Template Syntax

Variables

Top-level template variables are defined by the template's context type. You can use a dot (.) to access variable's attributes or methods. Reading from variables is subject to the usual borrowing policies. For example, {{ name }} will get the name field from the template context, while {{ user.name }} will get the name field of the user field from the template context.

Using constants in templates

You can use constants defined in your Rust code. For example if you have:

pub const MAX_NB_USERS: usize = 2;

defined in your crate root, you can then use it in your templates by using crate::MAX_NB_USERS:

<p>The user limit is {{ crate::MAX_NB_USERS }}.</p>
{% set value = 4 %}
{% if value > crate::MAX_NB_USERS %}
    <p>{{ value }} is bigger than MAX_NB_USERS.</p>
{% else %}
    <p>{{ value }} is less than MAX_NB_USERS.</p>
{% endif %}

Assignments

Inside code blocks, you can also declare variables or assign values to variables. Assignments can't be imported by other templates.

Assignments use the let tag:

{% let name = user.name %}
{% let len = name.len() %}

{% let val -%}
{% if len == 0 -%}
  {% let val = "foo" -%}
{% else -%}
  {% let val = name -%}
{% endif -%}
{{ val }}

Like Rust, Askama also supports shadowing variables.

{% let foo = "bar" %}
{{ foo }}

{% let foo = "baz" %}
{{ foo }}

You can declare variables as mutable with the mut keyword:

{# In this example, `foo` is an iterator. If you want to be able to iterate it,
you need it to be mutable #}
{% let mut foo = [1, 2].iter() %}
{{ foo.next().unwrap() }}

For compatibility with Jinja, set can be used in place of let.

Borrow rules

In some cases, the value of a variable initialization will be put behind a reference to prevent changing ownership. The rules are as follows:

  • If the value is an expression of more than one element (like x + 2), it WILL NOT BE put behind a reference.
  • If the value is a variable defined in the templates, it WILL NOT BE put behind a reference.
  • If the value has a filter applied to it (x|capitalize), it WILL NOT BE put behind a reference.
  • If the value is a field (x.y), it WILL BE put behind a reference.
  • If the expression ends with a question mark (like x?), it WILL NOT BE put behind a reference.

Filters

Values such as those obtained from variables can be post-processed using filters. Filters are applied to values using the pipe symbol (|) and may have optional extra arguments in parentheses. Filters can be chained, in which case the output from one filter is passed to the next.

For example, {{ "{:?}"|format(name|escape) }} will escape HTML characters from the value obtained by accessing the name field, and print the resulting string as a Rust literal.

The built-in filters are documented as part of the filters documentation.

To define your own filters, simply have a module named filters in scope of the context deriving a Template impl. Note that in case of name collision, the built in filters take precedence.

Filter blocks

You can apply a filter on a whole block at once using filter blocks:

{% filter lower %}
    {{ t }} / HELLO / {{ u }}
{% endfilter %}

The lower filter will be applied on the whole content.

Just like filters, you can combine them:

{% filter lower|capitalize %}
    {{ t }} / HELLO / {{ u }}
{% endfilter %}

In this case, lower will be called and then capitalize will be called on what lower returned.

Whitespace control

Askama considers all tabs, spaces, newlines and carriage returns to be whitespace. By default, it preserves all whitespace in template code, except that a single trailing newline character is suppressed. However, whitespace before and after expression and block delimiters can be suppressed by writing a minus sign directly following a start delimiter or leading into an end delimiter.

Here is an example:

{% if foo %}
  {{- bar -}}
{% else if -%}
  nothing
{%- endif %}

This discards all whitespace inside the if/else block. If a literal (any part of the template not surrounded by {% %} or {{ }}) includes only whitespace, whitespace suppression on either side will completely suppress that literal content.

If the whitespace default control is set to "suppress" and you want to preserve whitespace characters on one side of a block or of an expression, you need to use +. Example:

<a href="/" {#+ #}
   class="something">text</a>

In the above example, one whitespace character is kept between the href and the class attributes.

There is a third possibility. In case you want to suppress all whitespace characters except one ("minimize"), you can use ~:

{% if something ~%}
Hello
{%~ endif %}

To be noted, if one of the trimmed characters is a newline, then the only character remaining will be a newline.

Whitespace controls can also be defined by a configuration file or in the derive macro. These definitions follow the global-to-local preference:

  1. Inline (-, +, ~)
  2. Derive (#[template(whitespace = "suppress")])
  3. Configuration (in askama.toml, whitespace = "preserve")

Two inline whitespace controls may point to the same whitespace span. In this case, they are resolved by the following preference.

