Askama
Askama implements a template rendering engine based on Jinja.
It generates Rust code from your templates at compile time
based on a user-defined struct
to hold the template's context.
See below for an example, or read the book.
"Pretty exciting. I would love to use this already." -- Armin Ronacher, creator of Jinja
All feedback welcome. Feel free to file bugs, requests for documentation and any other feedback to the issue tracker or tweet me.
Askama was created by and is maintained by Dirkjan Ochtman. If you are in a position to support ongoing maintenance and further development or use it in a for-profit context, please consider supporting my open source work on Patreon.
Feature highlights
- Construct templates using a familiar, easy-to-use syntax
- Benefit from the safety provided by Rust's type system
- Template code is compiled into your crate for optimal performance
- Optional built-in support for Actix, Axum, Gotham, Mendes, Rocket, tide, and warp web frameworks
- Debugging features to assist you in template development
- Templates must be valid UTF-8 and produce UTF-8 when rendered
- IDE support available in JetBrains products
- Works on stable Rust
Supported in templates
- Template inheritance
- Loops, if/else statements and include support
- Macro support
- Variables (no mutability allowed)
- Some built-in filters, and the ability to use your own
- Whitespace suppressing with '-' markers
- Opt-out HTML escaping
- Syntax customization
Getting Started
First, add the following to your crate's Cargo.toml
:
# in section [dependencies]
askama = "0.12.1"
Now create a directory called templates
in your crate root.
In it, create a file called hello.html
, containing the following:
Hello, {{ name }}!
In any Rust file inside your crate, add the following:
use askama::Template; // bring trait in scope #[derive(Template)] // this will generate the code... #[template(path = "hello.html")] // using the template in this path, relative // to the `templates` dir in the crate root struct HelloTemplate<'a> { // the name of the struct can be anything name: &'a str, // the field name should match the variable name // in your template } fn main() { let hello = HelloTemplate { name: "world" }; // instantiate your struct println!("{}", hello.render().unwrap()); // then render it. }
You should now be able to compile and run this code.
Using integrations
To use one of the integrations, with axum as an example:
First, add this to your Cargo.toml
instead:
# in section [dependencies]
askama_axum = "0.4.0"
Then, import from askama_axum instead of askama:
#![allow(unused)] fn main() { use askama_axum::Template; }
This enables the implementation for axum's IntoResponse
trait,
so an instance of the template can be returned as a response.
For other integrations, import and use their crate accordingly.
Creating Templates
An Askama template is a struct
definition which provides the template
context combined with a UTF-8 encoded text file (or inline source, see
below). Askama can be used to generate any kind of text-based format.
The template file's extension may be used to provide content type hints.
A template consists of text contents, which are passed through as-is, expressions, which get replaced with content while being rendered, and tags, which control the template's logic. The template syntax is very similar to Jinja, as well as Jinja-derivatives like Twig or Tera.
#![allow(unused)] fn main() { #[derive(Template)] // this will generate the code... #[template(path = "hello.html")] // using the template in this path, relative // to the `templates` dir in the crate root struct HelloTemplate<'a> { // the name of the struct can be anything name: &'a str, // the field name should match the variable name // in your template } }
The template()
attribute
Askama works by generating one or more trait implementations for any
struct
type decorated with the #[derive(Template)]
attribute. The
code generation process takes some options that can be specified through
the template()
attribute. The following sub-attributes are currently
recognized:
-
path
(aspath = "foo.html"
): sets the path to the template file. The path is interpreted as relative to the configured template directories (by default, this is atemplates
directory next to yourCargo.toml
). The file name extension is used to infer an escape mode (see below). In web framework integrations, the path's extension may also be used to infer the content type of the resulting response. Cannot be used together withsource
.#![allow(unused)] fn main() { #[derive(Template)] #[template(path = "hello.html")] struct HelloTemplate<'a> { ... } }
-
source
(assource = "{{ foo }}"
): directly sets the template source. This can be useful for test cases or short templates. The generated path is undefined, which generally makes it impossible to refer to this template from other templates. Ifsource
