Template Designer Documentation =============================== This document describes the syntax and semantics of the template engine and will be most useful as reference to those creating Jinja templates. As the template engine is very flexible the configuration from the application might be slightly different from here in terms of delimiters and behavior of undefined values. Synopsis -------- A template is simply a text file. It can generate any text-based format (HTML, XML, CSV, LaTeX, etc.). It doesn't have a specific extension, ``.html`` or ``.xml`` are just fine. A template contains **variables** or **expressions**, which get replaced with values when the template is evaluated, and tags, which control the logic of the template. The template syntax is heavily inspired by Django and Python. Below is a minimal template that illustrates a few basics. We will cover the details later in that document: .. sourcecode:: html+jinja My Webpage

My Webpage

{{ a_variable }} This covers the default settings. The application developer might have changed the syntax from ``{% foo %}`` to ``<% foo %>`` or something similar. There are two kinds of delimiers. ``{% ... %}`` and ``{{ ... }}``. The first one is used to execute statements such as for-loops or assign values, the latter prints the result of the expression to the template. .. _variables: Variables --------- The application passes variables to the templates you can mess around in the template. Variables may have attributes or elements on them you can access too. How a variable looks like, heavily depends on the application providing those. You can use a dot (``.``) to access attributes of a variable, alternative the so-called "subscribe" syntax (``[]``) can be used. The following lines do the same: .. sourcecode:: jinja {{ foo.bar }} {{ foo['bar'] }} It's important to know that the curly braces are *not* part of the variable but the print statement. If you access variables inside tags don't put the braces around. If a variable or attribute does not exist you will get back an undefined value. What you can do with that kind of value depends on the application configuration, the default behavior is that it evaluates to an empty string if printed and that you can iterate over it, but every other operation fails. .. _filters: Filters ------- Variables can by modified by **filters**. Filters are separated from the variable by a pipe symbol (``|``) and may have optional arguments in parentheses. Multiple filters can be chained. The output of one filter is applied to the next. ``{{ name|striptags|title }}`` for example will remove all HTML Tags from the `name` and title-cases it. Filters that accept arguments have parentheses around the arguments, like a function call. This example will join a list by spaces: ``{{ list|join(', ') }}``. The :ref:`builtin-filters` below describes all the builtin filters. .. _tests: Tests ----- Beside filters there are also so called "tests" available. Tests can be used to test a variable against a common expression. To test a variable or expression you add `is` plus the name of the test after the variable. For example to find out if a variable is defined you can do ``name is defined`` which will then return true or false depening 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: .. sourcecode:: jinja {% if loop.index is divisibleby 3 %} {% if loop.index is divisibleby(3) %} The :ref:`builtin-tests` below descibes 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: .. sourcecode:: jinja {# note: disabled template because we no longer user this {% for user in users %} ... {% endfor %} #} 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: .. sourcecode:: html+jinja {% block head %} {% block title %}{% endblock %} - My Webpage {% endblock %}

{% block content %}{% endblock %}

In this example, the ``{% block %}`` tags define four blocks that child templates can fill in. All the `block` tag does is to tell the template engine that a child template may override those portions of the template. Child Template ~~~~~~~~~~~~~~ A child template might look like this: .. sourcecode:: html+jinja {% extends "base.html" %} {% block title %}Index{% endblock %} {% block head %} {{ super() }} {% endblock %} {% block content %}

Index

Welcome on my awsome homepage.

{% endblock %} The ``{% extends %}`` tag is the key here. It tells the template engine that this template "extends" another template. When the template system evaluates this template, first it locates the parent. The extends tag should be the first tag in the template. Everything before it is printed out normally and may cause confusion. The filename of the template depends on the template loader. For example the :class:`FileSystemLoader` allows you to access other templates by giving the filename. You can access templates in subdirectories with an slash: .. sourcecode:: jinja {% extends "layout/default.html" %} But this behavior can depend on the application embedding Jinja. Note that since the child template doesn't define the ``footer`` block, the value from the parent template is used instead. You can't define multiple ``{% block %}`` tags with the same name in the same template. This limitation exists because a block tag works in "both" directions. That is, a block tag doesn't just provide a hole to fill - it also defines the content that fills the hole in the *parent*. If there were two similarly-named ``{% block %}`` tags in a template, that template's parent wouldn't know which one of the blocks' content to use. If you want to print a block multiple times you can however use the special `self` variable and call the block with that name: .. sourcecode:: jinja {% block title %}{% endblock %}

