13. Expressions¶

13.1. 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 / 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, in Python, 42 and 42.0 are different (int and float, respectively).

[‘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.

13.2. 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 }}. (Just like from __future__ import division.)

//

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.

13.3. 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.

13.4. 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)

group an expression.

13.5. 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)

13.6. 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 }}