is specified,ext
must also be specified (see below). Cannot be used together withpath
.#![allow(unused)] fn main() { #[derive(Template)] #[template(source = "Hello {{ name }}")] struct HelloTemplate<'a> { name: &'a str, } }
-
ext
(asext = "txt"
): lets you specify the content type as a file extension. This is used to infer an escape mode (see below), and some web framework integrations use it to determine the content type. Cannot be used together withpath
.#![allow(unused)] fn main() { #[derive(Template)] #[template(source = "Hello {{ name }}", ext = "txt")] struct HelloTemplate<'a> { name: &'a str, } }
-
print
(asprint = "code"
): enable debugging by printing nothing (none
), the parsed syntax tree (ast
), the generated code (code
) orall
for both. The requested data will be printed to stdout at compile time.#![allow(unused)] fn main() { #[derive(Template)] #[template(path = "hello.html", print = "all")] struct HelloTemplate<'a> { ... } }
-
block
(asblock = "block_name"
): renders the block by itself. Expressions outside of the block are not required by the struct, and inheritance is also supported. 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.#![allow(unused)] fn main() { #[derive(Template)] #[template(path = "hello.html", block = "hello")] struct HelloTemplate<'a> { ... } }
-
escape
(asescape = "none"
): override the template's extension used for the purpose of determining the escaper for this template. See the section on configuring custom escapers for more information.#![allow(unused)] fn main() { #[derive(Template)] #[template(path = "hello.html", escape = "none")] struct HelloTemplate<'a> { ... } }
-
syntax
(assyntax = "foo"
): set the syntax name for a parser defined in the configuration file. The default syntax , "default", is the one provided by Askama.#![allow(unused)] fn main() { #[derive(Template)] #[template(path = "hello.html", syntax = "foo")] struct HelloTemplate<'a> { ... } }
-
config
(asconfig = "config_file_path"
): set the path for the config file to be used. The path is interpreted as relative to your crate root.#![allow(unused)] fn main() { #[derive(Template)] #[template(path = "hello.html", config = "config.toml")] struct HelloTemplate<'a> { ... } }
Debugging and Troubleshooting
You can view the parse tree for a template as well as the generated code by
changing the template
attribute item list for the template struct:
#![allow(unused)] fn main() { #[derive(Template)] #[template(path = "hello.html", print = "all")] struct HelloTemplate<'a> { ... } }
The print
key can take one of four values:
none
(the default value)ast
(print the parse tree)code
(print the generated code)all
(print both parse tree and code)
The resulting output will be printed to stderr
during the compilation process.
The parse tree looks like this for the example template:
[Lit("", "Hello,", " "), Expr(WS(false, false), Var("name")),
Lit("", "!", "\n")]
The generated code looks like this:
#![allow(unused)] fn main() { impl < 'a > ::askama::Template for HelloTemplate< 'a > { fn render_into(&self, writer: &mut ::std::fmt::Write) -> ::askama::Result<()> { write!( writer, "Hello, {expr0}!", expr0 = &::askama::MarkupDisplay::from(&self.name), )?; Ok(()) } fn extension() -> Option<&'static str> { Some("html") } } impl < 'a > ::std::fmt::Display for HelloTemplate< 'a > { fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result { ::askama::Template::render_into(self, f).map_err(|_| ::std::fmt::Error {}) } } }
Configuration
At compile time, Askama will read optional configuration values from
askama.toml
in the crate root (the directory where Cargo.toml
can
be found). Currently, this covers the directories to search for templates,
custom syntax configuration and escaper configuration.
This example file demonstrates the default configuration:
[general]
# Directories to search for templates, relative to the crate root.
dirs = ["templates"]
# Unless you add a `-` in a block, whitespace characters won't be trimmed.
whitespace = "preserve"
Whitespace control
In the default configuration, you can use the -
operator to indicate that
whitespace should be suppressed before or after a block. For example:
<div>
{%- if something %}
Hello
{% endif %}
In the template above, only the whitespace between <div>
and {%-
will be
suppressed. If you set whitespace
to "suppress"
, whitespace characters before
and after each block will be suppressed by default. To preserve the whitespace
characters, you can use the +
operator:
{% if something +%}
Hello
{%+ endif %}
In this example, Hello
will be surrounded with newline characters.
There is a third possibility: in case you want to suppress all whitespace
characters except one, 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.
