Tera, a rust template system11.01.24

Getting started

To use Tera in your Rust projects, simply add it to your Cargo.toml:

tera = "1"

By default, Tera comes with some additional dependencies required for the truncate, date, filesizeformat, slugify, urlencode and urlencode_strict filters as well as for the now function. You can disable them by setting the following in your Cargo.toml:

tera = { version = "1", default-features = false }

And add the following to your lib.rs or main.rs if you are not using Rust 2018 edition or later:

extern crate tera;

You can view everything Tera exports on the API docs.
Usage

The primary method of using Tera is to load and parse all the templates in a given glob.

Let's take the following directory as example.

templates/
  hello.html
  index.html
  products/
    product.html
    price.html

Assuming the Rust file is at the same level as the templates folder, we can get a Tera instance that way:

use tera::Tera;

// Use globbing
let tera = match Tera::new("templates/**/*.html") {
    Ok(t) => t,
    Err(e) => {
        println!("Parsing error(s): {}", e);
        ::std::process::exit(1);
    }
};

Compiling templates is a step that is meant to only happen once: use something like lazy_static to define a constant instance.

lazy_static! {
    pub static ref TEMPLATES: Tera = {
        let mut tera = match Tera::new("examples/basic/templates/**/*") {
            Ok(t) => t,
            Err(e) => {
                println!("Parsing error(s): {}", e);
                ::std::process::exit(1);
            }
        };
        tera.autoescape_on(vec![".html", ".sql"]);
        tera.register_filter("do_nothing", do_nothing_filter);
        tera
    };
}

You need two things to render a template: a name and a context. If you are using globs, Tera will automatically remove the glob prefix from the template names. To use our example from before, the template name for the file located at templates/hello.html will be hello.html.

The context can either be a data structure that implements the Serialize trait from serde_json or an instance of tera::Context:

use tera::Context;
// Using the tera Context struct
let mut context = Context::new();
context.insert("product", &product);
context.insert("vat_rate", &0.20);
tera.render("products/product.html", &context)?;

#[derive(Serialize)]
struct Product {
    name: String
}
// or a struct
tera.render("products/product.html", &Context::from_serialize(&product)?)?;

Auto-escaping

By default, Tera will auto-escape all content in files ending with ".html", ".htm" and ".xml". Escaping follows the recommendations from OWASP.

You can override that or completely disable auto-escaping by calling the autoescape_on method:

// escape only files ending with `.php.html`
tera.autoescape_on(vec![".php.html"]);
// disable autoescaping completely
tera.autoescape_on(vec![]);

Tera does not perform contextual auto-escaping, e.g. by parsing the template to know whether to escape JS, CSS or HTML (see https://rawgit.com/mikesamuel/sanitized-jquery-templates/trunk/safetemplate.html for more details on that).
Advanced usage
Extending another instance

If you are using a framework or a library using Tera, chances are they provide their own Tera instance with some built-in templates, filters, global functions or testers. Tera offers an extend method that will extend your own instance with everything mentioned before:

let mut tera = Tera::new(&tpl_glob).chain_err(|| "Error parsing templates")?;
// ZOLA_TERA is an instance present in a library
tera.extend(&ZOLA_TERA)?;

If anything - templates, filters, etc - with the same name exists in both instances, Tera will only keep yours.
Reloading

If you are watching a directory and want to reload templates on change (editing/adding/removing a template), Tera gives the full_reload method:

tera.full_reload()?;

Note that reloading is only available if you are loading templates with a glob.
Loading templates from strings

Tera allows you to load templates not only from files but also from plain strings.

// one template only
let mut tera = Tera::default();
tera.add_raw_template("hello.html", "the body")?;

// many templates
let mut tera = Tera::default();
tera.add_raw_templates(vec![
    ("grandparent", "{% block hey %}hello{% endblock hey %}"),
    ("parent", "{% extends \"grandparent\" %}{% block hey %}Parent{% endblock hey %}"),
])?;

If some templates are related, for example one extending the other, you will need to use the add_raw_templates method as Tera will error if it find inconsistencies such as extending a template that Tera doesn't know about.
Render a one off template

Want to render a single template, for example one coming from a user? The one_off function is there for that.