  1. Suppress (-)
  2. Minimize (~)
  3. Preserve (+)

Functions

There are several ways that functions can be called within templates, depending on where the function definition resides. These are:

  • Template struct fields
  • Static functions
  • Struct/Trait implementations

Template struct field

When the function is a field of the template struct, we can simply call it by invoking the name of the field, followed by parentheses containing any required arguments. For example, we can invoke the function foo for the following MyTemplate struct:

#[derive(Template)]
#[template(source = "{{ foo(123) }}", ext = "txt")]
struct MyTemplate {
  foo: fn(u32) -> String,
}

However, since we'll need to define this function every time we create an instance of MyTemplate, it's probably not the most ideal way to associate some behavior for our template.

Static functions

When a function exists within the same Rust module as the template definition, we can invoke it using the self path prefix, where self represents the scope of the module in which the template struct resides.

For example, here we call the function foo by writing self::foo(123) within the MyTemplate struct source:

fn foo(val: u32) -> String {
  format!("{}", val)
}

#[derive(Template)]
#[template(source = "{{ self::foo(123) }}", ext = "txt")]
struct MyTemplate;

This has the advantage of being able to share functionality across multiple templates, without needing to expose the function publicly outside of its module.

However, we are not limited to local functions defined within the same module. We can call any public function by specifying the full path to that function within the template source. For example, given a utilities module such as:

// src/templates/utils/mod.rs

pub fn foo(val: u32) -> String {
  format!("{}", val)
}

Within our MyTemplate source, we can call the foo function by writing:

// src/templates/my_template.rs

#[derive(Template)]
#[template(source = "{{ crate::templates::utils::foo(123) }}", ext = "txt")]
struct MyTemplate;

Struct / trait implementations

Finally, we can call methods of our template struct:

#[derive(Template)]
#[template(source = "{{ foo(123) }}", ext = "txt")]
struct MyTemplate {
  count: u32,
};

impl MyTemplate {
  fn foo(&self, val: u32) -> String {
    format!("{} is the count, {} is the value", self.count, val)
  }
}

You can also use self.foo(123), or even Self::foo(self, 123), as you see fit.

Similarly, using the Self path, we can also call any method belonging to a trait that has been implemented for our template struct:

trait Hello {
  fn greet(name: &str) -> String;
}

#[derive(Template)]
#[template(source = r#"{{ Self::greet("world") }}"#, ext = "txt")]
struct MyTemplate;

impl Hello for MyTemplate {
  fn greet(name: &str) -> String {
    format!("Hello {}", name)
  }
}

If you want to call a closure which is a field, you'll need to follow Rust's syntax by surrounding the call with parens:

#[derive(Template)]
#[template(source = "{{ (closure)(12) }}", ext = "txt")]
struct MyTemplate {
    closure: fn(i32) -> i32,
}

Calling functions

If you only provide a function name, askama will assume it's a method. If you want to call a function, you will need to use a path instead:

{# This is the equivalent of `self.method()`. #}
{{ method() }}
{# This is the equivalent of `self::function()`, which will call the
`function` function from the current module. #}
{{ self::function() }}
{# This is the equivalent of `super::b::f()`. #}
{{ super::b::f() }}

Template inheritance

Template inheritance allows you to build a base template with common elements that can be shared by all inheriting templates. A base template defines blocks that child templates can override.

Base template

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>{% block title %}{{ title }} - My Site{% endblock %}</title>
    {% block head %}{% endblock %}
  </head>
  <body>
    <div id="content">
      {% block content %}<p>Placeholder content</p>{% endblock %}
    </div>
  </body>
</html>

The block tags define three blocks that can be filled in by child templates. The base template defines a default version of the block. A base template must define one or more blocks in order to enable inheritance. Blocks can only be specified at the top level of a template or inside other blocks, not inside if/else branches or in for-loop bodies.

It is also possible to use the name of the block in endblock (both in declaration and use):

{% block content %}<p>Placeholder content</p>{% endblock content %}

Child template

Here's an example child template:

{% extends "base.html" %}

{% block title %}Index{% endblock %}

{% block head %}
  <style>
  </style>
{% endblock %}

{% block content %}
  <h1>Index</h1>
  <p>Hello, world!</p>
  {{ super() }}
{% endblock %}

The extends tag tells the code generator that this template inherits from another template. It will search for the base template relative to itself before looking relative to the template base directory. It will render the top-level content from the base template, and substitute blocks from the base template with those from the child template. Inside a block in a child template, the super() macro can be called to render the parent block's contents.