If you want this to be the default behaviour, you can set whitespace
to
"minimize"
.
To be noted: you can also configure whitespace
directly into the template
derive proc macro:
#![allow(unused)] fn main() { #[derive(Template)] #[template(whitespace = "suppress")] pub struct SomeTemplate; }
If you configure whitespace
directly into the template
derive proc-macro,
it will take precedence over the one in your configuration file. So in this
case, if you already set whitespace = "minimize"
into your configuration file,
it will be replaced by suppress
for this template.
Custom syntaxes
Here is an example that defines two custom syntaxes:
[general]
default_syntax = "foo"
[[syntax]]
name = "foo"
block_start = "%{"
comment_start = "#{"
expr_end = "^^"
[[syntax]]
name = "bar"
block_start = "%%"
block_end = "%%"
comment_start = "%#"
expr_start = "%{"
A syntax block consists of at least the attribute name
which uniquely
names this syntax in the project.
The following keys can currently be used to customize template syntax:
block_start
, defaults to{%
block_end
, defaults to%}
comment_start
, defaults to{#
comment_end
, defaults to#}
expr_start
, defaults to{{
expr_end
, defaults to}}
Values must be at least two characters long. If a key is omitted, the value from the default syntax is used.
Here is an example of a custom escaper:
[[escaper]]
path = "::tex_escape::Tex"
extensions = ["tex"]
An escaper block consists of the attributes path
and extensions
. path
contains a Rust identifier that must be in scope for templates using this
escaper. extensions
defines a list of file extensions that will trigger
the use of that escaper. Extensions are matched in order, starting with the
first escaper configured and ending with the default escapers for HTML
(extensions html
, htm
, xml
, j2
, jinja
, jinja2
) and plain text
(no escaping; md
, yml
, none
, txt
, and the empty string). Note that
this means you can also define other escapers that match different extensions
to the same escaper.
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:
#![allow(unused)] fn main() { 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 }}
For compatibility with Jinja, set
can be used in place of let
.
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:
- Inline (
-
,+
,~
) - Derive (
#[template(whitespace = "suppress")]
) - 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.
- Suppress (
-
) - Minimize (
~
) - 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:
#![allow(unused)] fn main() { #[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 behaviour 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:
#![allow(unused)] fn main() { 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:
#![allow(unused)] fn main() { // 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:
#![allow(unused)] fn main() { // src/templates/my_template.rs #[derive(Template)] #[template(source = "{{ crate::templates::utils::foo(123) }}", ext = "txt")] struct MyTemplate; }
Struct / trait implementations
Finally, we can invoke functions that are implementation methods of our
template struct, by referencing Self
(note the uppercase S
) as the path,
before calling our function:
#![allow(unused)] fn main() { #[derive(Template)] #[template(source = "{{ Self::foo(self, 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) } } }
If the implemented method requires a reference to the struct itself,
such as is demonstrated in the above example, we can pass self
(note the lowercase s
) as the first argument.
Similarly, using the Self
path, we can also call any method belonging
to a trait that has been implemented for our template struct:
#![allow(unused)] fn main() { 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) } } }
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>
{% call 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.
#![allow(unused)] fn main() { #[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(), "// my <html> is "unsafe" & \ should be 'escaped'" ); }
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
{% 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 %}
Match
In order to deal with Rust enum
s 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 can optionally contain some whitespace (but
no other literal content), followed by a number of when
blocks
and an optional else
block. Each when
block must name a list of
matches ((val)
), optionally introduced with a variant name. The
else
block is equivalent to matching on _
(matching anything).
Struct-like enum variants are supported from version 0.8, with the list
of matches surrounded by curly braces instead ({ field }
). New names
for the fields can be specified after a colon in the list of matches
({ field: val }
).
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 precedence order as the Rust compiler.
Expressions can be grouped using parentheses.
The HTML special characters &
, <
and >
will be replaced with their
character entities unless the escape
mode is disabled for a template.
Methods can be called on variables that are in scope, including self
.
{{ 3 * 4 / 2 }}
{{ 26 / 2 % 7 }}
{{ 3 % 2 * 6 }}
{{ 1 * 2 + 4 }}
{{ 11 - 15 / 3 }}
{{ 4 + 5 % 3 }}
{{ 4 | 2 + 5 & 2 }}
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.