// The last parameter is whether we want to autoescape the template or not.
// Should be true in 99% of the cases for HTML
let context = Context::new();
// add stuff to context
let result = Tera::one_off(user_tpl, context, true);

Templates
Introduction
Tera Basics

A Tera template is just a text file where variables and expressions get replaced with values when it is rendered. The syntax is based on Jinja2 and Django templates.

There are 3 kinds of delimiters and those cannot be changed:

{{ and }} for expressions
    {% and %} for statements
    {# and #} for comments

Raw

Tera will consider all text inside the raw block as a string and won't try to render what's inside. Useful if you have text that contains Tera delimiters.

{% raw %}
  Hello {{ name }}
{% endraw %}

would be rendered as Hello {{ name }}.
Whitespace control

Tera comes with easy to use whitespace control: use {%- if you want to remove all whitespace before a statement and -%} if you want to remove all whitespace after.

For example, let's look at the following template:

{% set my_var = 2 %}
{{ my_var }}

will have the following output:

If we want to get rid of the empty line, we can write the following:

{% set my_var = 2 -%}
{{ my_var }}

Comments

To comment out part of the template, wrap it in {# #}. Anything in between those tags will not be rendered.

{# A comment #}

Data structures
Literals

Tera has a few literals that can be used:

booleans: true (or True) and false (or False)
    integers
    floats
    strings: text delimited by "", '' or ``
    arrays: a comma-separated list of literals and/or idents surrounded by [ and ] (trailing comma allowed)

Variables

Variables are defined by the context given when rendering a template. If you'd like to define your own variables, see the Assignments section.

You can render a variable by using the {{ name }}.

Trying to access or render a variable that doesn't exist will result in an error.

A magical variable is available in every template if you want to print the current context: __tera_context.
Dot notation:

Construct and attributes can be accessed by using the dot (.) like {{ product.name }}. Specific members of an array or tuple are accessed by using the .i notation, where i is a zero-based index. In dot notation variable can not be used after the dot (.).
Square bracket notation:

A more powerful alternative to (.) is to use square brackets ([ ]). Variables can be rendered using the notation {{product['name']}} or {{product["name"]}}.

If the item is not in quotes it will be treated as a variable. Assuming you have the following objects in your context product = Product{ name: "Fred" } and my_field = "name", calling {{product[my_field]}} will resolve to: {{product.name}}.

Only variables evaluating to string or integer number can be used as index: anything else will be an error.
Expressions

Tera allows expressions almost everywhere.
Math

You can do some basic math in Tera but it shouldn't be abused other than the occasional + 1 or similar. Math operations are only allowed with numbers, using them on any other kind of values will result in an error. You can use the following operators:

+: adds 2 values together, {{ 1 + 1 }} will print 2
    -: performs a subtraction, {{ 2 - 1 }} will print 1
    /: performs a division, {{ 10 / 2 }} will print 5
    *: performs a multiplication, {{ 5 * 2 }} will print 10
    %: performs a modulo, {{ 2 % 2 }} will print 0

The priority of operations is the following, from lowest to highest:

+ and -
    * and / and %

Comparisons

==: checks whether the values are equal
    !=: checks whether the values are different
    >=: true if the left value is equal or greater to the right one
    <=: true if the right value is equal or greater to the left one
    >: true if the left value is greater than the right one
    <: true if the right value is greater than the left one

Logic

and: true if the left and right operands are true
    or: true if the left or right operands are true
    not: negate an expression

Concatenation

You can concatenate several strings/numbers/idents using the ~ operator.

An ident resolving to something other than a string or a number will raise an error.
in checking

You can check whether a left side is contained in a right side using the in operator.

Only literals/variables resulting in an array, a string and an object are supported in the right hand side: everything else will raise an error. While in the left hand side only literals/variables resulting in a number, a string and a boolean are supported.
Manipulating data
Assignments

You can assign values to variables during the rendering. Assignments in for loops and macros are scoped to their context but assignments outside of those will be set in the global context. Furthermore, assignments in for loop are valid until the end of the current iteration only.