Because top-level content from the child template is thus ignored, the extends tag doesn't support whitespace control:

{%- extends "base.html" +%}

The above code is rejected because we used - and +. For more information about whitespace control, take a look here.

Block fragments

Additionally, a block can be rendered by itself. This can be useful when you need to decompose your template for partial rendering, without needing to extract the partial into a separate template or macro. This can be done with the block parameter.

#[derive(Template)]
#[template(path = "...", block = "my_block")]
struct BlockFragment {
    name: String,
}

HTML escaping

Askama by default escapes variables if it thinks it is rendering HTML content. It infers the escaping context from the extension of template filenames, escaping by default if the extension is one of html, htm, or xml. When specifying a template as source in an attribute, the ext attribute parameter must be used to specify a type. Additionally, you can specify an escape mode explicitly for your template by setting the escape attribute parameter value (to none or html).

Askama escapes <, >, &, ", and ', according to the OWASP escaping recommendations. Use the safe filter to prevent escaping for a single expression, or the escape (or e) filter to escape a single expression in an unescaped context.

#[derive(Template)]
#[template(source = "{{strvar}}")]
struct TestTemplate {
    strvar: String,
}

fn main() {
    let s = TestTemplate {
        strvar: "// my <html> is \"unsafe\" & should be 'escaped'".to_string(),
    };
    assert_eq!(
        s.render().unwrap(),
        "&#x2f;&#x2f; my &lt;html&gt; is &quot;unsafe&quot; &amp; \
         should be &#x27;escaped&#x27;"
    );
}

Control structures

For

Loop over each item in an iterator. For example:

<h1>Users</h1>
<ul>
{% for user in users %}
  <li>{{ user.name|e }}</li>
{% endfor %}
</ul>

Inside for-loop blocks, some useful variables are accessible:

  • loop.index: current loop iteration (starting from 1)
  • loop.index0: current loop iteration (starting from 0)
  • loop.first: whether this is the first iteration of the loop
  • loop.last: whether this is the last iteration of the loop
<h1>Users</h1>
<ul>
{% for user in users %}
   {% if loop.first %}
   <li>First: {{user.name}}</li>
   {% else %}
   <li>User#{{loop.index}}: {{user.name}}</li>
   {% endif %}
{% endfor %}
</ul>

If

The if statement essentially mirrors Rust's if expression, and is used as you might expect:

{% if users.len() == 0 %}
  No users
{% else if users.len() == 1 %}
  1 user
{% elif users.len() == 2 %}
  2 users
{% else %}
  {{ users.len() }} users
{% endif %}

If Let

Additionally, if let statements are also supported and similarly mirror Rust's if let expressions:

{% if let Some(user) = user %}
  {{ user.name }}
{% else %}
  No user
{% endif %}

is (not) defined

You can use is (not) defined to ensure a variable exists (or not):

{% if x is defined %}
  x is defined!
{% endif %}
{% if y is not defined %}
  y is not defined
{% else %}
  y is defined
{% endif %}

You can combine conditions with this feature and even use it in expressions:

{% if x is defined && x == "12" && y == Some(true) %}
...
{% endif %}

<script>
// It will generate `const x = true;` (or false is `x` is not defined).
const x = {{ x is defined }};
</script>

Due to proc-macro limitations, askama can only see the fields of your current type and the variables declared in the templates. Because of this, you can not check if a field or a function is defined:

{% if x.y is defined %}
  This code will not compile
{% endif %}

Match

In order to deal with Rust enums in a type-safe way, templates support match blocks from version 0.6. Here is a simple example showing how to expand an Option:

{% match item %}
  {% when Some with ("foo") %}
    Found literal foo
  {% when Some with (val) %}
    Found {{ val }}
  {% when None %}
{% endmatch %}

That is, a {% match %} block may contain whitespaces (but no other literal content) and comment blocks, followed by a number of {% when %} blocks and an optional {% else %} block.

Like in Rust, the matching is done against a pattern. Such a pattern may be a literal, e.g.