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.
#![allow(unused)] fn main() { 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") }
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.
Including self does not work, see #105 and #220 .
#![allow(unused)] fn main() { use askama::Template; #[derive(Template)] #[template(source = r#" //! {% for item in children %} {{ item }} {% 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) %}
:
{% macro heading(arg) %}
<h1>{{arg}}</h1>
{% endmacro %}
{% call heading(s) %}
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) %}
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") %}
You can use whitespace characters around =
:
{% call heading(bold = "something", arg = "title") %}
You can mix named and non-named arguments when calling a macro:
{% call heading("title", bold="something") %}
However please note than 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") %}
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") %}
{# Equivalent of: #}
{% call heading("something", "title", "b", arg4="ah") %}
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:
#![allow(unused)] fn main() { 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) }}
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.
Note that the pipe symbol must not be surrounded by spaces;
otherwise, it will be interpreted as the BitOr
operator.
Filters can be chained, in which case the output from one filter
is passed to the next.
{{ "HELLO"|lower }}
Askama has a collection of built-in filters, documented below, but can also include custom filters.
Additionally, the json
filter is included in the built-in filters, but is disabled by default.
Enable it with Cargo features (see below for more information).
Table of contents
-
Built-in filters:
abs
,as_ref
,capitalize
,center
,deref
,escape|e
,filesizeformat
,fmt
,format
,indent
,join
,linebreaks
,linebreaksbr
,lower|lowercase
,safe
,trim
,truncate
,upper|uppercase
,wordcount
Built-In Filters
abs
Returns the absolute value.
{{ -2|abs }}
Output:
2
as_ref
Creates a reference to the given argument.
{{ "a"|as_ref }}
{{ self.x|as_ref }}
will become:
&"a"
&self.x
capitalize
Capitalize a value. The first character will be uppercase, all others lowercase:
{{ "hello"|capitalize }}
Output:
Hello
center
Centers the value in a field of a given width:
-{{ "a"|center(5) }}-
Output:
- a -
deref
Dereferences the given argument.
{% let s = String::from("a")|as_ref %}
{% if s|deref == String::from("b") %}
{% endif %}
will become:
let s = &String::from("a");
if *s == String::from("b") {}
escape | e
Escapes HTML characters in strings:
{{ "Escape <>&"|e }}
Output:
Escape <>&
Optionally, it is possible to specify and override which escaper is used.
Consider a template where the escaper is configured as escape = "none"
.
However, somewhere escaping using the HTML escaper is desired.
Then it is possible to override and use the HTML escaper like this:
{{ "Don't Escape <>&"|escape }}
{{ "Don't Escape <>&"|e }}
{{ "Escape <>&"|escape("html") }}
{{ "Escape <>&"|e("html") }}
Output:
Don't Escape <>&
Don't Escape <>&
Escape <>&
Escape <>&
filesizeformat
Returns adequate string representation (in KB, ..) of number of bytes:
{{ 1000|filesizeformat }}
Output:
1 KB
fmt
Formats arguments according to the specified format
The second argument to this filter must be a string literal (as in normal
Rust). The two arguments are passed through to format!()
by
the Askama code generator, but the order is swapped to support filter
composition.
{{ value|fmt("{:?}") }}
As an example, this allows filters to be composed like the following.
Which is not possible using the format
filter.
{{ value|capitalize|fmt("{:?}") }}
format
Formats arguments according to the specified format.
The first argument to this filter must be a string literal (as in normal Rust).
All arguments are passed through to format!()
by the Askama code generator.
{{ "{:?}"|format(var) }}
indent
Indent newlines with width spaces.
{{ "hello\nfoo\nbar"|indent(4) }}
Output:
hello
foo
bar
join
Joins iterable into a string separated by provided argument.
array = &["foo", "bar", "bazz"]
{{ array|join(", ") }}
Output:
foo, bar, bazz
linebreaks
Replaces line breaks in plain text with appropriate HTML.
A single newline becomes an HTML line break <br>
and a new line followed by a blank line becomes a paragraph break <p>
.
{{ "hello\nworld\n\nfrom\naskama"|linebreaks }}
Output:
<p>hello<br />world</p><p>from<br />askama</p>
linebreaksbr
Converts all newlines in a piece of plain text to HTML line breaks.