{% block body %}{% endblock %} Unlike Python Jinja does not support multiple inheritance. So you can only have one extends tag called per rendering. Super Blocks ~~~~~~~~~~~~ It's possible to render the contents of the parent block by calling `super`. This gives back the results of the parent block: .. sourcecode:: jinja {% block sidebar %}

... {{ super() }} {% endblock %} HTML Escaping ------------- When generating HTML from templates, there's always a risk that a variable will include characters that affect the resulting HTML. There are two approaches: manually escaping each variable or automatically escaping everything by default. Jinja supports both, but what is used depends on the application configuration. The default configuaration is no automatic escaping for various reasons: - escaping everything except of safe values will also mean that Jinja is escaping variables known to not include HTML such as numbers which is a huge performance hit. - The information about the safety of a variable is very fragile. It could happen that by coercing safe and unsafe values the return value is double escaped HTML. Working with Manual Escaping ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If manual escaping is enabled it's **your** responsibility to escape variables if needed. What to escape? If you have a variable that *may* include any of the following chars (``>``, ``<``, ``&``, or ``"``) you **have to** escape it unless the variable contains well-formed and trusted HTML. Escaping works by piping the variable through the ``|e`` filter: ``{{ user.username|e }}``. Working with Automatic Escaping ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When automatic escaping is enabled everything is escaped by default except for values explicitly marked as safe. Those can either be marked by the application or in the template by using the `|safe` filter. The main problem with this approach is that Python itself doesn't have the concept of tainted values so the information if a value is safe or unsafe can get lost. If the information is lost escaping will take place which means that you could end up with double escaped contents. Double escaping is easy to avoid however, just relay 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 Loops ~~~~~~~~~ Loop over each item in a sequece. For example, to display a list of users provided in a variable called `users`: .. sourcecode:: html+jinja

Members

{{ user.username|e }}

Inside of a for loop block you can access some special variables: +-----------------------+---------------------------------------------------+ | Variable | Description | +=======================+===================================================+ | `loop.index` | The current iteration of the loop. (1 indexed) | +-----------------------+---------------------------------------------------+ | `loop.index0` | The current iteration of the loop. (0 indexed) | +-----------------------+---------------------------------------------------+ | `loop.revindex` | The number of iterations from the end of the loop | | | (1 indexed) | +-----------------------+---------------------------------------------------+ | `loop.revindex0` | The number of iterations from the end of the loop | | | (0 indexed) | +-----------------------+---------------------------------------------------+ | `loop.first` | True if first iteration. | +-----------------------+---------------------------------------------------+ | `loop.last` | True if last iteration. | +-----------------------+---------------------------------------------------+ | `loop.length` | The number of items in the sequence. | +-----------------------+---------------------------------------------------+ | `loop.cycle` | A helper function to cycle between a list of | | | sequences. See the explanation below. | +-----------------------+---------------------------------------------------+ Within a for-loop, it's psosible to cycle among a list of strings/variables each time through the loop by using the special `loop.cycle` helper: .. sourcecode:: html+jinja {% for row in rows %}

{% endfor %} .. _loop-filtering: Unlike in Python it's not possible to `break` or `continue` in a loop. You can however filter the sequence during iteration which allows you to skip items. The following example skips all the users which are hidden: .. sourcecode:: html+jinja {% for user in users if not user.hidden %}

{% 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`: .. sourcecode:: html+jinja

{{ user.username|e }}
no users found

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: .. sourcecode:: html+jinja {% if users %}