{% match multiple_choice_answer %}
  {% when 3 %} Correct!
  {% else %} Sorry, the right answer is "3".
{% endmatch %}

Or some more complex type, such as a Result<T, E>:

{% match result %}
  {% when Ok(val) %} Good: {{ val }}.
  {% when Err(err) %} Bad: {{ err }}.
{% endmatch %}

Using the placeholder _ to match against any value without capturing the datum, works too. The wildcard operator .. is used to match against an arbitrary amount of items, and the same restrictions as in Rust, e.g. that it can be used only once in a slice or struct:

{% match list_of_ints %}
  {% when [first, ..] %} The list starts with a {{ first }}
  {% when _ %} The list is empty.
{% endmatch %}

The {% else %} node is syntactical sugar for {% when _ %}. If used, it must come last, after all other {% when %} blocks:

{% match answer %}
  {% when Ok(42) %} The answer is "42".
  {% else %} No answer wrong answer?
{% endmatch %}

A {% match %} must be exhaustive, i.e. all possible inputs must have a case. This is most easily done by providing an {% else %} case, if not all possible values need an individual handling.

Because a {% match %} block could not generate valid code otherwise, you have to provide at least one {% when %} case and/or an {% else %} case.

You can also match against multiple alternative patterns at once:

{% match number %}
  {% when 1 | 4 | 86 %} Some numbers
  {% when n %} Number is {{ n }}
{% endmatch %}

For better interoperability with linters and auto-formatters like djLint, you can also use an optional {% endwhen %} node to close a {% when %} case:

{% match number %}
  {% when 0 | 2 | 4 | 6 | 8 %}
    even
  {% endwhen %}
  {% when 1 | 3 | 5 | 7 | 9 %}
    odd
  {% endwhen %}
  {% else }
    unknown
{% endmatch %}

Referencing and dereferencing variables

If you need to put something behind a reference or to dereference it, you can use & and * operators:

{% let x = &"bla" %}
{% if *x == "bla" %}
Just talking
{% else if x == &"another" %}
Another?!
{% endif %}

They have the same effect as in Rust and you can put multiple of them:

{% let x = &&"bla" %}
{% if *&**x == "bla" %}
You got it
{% endif %}

Include

The include statement lets you split large or repetitive blocks into separate template files. Included templates get full access to the context in which they're used, including local variables like those from loops:

{% for i in iter %}
  {% include "item.html" %}
{% endfor %}

* Item: {{ i }}

The path to include must be a string literal, so that it is known at compile time. Askama will try to find the specified template relative to the including template's path before falling back to the absolute template path. Use include within the branches of an if/else block to use includes more dynamically.

Expressions

Askama supports string literals ("foo") and integer literals (1). It supports almost all binary operators that Rust supports, including arithmetic, comparison and logic operators. The parser applies the same operator precedence as the Rust compiler. Expressions can be grouped using parentheses.

{{ 3 * 4 / 2 }}
{{ 26 / 2 % 7 }}
{{ 3 % 2 * 6 }}
{{ 1 * 2 + 4 }}
{{ 11 - 15 / 3 }}
{{ (4 + 5) % 3 }}

The HTML special characters &, < and > will be replaced with their character entities unless the escape mode is disabled for a template, or the filter |safe is used.

Methods can be called on variables that are in scope, including self.

Warning: if the result of an expression (a {{ }} block) is equivalent to self, this can result in a stack overflow from infinite recursion. This is because the Display implementation for that expression will in turn evaluate the expression and yield self again.

Expressions containing bit-operators

In Askama, the binary AND, OR, and XOR operators (called &, |, ^ in Rust, resp.), are renamed to bitand, bitor, xor to avoid confusion with filter expressions. They still have the same operator precedence as in Rust. E.g. to test if the least significant bit is set in an integer field:

{% if my_bitset bitand 1 != 0 %}
    It is set!
{% endif %}

Type conversion

You can use the as operator in {{ … }} expressions, and {% … %} blocks. It works the same as in Rust, but with some deliberate restrictions:

  • You can only use primitive types like i32 or f64 both as source variable type and as target type.
  • If the source is a reference to a primitive type, e.g. &&&bool, then askama automatically dereferences the value until it gets the underlying bool.

String concatenation

As a short-hand for {{ a }}{{ b }}{{ c }} you can use the concat operator ~: {{ a ~ b ~ c }}. The tilde ~ has to be surrounded by spaces to avoid confusion with the whitespace control operator.

Templates in templates

Using expressions, it is possible to delegate rendering part of a template to another template. This makes it possible to inject modular template sections into other templates and facilitates testing and reuse.

use askama::Template;
#[derive(Template)]
#[template(source = "Section 1: {{ s1 }}", ext = "txt")]
struct RenderInPlace<'a> {
   s1: SectionOne<'a>
}

#[derive(Template)]
#[template(source = "A={{ a }}\nB={{ b }}", ext = "txt")]
struct SectionOne<'a> {
   a: &'a str,
   b: &'a str,
}

let t = RenderInPlace { s1: SectionOne { a: "a", b: "b" } };
assert_eq!(t.render().unwrap(), "Section 1: A=a\nB=b")

Note that if your inner template like SectionOne renders HTML content, then you may want to disable escaping when injecting it into an outer template, e.g. {{ s1|safe }}. Otherwise it will render the HTML content literally, because askama escapes HTML variables by default.