{% set my_var = "hello" %}
{% set my_var = 1 + 4 %}
{% set my_var = some_var %}
{% set my_var = macros::some_macro() %}
{% set my_var = global_fn() %}
{% set my_var = [1, true, some_var | round] %}

If you want to assign a value in the global context while in a for loop, you can use set_global:

{% set_global my_var = "hello" %}
{% set_global my_var = 1 + 4 %}
{% set_global my_var = some_var %}
{% set_global my_var = macros::some_macro() %}
{% set_global my_var = global_fn() %}
{% set_global my_var = [1, true, some_var | round] %}

Outside of a for loop, set_global is exactly the same as set.
Filters

You can modify variables using filters. Filters are separated from the variable by a pipe symbol (|) and may have named arguments in parentheses. Multiple filters can be chained: the output of one filter is applied to the next.

For example, {{ name | lower | replace(from="doctor", to="Dr.") }} will take a variable called name, make it lowercase and then replace instances of doctor by Dr.. It is equivalent to replace(lower(name), from="doctor", to="Dr.") if we were to look at it as functions.

Calling filters on an incorrect type like trying to capitalize an array or using invalid types for arguments will result in an error.

Filters are functions with the fn(Value, HashMap<String, Value>) -> Result<Value> definition and custom ones can be added like so:

tera.register_filter("upper", string::upper);

While filters can be used in math operations, they will have the lowest priority and therefore might not do what you expect:

{{ 1 + a | length }}
// is equal to
{{ (1 + a) | length } // this will probably error

// This will do what you wanted initially
{{ a | length + 1 }}

Tera has many built-in filters that you can use.
Filter sections

Whole sections can also be processed by filters if they are encapsulated in {% filter name %} and {% endfilter %} tags where name is the name of the filter:

{% filter upper %}
    Hello
{% endfilter %}

This example transforms the text Hello in all upper-case (HELLO).

Filter sections can also contain block sections like this:

{% filter upper %}
  {% block content_to_be_upper_cased %}
    This will be upper-cased
  {% endblock content_to_be_upper_cased %} 
{% endfilter %}

Tests

Tests can be used against an expression to check some condition on it and are made in if blocks using the is keyword. For example, you would write the following to test if an expression is odd:

{% if my_number is odd %}
 Odd
{% endif %}

Tests can also be negated:

{% if my_number is not odd %}
 Even
{% endif %}

Tests are functions with the fn(Option<Value>, Vec<Value>) -> Result<bool> definition and custom ones can be added like so:

tera.register_tester("odd", testers::odd);

Tera has many built-in tests that you can use.
Functions

Functions are Rust code that return a Result<Value> from the given params.

Quite often, functions will need to capture some external variables, such as a url_for global function needing the list of URLs for example.

Here's an example on how to implement a very basic function:

fn make_url_for(urls: BTreeMap<String, String>) -> impl Function {
    Box::new(move |args| -> Result<Value> {
        match args.get("name") {
            Some(val) => match from_value::<String>(val.clone()) {
                Ok(v) =>  Ok(to_value(urls.get(&v).unwrap()).unwrap()),
                Err(_) => Err("oops".into()),
            },
            None => Err("oops".into()),
        }
    })
}

You then need to add it to Tera:

tera.register_function("url_for", make_url_for(urls));

And you can now call it from a template:

You can also implement the trait directly if you have more complex requirements.

Currently functions can be called in two places in templates:

variable block: {{ url_for(name="home") }}
    for loop container: {% for i in range(end=5) %}

Tera comes with some built-in functions.
Control structures
If

Conditionals are fully supported and are identical to the ones in Python.