{{ "hello\nworld\n\nfrom\naskama"|linebreaks }}
Output:
hello<br />world<br /><br />from<br />askama
paragraphbreaks
A new line followed by a blank line becomes <p>
, but, unlike linebreaks
, single new lines are ignored and no <br/>
tags are generated.
Consecutive double line breaks will be reduced down to a single paragraph break.
This is useful in contexts where changing single line breaks to line break tags would interfere with other HTML elements, such as lists and nested <div>
tags.
{{ "hello\nworld\n\nfrom\n\n\n\naskama"|paragraphbreaks }}
Output:
<p>hello\nworld</p><p>from</p><p>askama</p>
lower | lowercase
Converts to lowercase.
{{ "HELLO"|lower }}
Output:
hello
safe
Marks a string (or other Display type) as safe. By default all strings are escaped according to the format.
{{ "<p>I'm Safe</p>"|safe }}
Output:
<p>I'm Safe</p>
trim
Strip leading and trailing whitespace.
{{ " hello "|trim }}
Output:
hello
truncate
Limit string length, appends '...' if truncated.
{{ "hello"|truncate(2) }}
Output:
he...
upper | uppercase
Converts to uppercase.
{{ "hello"|upper }}
Output:
HELLO
wordcount
Count the words in that string.
{{ "askama is sort of cool"|wordcount }}
Output:
5
Optional / feature gated filters
The following filters can be enabled by requesting the respective feature in the Cargo.toml dependencies section, e.g.
[dependencies]
askama = { version = "0.11.2", features = "serde-json" }
json
| tojson
Enabling the serde-json
feature will enable the use of the json
filter.
This will output formatted JSON for any value that implements the required
Serialize
trait.
The generated string does not contain ampersands &
, chevrons < >
, or apostrophes '
.
To use it in a <script>
you can combine it with the safe filter.
In HTML attributes, you can either use it in quotation marks "{{data|json}}"
as is,
or in apostrophes with the (optional) safe filter '{{data|json|safe}}'
.
In HTML texts the output of e.g. <pre>{{data|json|safe}}</pre>
is safe, too.
Good: <li data-extra="{{data|json}}">…</li>
Good: <li data-extra='{{data|json|safe}}'>…</li>
Good: <pre>{{data|json|safe}}</pre>
Good: <script>var data = {{data|json|safe}};</script>
Bad: <li data-extra="{{data|json|safe}}">…</li>
Bad: <script>var data = {{data|json}};</script>
Bad: <script>var data = "{{data|json|safe}}";</script>
Ugly: <script>var data = "{{data|json}}";</script>
Ugly: <script>var data = '{{data|json|safe}}';</script>
Custom Filters
To define your own filters, simply have a module named filters
in scope of the context deriving a Template
impl
and define the filters as functions within this module.
The functions must have at least one argument and the return type must be ::askama::Result<T>
.
Although there are no restrictions on T
for a single filter,
the final result of a chain of filters must implement Display
.
The arguments to the filters are passed as follows.
The first argument corresponds to the expression they are applied to.
Subsequent arguments, if any, must be given directly when calling the filter.
The first argument may or may not be a reference, depending on the context in which the filter is called.
To abstract over ownership, consider defining your argument as a trait bound.
For example, the trim
built-in filter accepts any value implementing Display
.
Its signature is similar to fn trim(s: impl std::fmt::Display) -> ::askama::Result<String>
.
Note that built-in filters have preference over custom filters, so, in case of name collision, the built-in filter is applied.
Examples
Implementing a filter that replaces all instances of "oo"
for "aa"
.
use askama::Template; #[derive(Template)] #[template(source = "{{ s|myfilter }}", ext = "txt")] struct MyFilterTemplate<'a> { s: &'a str, } // Any filter defined in the module `filters` is accessible in your template. mod filters { // This filter does not have extra arguments pub fn myfilter<T: std::fmt::Display>(s: T) -> ::askama::Result<String> { let s = s.to_string(); Ok(s.replace("oo", "aa")) } } fn main() { let t = MyFilterTemplate { s: "foo" }; assert_eq!(t.render().unwrap(), "faa"); }
Implementing a filter that replaces all instances of "oo"
for n
times "a"
.