See the example render in place using a vector of templates in a for block.

Comments

Askama supports block comments delimited by {# and #}.

{# A Comment #}

Like Rust, Askama also supports nested block comments.

{#
A Comment
{# A nested comment #}
#}

Recursive Structures

Recursive implementations should preferably use a custom iterator and use a plain loop. If that is not doable, call .render() directly by using an expression as shown below.

use askama::Template;

#[derive(Template)]
#[template(source = r#"
{{ name }} {
{% for item in children %}
   {{ item.render()? }}
{% endfor %}
}
"#, ext = "html", escape = "none")]
struct Item<'a> {
    name: &'a str,
    children: &'a [Item<'a>],
}

Macros

You can define macros within your template by using {% macro name(args) %}, ending with {% endmacro %}.

You can then call it with {% call name(args) %}, ending with {% endcall %}:

{% macro heading(arg) %}

<h1>{{arg}}</h1>

{% endmacro %}

{% call heading(s) %}{% endcall %}

You can place macros in a separate file and use them in your templates by using {% import %}:

{%- import "macro.html" as scope -%}

{% call scope::heading(s) %}{% endcall %}

You can optionally specify the name of the macro in endmacro:

{% macro heading(arg) %}<p>{{arg}}</p>{% endmacro heading %}

You can also specify arguments by their name (as defined in the macro):

{% macro heading(arg, bold) %}

<h1>{{arg}} <b>{{bold}}</b></h1>

{% endmacro %}

{% call heading(bold="something", arg="title") %}{% endcall%}

You can use whitespace characters around =:

{% call heading(bold = "something", arg = "title") %}{% endcall %}

You can mix named and non-named arguments when calling a macro:

{% call heading("title", bold="something") %}{% endcall %}

However please note that named arguments must always come last.

Another thing to note, if a named argument is referring to an argument that would be used for a non-named argument, it will error:

{% macro heading(arg1, arg2, arg3, arg4) %}
{% endmacro %}

{% call heading("something", "b", arg4="ah", arg2="title") %}{% endcall %}

In here it's invalid because arg2 is the second argument and would be used by "b". So either you replace "b" with arg3="b" or you pass "title" before:

{% call heading("something", arg3="b", arg4="ah", arg2="title") %}{% endcall %}
{# Equivalent of: #}
{% call heading("something", "title", "b", arg4="ah") %}{% endcall %}

Default value

You can specify default values for your macro arguments:

{% macro heading(arg1, arg2 = "something") %}
{% endmacro %}

Then if you don't pass a value for this argument, its default value will be used:

{# We only specify `arg1`, so `arg2` will be "something" #}
{% call heading(1) %}{% endcall %}
{# We specify both `arg1` and `arg2` so no default value is used #}
{% call heading(1, 2) %}{% endcall %}

Call

You can use the content in the call block directly inside the macro by using {{ caller() }}:

{% 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 %}

Here is an example with a call block using arguments:

{% macro dump_users(users) -%}
    <ul>
    {%- for user in users %}
        <li><p>{{ user.username }}</p>{{ caller(user) }}</li>
    {%- endfor %}
    </ul>
{%- endmacro %}

{% call(user) dump_users(list_of_user) %}
    <dl>
        <dt>Realname</dt>
        <dd>{{ user.realname }}</dd>
        <dt>Description</dt>
        <dd>{{ user.description }}</dd>
    </dl>
{% endcall %}

Calling Rust macros

It is possible to call rust macros directly in your templates:

{% let s = format!("{}", 12) %}

One important thing to note is that contrary to the rest of the expressions, Askama cannot know if a token given to a macro is a variable or something else, so it will always default to generate it "as is". So if you have:

macro_rules! test_macro{
    ($entity:expr) => {
        println!("{:?}", &$entity);
    }
}

#[derive(Template)]
#[template(source = "{{ test_macro!(entity) }}", ext = "txt")]
struct TestTemplate<'a> {
    entity: &'a str,
}

It will not compile, telling you it doesn't know entity. It didn't infer that entity was a field of the current type unlike usual. You can go around this limitation by binding your field's value into a variable:

{% let entity = entity %}
{{ test_macro!(entity) }}