently is. Starts at level 1
loop.depth0 - Indicates how deep in a recursive loop the rendering currently is. Starts at level 0
loop.previtem - The item from the previous iteration of the loop. Undefined during the first iteration.
loop.nextitem - The item from the following iteration of the loop. Undefined during the last iteration.
loop.changed(*val) - True if previously called with a different value (or not called at all).

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 %}

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>

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(…) }}

Please note that assignments in loops will be cleared at the end of the iteration and cannot outlive the loop scope. Older versions of Jinja had a bug where in some circumstances it appeared that assignments would work. This is not supported. See Assignments for more information about how to deal with this.

If all you want to do is check whether some value has changed since the last iteration or will change in the next iteration, you can use previtem and nextitem:

{% for value in values %}
    {% if loop.previtem is defined and value > loop.previtem %}
        The value just increased!
    {% endif %}
    {{ value }}
    {% if loop.nextitem is defined and loop.nextitem > value %}
        The value will increase even more!
    {% endif %}
{% endfor %}

If you only care whether the value changed at all, using changed is even easier:

{% for entry in entries %}
    {% if loop.changed(entry.category) %}
        <h2>{{ entry.category }}</h2>
    {% endif %}
    <p>{{ entry.message }}</p>
{% endfor %}

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 and 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.

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.

Call

In some cases it can be useful to pass a macro to another macro. For this purpose, you can use the special callblock. 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 %}

Filter

Filter sections allow you to apply regular Jinja 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() %}

Scoping Behavior

Please keep in mind that it is not possible to set variables inside a block and have them show up outside of it. This also applies to loops. The only exception to that rule are if statements which do not introduce a scope. As a result the following template is not going to do what you might expect:

{% set iterated = false %}
{% for item in seq %}
    {{ item }}
    {% set iterated = true %}
{% endfor %}
{% if not iterated %} did not iterate {% endif %}

It is not possible with Jinja syntax to do this. Instead use alternative constructs like the loop else block or the special loop variable:

{% for item in seq %}
    {{ item }}
{% else %}
    did not iterate
{% endfor %}

More complex use cases can be handled using namespace objects which allow propagating of changes across scopes:

{% set ns = namespace(found=false) %}
{% for item in items %}
    {% if item.check_something() %}
        {% set ns.found = true %}
    {% endif %}
    * {{ item.title }}
{% endfor %}
Found item having something: {{ ns.found }}

Note that the obj.attr notation in the set tag is only allowed for namespace objects; attempting to assign an attribute on any other object will raise an exception.

Block Assignments

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.

The block assignment also supports filters.

Example:

{% set reply | wordwrap %}
    You wrote:
    {{ message }}
{% endset %}

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.

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.

Include

The include tag is useful to include a template and return the rendered contents of that file into the current namespace:

{% include site.key+':header.html' %}
    Body
{% include site.key+':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.

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 site.key+":sidebar.html" ignore missing %}
{% include site.key+":sidebar.html" ignore missing with context %}
{% include site.key+":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 [site.key+':page_detailed.html', site.key+':page.html'] %}
{% include [
    site.key+':special_sidebar.html', site.key+':sidebar.html'] 
    ignore missing %}

Import

Jinja 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 site.key+':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 site.key+':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.

Import Context Behavior

By default, included templates are passed the current context and imported templates are 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 behavior can be changed 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 are two examples:

{% from 'forms.html' import input with context %}
{% include 'header.html' without context %}

Expressions

Jinja allows basic expressions everywhere. These work very similarly to regular Python; 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 (e.g. as arguments to function calls and filters, or just to extend or include a template).
42 / 123_456 - Integers are whole numbers without a decimal part. The ‘_’ character can be used to separate groups for legibility.
42.23 / 42.1e2 / 123_456.789 - Floating point numbers can be written using a ‘.’ as a decimal mark. They can also be written in scientific notation with an upper or lower case ‘e’ to indicate the exponent part. The ‘_’ character can be used to separate groups for legibility, but cannot be used in the exponent part.
['list', 'of', 'objects'] - Everything between two brackets is a list. Lists are useful for storing sequential data to be iterated over. For example, you can easily create a list of links using lists and tuples for (and 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 that cannot be modified (“immutable”). If a tuple only has one item, it must be followed by a comma (('1-tuple',)). Tuples are usually used to represent items of two or more elements. See the list 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.

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-see 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, and if expressions, it can be useful to combine multiple expressions:

and - Return true if the left and the right operand are true.
or - Return true if the left or the right operand are true.
not - negate a statement (see below).
(expr) - Parentheses group an expression.

Other Operators

The following operators are very useful but don’t fit into any of the other two categories:

in - Perform a 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 set to '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 site.key+'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:

{{ "[{}]".format(page.title) if page.title }}

Python Methods

You can also use any of the methods of defined on a variable’s type. The value returned from the method invocation is used as the value of the expression. Here is an example that uses methods defined on strings (where page.title is a string):

{{ page.title.capitalize() }}

This works for methods on user-defined types. For example, if variable f of type Foo has a method bar defined on it, you can do the following:

{{ f.bar(value) }}

Operator methods also work as expected. For example, % implements printf-style for strings:

{{ "Hello, %s!" % name }}

Although you should prefer the .format method for that case (which is a bit contrived in the context of rendering a template):

{{ "Hello, {}!".format(name) }}

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)and range(0, 4, 1) return [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, e.g. 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. By default, five paragraphs of HTML are generated with each paragraph between 20 and 100 words. If html is False, 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').

cycler(*items) - Cycle through values by yielding them one at a time, then restarting once the end is reached.

Similar to loop.cycle, but can be used outside loops or across multiple loops. For example, render a list of folders and files in a list, alternating giving them “odd” and “even” classes.

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

current - Return the current item. Equivalent to the item that will be returned next time next() is called.
next() - Return the current item, then advance current to the next item.
reset() - Resets the current item to the first item.

namespace(...) - Creates a new container that allows attribute assignment using the {% set %} tag:

{% set ns = namespace() %}
{% set ns.foo = 'bar' %}

The main purpose of this is to allow carrying a value from within a loop body to an outer scope. Initial values can be provided as a dict, as keyword arguments, or both (same behavior as Python’s dict constructor):

{% set ns = namespace(found=false) %}
{% for item in items %}
    {% if item.check_something() %}
        {% set ns.found = true %}
    {% endif %}
    * {{ item.title }}
{% endfor %}
Found item having something: {{ ns.found }}

Expression Statement

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

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

Loop Controls

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’s a loop that skips every second item:

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

Likewise, a loop that stops processing after the 10th iteration:

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

Note that loop.index starts with 1, and loop.index0 starts with 0 (See: For).

Debug statement

A {% debug %} tag will be available to dump the current context as well as the available filters and tests. This is useful to see what’s available to use in the template without setting up a debugger.

<pre>{% debug %}</pre>

{'context': {'cycler': <class 'jinja2.utils.Cycler'>,
             ...,
             'namespace': <class 'jinja2.utils.Namespace'>},
 'filters': ['abs', 'attr', 'batch', 'capitalize', 'center', 'count', 'd',
             ..., 'urlencode', 'urlize', 'wordcount', 'wordwrap', 'xmlattr'],
 'tests': ['!=', '<', '<=', '==', '>', '>=', 'callable', 'defined',
           ..., 'odd', 'sameas', 'sequence', 'string', 'undefined', 'upper']}

With statement

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

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

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

PreviousWorking with files NextUnderstanding controllers

Last updated 4 years ago

Was this helpful?