use askama::Template; #[derive(Template)] #[template(source = "{{ s|myfilter(4) }}", ext = "txt")] struct MyFilterTemplate<'a> { s: &'a str, } // Any filter defined in the module `filters` is accessible in your template. mod filters { // This filter requires a `usize` input when called in templates pub fn myfilter<T: std::fmt::Display>(s: T, n: usize) -> ::askama::Result<String> { let s = s.to_string(); let mut replace = String::with_capacity(n); replace.extend((0..n).map(|_| "a")); Ok(s.replace("oo", &replace)) } } fn main() { let t = MyFilterTemplate { s: "foo" }; assert_eq!(t.render().unwrap(), "faaaa"); }
Integrations
Rocket integration
In your template definitions, replace askama::Template
with
askama_rocket::Template
.
Enabling the with-rocket
feature appends an implementation of Rocket's
Responder
trait for each template type. This makes it easy to trivially
return a value of that type in a Rocket handler. See
the example
from the Askama test suite for more on how to integrate.
In case a run-time error occurs during templating, a 500 Internal Server Error
Status
value will be returned, so that this can be further
handled by your error catcher.
Actix-web integration
In your template definitions, replace askama::Template
with
askama_actix::Template
.
Enabling the with-actix-web
feature appends an implementation of Actix-web's
Responder
trait for each template type. This makes it easy to trivially return
a value of that type in an Actix-web handler. See
the example
from the Askama test suite for more on how to integrate.
Axum integration
In your template definitions, replace askama::Template
with
askama_axum::Template
.
Enabling the with-axum
feature appends an implementation of Axum's
IntoResponse
trait for each template type. This makes it easy to trivially
return a value of that type in a Axum handler. See
the example
from the Askama test suite for more on how to integrate.
In case of a run-time error occurring during templating, the response will be of the same
signature, with a status code of 500 Internal Server Error
, mime */*
, and an empty Body
.
This preserves the response chain if any custom error handling needs to occur.
Warp integration
In your template definitions, replace askama::Template
with
askama_warp::Template
.
Enabling the with-warp
feature appends an implementation of Warp's Reply
trait for each template type. This makes it simple to return a template from
a Warp filter. See the example
from the Askama test suite for more on how to integrate.
Performance
Slow Debug Recompilations
If you experience slow compile times when iterating with lots of templates, you can compile Askama's derive macros with a higher optimization level. This can speed up recompilation times dramatically.
Add the following to Cargo.toml
or .cargo/config.toml
:
#![allow(unused)] fn main() { [profile.dev.package.askama_derive] opt-level = 3 }
This may affect clean compile times in debug mode, but incremental compiles will be faster.
Template Expansion
This chapter will explain how the different parts of the templates are translated into Rust code.
⚠️ Please note that the generated code might change in the future so the following examples might not be up-to-date.
Basic explanations
When you add #[derive(Template)]
and #[template(...)]
on your type, the
Template
derive proc-macro will then generate an implementation of the
askama::Template
trait which will be a Rust version of the template.
It will also implement the std::fmt::Display
trait on your type which will
internally call the askama::Template
trait.
Let's take a small example:
#![allow(unused)] fn main() { #[derive(Template)] #[template(source = "{% set x = 12 %}", ext = "html")] struct Mine; }
will generate:
#![allow(unused)] fn main() { impl ::askama::Template for YourType { fn render_into( &self, writer: &mut (impl ::std::fmt::Write + ?Sized), ) -> ::askama::Result<()> { let x = 12; ::askama::Result::Ok(()) } const EXTENSION: ::std::option::Option<&'static ::std::primitive::str> = Some( "html", ); const SIZE_HINT: ::std::primitive::usize = 0; const MIME_TYPE: &'static ::std::primitive::str = "text/html; charset=utf-8"; } impl ::std::fmt::Display for YourType { #[inline] fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result { ::askama::Template::render_into(self, f).map_err(|_| ::std::fmt::Error {}) } } }
For simplicity, we will only keep the content of the askama::Template::render_into
function from now on.