{{ user.username|e }}

{% endif %} For multiple branches `elif` and `else` can be used like in Python. You can use more complex :ref:`expressions` there too. .. sourcecode:: html+jinja {% if kenny.sick %} Kenny is sick. {% elif kenny.dead %} You killed Kenny! You bastard!!! {% else %} Kenny looks okay --- so far {% endif %} If can also be used as :ref:`inline expression ` and for :ref:`loop filtering `. .. _expressions: Expressions ----------- Jinja allows basic expressions everywhere. These work very similar to regular Python and even if you're not working with Python you should feel comfortable with it. Literals ~~~~~~~~ The simplest form of expressions are literals. Literals are representations for Python objects such as strings and numbers. The following literals exist: ``"Hello World"`` Everything between two double or single quotes is a string. They are useful whenever you need a string in the template (for example as arguments to function calls, filters or just to extend or include a template). ``42`` / ``42.23`` Integers and floating point numbers are created by just writing the number down. If a dot is present the number is a float, otherwise an integer. Keep in mind that for Python ``42`` and ``42.0`` is something different. ``['list', 'of', 'objects']`` Everything between two brackets is a list. Lists are useful to store sequential data in or to iterate over them. For example you can easily create a list of links using lists and tuples with a for loop. .. sourcecode:: html+jinja

{{ caption }}

``('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', 'keys': 'and', 'value': 'pairs'}`` A dict in Python is a structure that combines keys and values. Keys must be unique and always have exactly one value. Dicts are rarely used in templates, they are useful in some rare cases such as the :func:`xmlattr` filter. ``true`` and ``false`` true is always true and false is always false. Keep in mind that those literals are lowercase! Math ~~~~ Jinja allows you to calculate with values. This is rarely useful in templates but exists for completeness sake. The following operators are supported: ``+`` Adds two objects with each other. Usually numbers but if both objects are strings or lists you can concatenate them this way. This however is not the preferred way to concatenate strings! For string concatenation have a look at the ``~`` operator. ``{{ 1 + 1 }}`` is ``2``. ``-`` Substract two numbers from each other. ``{{ 3 - 2 }}`` is ``1``. ``/`` Divide two numbers. The return value will be a floating point number. ``{{ 1 / 2 }}`` is ``{{ 0.5 }}``. ``//`` Divide two numbers and return the truncated integer result. ``{{ 20 / 7 }}`` is ``2``. ``%`` Calculate the remainder of an integer division between the left and right operand. ``{{ 11 % 7 }}`` is ``4``. ``*`` Multiply the left operand with the right one. ``{{ 2 * 2 }}`` would return ``4``. This can also be used to repeat string multiple times. ``{{ '=' * 80 }}`` would print a bar of 80 equal signs. ``**`` Raise the left operand to the power of the right operand. ``{{ 2**3 }}`` would return ``8``. Logic ~~~~~ For `if` statements / `for` filtering or `if` expressions it can be useful to combine group multiple expressions: ``and`` Return true if the left and the right operand is true. ``or`` Return true if the left or the right operand is true. ``not`` negate a statement (see below). ``()`` (group) group an expression. Note that there is no support for any bit operations or something similar. - special note regarding ``not``: The ``is`` and ``in`` operators support negation using an infix notation too: ``foo is not bar`` and ``foo not in bar`` instead of ``not foo is bar`` and ``not foo in bar``. All other expressions require a prefix notation: ``not (foo and bar).`` Other Operators ~~~~~~~~~~~~~~~ The following operators are very useful but don't fit into any of the other two categories: ``in`` Perform sequence / mapping containment test. Returns true if the left operand is contained in the right. ``{{ 1 in [1, 2, 3] }}`` would for example return true. ``is`` Performs a :ref:`tests `. ``|`` Applies a :ref:`filters `. ``~`` Converts all operands into strings and concatenates them. ``{{ "Hello " ~ name ~ "!" }}`` would return (assuming `name` is ``'John'``) ``Hello John!``. ``()`` (call) Call a callable: ``{{ post.render() }}``. Inside of the parentheses you can use arguments and keyword arguments like in python: ``{{ post.render(user, full=true) }}``. ``.`` / ``[]`` Get an attribute of an object. (See :ref:`variables`) .. _if-expression: If Expression ~~~~~~~~~~~~~ blah .. _builtin-filters: List of Builtin Filters ----------------------- .. jinjafilters:: .. _builtin-tests: List of Builtin Tests --------------------- .. jinjatests::

My Webpage

Index

{{ self.title() }}

Table Of Contents

Members