Text content
If you have "text content" (for example HTML) in your template:
<h1>{{ title }}</h1>
It will generate it like this:
#![allow(unused)] fn main() { writer .write_fmt( format_args!( "<h1>{0}</h1>", &::askama::MarkupDisplay::new_unsafe(&(self.title), ::askama::Html), ), )?; ::askama::Result::Ok(()) }
About MarkupDisplay
: we need to use this type in order to prevent generating
invalid HTML. Let's take an example: if title
is "<a>"
and we display it as
is, in the generated HTML, you won't see <a>
but instead a new HTML element
will be created. To prevent this, we need to escape some characters.
In this example, <a>
will become <a>
. And this is why there is the
safe
builtin filter, in case you want it to be displayed as is.
Variables
Variables creation
If you create a variable in your template, it will be created in the generated Rust code as well. For example:
{% set x = 12 %}
{% let y = x + 1 %}
will generate:
#![allow(unused)] fn main() { let x = 12; let y = x + 1; ::askama::Result::Ok(()) }
Variables usage
By default, variables will reference a field from the type on which the askama::Template
trait is implemented:
{{ y }}
This template will expand as follows:
#![allow(unused)] fn main() { writer .write_fmt( format_args!( "{0}", &::askama::MarkupDisplay::new_unsafe(&(self.y), ::askama::Html), ), )?; ::askama::Result::Ok(()) }
This is why if the variable is undefined, it won't work with Askama and why we can't check if a variable is defined or not.
You can still access constants and statics by using paths. Let's say you have in your Rust code:
#![allow(unused)] fn main() { const FOO: u32 = 0; }
Then you can use them in your template by referring to them with a path:
{{ crate::FOO }}{{ super::FOO }}{{ self::FOO }}
It will generate:
#![allow(unused)] fn main() { writer .write_fmt( format_args!( "{0}{1}{2}", &::askama::MarkupDisplay::new_unsafe(&(crate::FOO), ::askama::Html), &::askama::MarkupDisplay::new_unsafe(&(super::FOO), ::askama::Html), &::askama::MarkupDisplay::new_unsafe(&(self::FOO), ::askama::Html), ), )?; ::askama::Result::Ok(()) }
(Note: crate::
is to get an item at the root level of the crate, super::
is
to get an item in the parent module and self::
is to get an item in the
current module.)
You can also access items from the type that implements Template
as well using
as Self::
, it'll use the same logic.
Control blocks
if/else
The generated code can be more complex than expected, as seen with if
/else
conditions:
{% if x == "a" %}
gateau
{% else %}
tarte
{% endif %}
It will generate:
#![allow(unused)] fn main() { if *(&(self.x == "a") as &bool) { writer.write_str("gateau")?; } else { writer.write_str("tarte")?; } ::askama::Result::Ok(()) }
Very much as expected except for the &(self.x == "a") as &bool
. Now about why
the as &bool
is needed:
The following syntax *(&(...) as &bool)
is used to trigger Rust's automatic
dereferencing, to coerce e.g. &&&&&bool
to bool
. First &(...) as &bool
coerces e.g. &&&bool
to &bool
. Then *(&bool)
finally dereferences it to
bool
.
In short, it allows to fallback to a boolean as much as possible, but it also explains why you can't do:
{% set x = "a" %}
{% if x %}
{{ x }}
{% endif %}
Because it fail to compile because:
error[E0605]: non-primitive cast: `&&str` as `&bool`
if let
{% if let Some(x) = x %}
{{ x }}
{% endif %}
will generate:
#![allow(unused)] fn main() { if let Some(x) = &(self.x) { writer .write_fmt( format_args!( "{0}", &::askama::MarkupDisplay::new_unsafe(&(x), ::askama::Html), ), )?; } }
Loops
{% for user in users %}
{{ user }}
{% endfor %}
will generate:
#![allow(unused)] fn main() { { let _iter = (&self.users).into_iter(); for (user, _loop_item) in ::askama::helpers::TemplateLoop::new(_iter) { writer .write_fmt( format_args!( "\n {0}\n", &::askama::MarkupDisplay::new_unsafe(&(user), ::askama::Html), ), )?; } } ::askama::Result::Ok(()) }
Now let's see what happens if you add an else
condition:
{% for user in x if x.len() > 2 %}
{{ user }}
{% else %}
{{ x }}
{% endfor %}
Which generates:
#![allow(unused)] fn main() { { let mut _did_loop = false; let _iter = (&self.users).into_iter(); for (user, _loop_item) in ::askama::helpers::TemplateLoop::new(_iter) { _did_loop = true; writer .write_fmt( format_args!( "\n {0}\n", &::askama::MarkupDisplay::new_unsafe(&(user), ::askama::Html), ), )?; } if !_did_loop { writer .write_fmt( format_args!( "\n {0}\n", &::askama::MarkupDisplay::new_unsafe( &(self.x), ::askama::Html, ), ), )?; } } ::askama::Result::Ok(()) }
It creates a _did_loop
variable which will check if we entered the loop. If
we didn't (because the iterator didn't return any value), it will enter the
else
condition by checking if !_did_loop {
.
We can extend it even further if we add an if
condition on our loop:
{% for user in users if users.len() > 2 %}
{{ user }}
{% else %}
{{ x }}
{% endfor %}
which generates:
#![allow(unused)] fn main() { { let mut _did_loop = false; let _iter = (&self.users).into_iter(); let _iter = _iter.filter(|user| -> bool { self.users.len() > 2 }); for (user, _loop_item) in ::askama::helpers::TemplateLoop::new(_iter) { _did_loop = true; writer .write_fmt( format_args!( "\n {0}\n", &::askama::MarkupDisplay::new_unsafe(&(user), ::askama::Html), ), )?; } if !_did_loop { writer .write_fmt( format_args!( "\n {0}\n", &::askama::MarkupDisplay::new_unsafe( &(self.x), ::askama::Html, ), ), )?; } } ::askama::Result::Ok(()) }
It generates an iterator but filters it based on the if
condition (users.len() > 2
).
So once again, if the iterator doesn't return any value, we enter the else
condition.
Of course, if you only have a if
and no else
, the generated code is much
shorter:
{% for user in users if users.len() > 2 %}
{{ user }}
{% endfor %}
Which generates:
#![allow(unused)] fn main() { { let _iter = (&self.users).into_iter(); let _iter = _iter.filter(|user| -> bool { self.users.len() > 2 }); for (user, _loop_item) in ::askama::helpers::TemplateLoop::new(_iter) { writer .write_fmt( format_args!( "\n {0}\n", &::askama::MarkupDisplay::new_unsafe(&(user), ::askama::Html), ), )?; } } ::askama::Result::Ok(()) }
Filters
Example of using the abs
built-in filter:
{{ -2|abs }}
Which generates:
#![allow(unused)] fn main() { writer .write_fmt( format_args!( "{0}", &::askama::MarkupDisplay::new_unsafe( &(::askama::filters::abs(-2)?), ::askama::Html, ), ), )?; ::askama::Result::Ok(()) }
The filter is called with -2
as first argument. You can add further arguments
to the call like this:
{{ "a"|indent(4) }}
Which generates:
#![allow(unused)] fn main() { writer .write_fmt( format_args!( "{0}", &::askama::MarkupDisplay::new_unsafe( &(::askama::filters::indent("a", 4)?), ::askama::Html, ), ), )?; ::askama::Result::Ok(()) }
No surprise there, 4
is added after "a"
. Now let's check when we chain the filters:
{{ "a"|indent(4)|capitalize }}
Which generates:
#![allow(unused)] fn main() { writer .write_fmt( format_args!( "{0}", &::askama::MarkupDisplay::new_unsafe( &(::askama::filters::capitalize( &(::askama::filters::indent("a", 4)?), )?), ::askama::Html, ), ), )?; ::askama::Result::Ok(()) }
As expected, capitalize
's first argument is the value returned by the indent
call.
Macros
This code:
{% macro heading(arg) %}
<h1>{{arg}}</h1>
{% endmacro %}
{% call heading("title") %}
generates:
#![allow(unused)] fn main() { { let (arg) = (("title")); writer .write_fmt( format_args!( "\n<h1>{0}</h1>\n", &::askama::MarkupDisplay::new_unsafe(&(arg), ::askama::Html), ), )?; } ::askama::Result::Ok(()) }
As you can see, the macro itself isn't present in the generated code, only its internal code is generated as well as its arguments.