{% if price < 10 or always_show %}
   Price is {{ price }}.
{% elif price > 1000 and not rich %}
   That's expensive!
{% else %}
    N/A
{% endif %}

Undefined variables are considered falsy. This means that you can test for the presence of a variable in the current context by writing:

{% if my_var %}
    {{ my_var }}
{% else %}
    Sorry, my_var isn't defined.
{% endif %}

Every if statement has to end with an endif tag.
For

Loop over items in a array:

{% for product in products %}
  {{loop.index}}. {{product.name}}
{% endfor %}

Or on characters of a string:

{% for letter in name %}
  {% if loop.index % 2 == 0%}
    <span style="color:red">{{ letter }}</span>
  {% else %}
    <span style="color:blue">{{ letter }}</span>
  {% endif %}
{% endfor %}

A few special variables are available inside for loops:

loop.index: current iteration 1-indexed
    loop.index0: current iteration 0-indexed
    loop.first: whether this is the first iteration
    loop.last: whether this is the last iteration

Every for statement has to end with an endfor tag.

You can also loop on maps and structs using the following syntax:

{% for key, value in products %}
  {{key}}. {{value.name}}
{% endfor %}

key and value can be named however you want, they just need to be separated with a comma.

If you are iterating on an array, you can also apply filters to the container:

{% for product in products | reverse %}
  {{loop.index}}. {{product.name}}
{% endfor %}

You can also iterate on array literals:

{% for a in [1,2,3,] %}
  {{a}}
{% endfor %}

Lastly, you can set a default body to be rendered when the container is empty:

{% for product in products %}
  {{loop.index}}. {{product.name}}
{% else %}
  No products.  
{% endfor %}

Loop Controls

Within a loop, break and continue may be used to control iteration.

To stop iterating when target_id is reached:

{% for product in products %}
  {% if product.id == target_id %}{% break %}{% endif %}
  {{loop.index}}. {{product.name}}
{% endfor %}

To skip even-numbered items:

{% for product in products %}
  {% if loop.index is even %}{% continue %}{% endif %}
  {{loop.index}}. {{product.name}}
{% endfor %}

Include

You can include a template to be rendered using the current context with the include tag.

{% include "included.html" %}

The template path needs to be a static string. This is invalid:

{% include "partials/" ~ name ~ ".html" %}

Tera doesn't offer passing a custom context to the include tag. If you want to do that, use macros.

While you can set values in included templates, those values only exist while rendering them: the template calling include doesn't see them.

You can mark an include with ignore missing, so that Tera will ignore the statement if the template to be included does not exist.

{% include "header.html" ignore missing %}

You can also provide a list of templates that are checked for existence before inclusion. The first template that exists will be included. If ignore missing is given, it will fall back to rendering nothing if none of the templates exist.

{% include ["custom/header.html", "header.html"] %}
{% include ["special_sidebar.html", "sidebar.html"] ignore missing %}

Note: include works similar to how it does in other engines like Jinja, with the exception that the current version of Tera doesn't allow inheritance within included files. Practically speaking this means you have to choose between using includes or extends to organise your site, without mixing them.
Macros

Think of macros as functions or components that you can call and return some text.

They are defined as follows:

{% macro input(label, type="text") %}
    <label>
        {{ label }}
        <input type="{{type}}" />
    </label>
{% endmacro input %}

As shown in the example above, macro arguments can have a default literal value.

If a macro is defined in a separate file, you need to import the file containing the macros:

{% import "macros.html" as macros %}

You can name that file namespace (macros in the example) anything you want. A macro is called like this:

// namespace::macro_name(**kwargs)
{{ macros::input(label="Name", type="text") }}

Do note that macros, like filters, require keyword arguments. Use the self namespace when calling a macro defined in the same file. Macros must be defined top-level (they cannot be nested in an if, for, etc.) and should only reference arguments, not template variables directly.

Macros can be called recursively but there is no limit to recursion so make sure your macro ends.

Here's an example of a recursive macro:

{% macro factorial(n) %}
  {% if n > 1 %}{{ n }} - {{ self::factorial(n=n-1) }}{% else %}1{% endif %}
{% endmacro factorial %}

A macro's body can contain all normal Tera syntax with the exception of macros definition, block and extends.
Inheritance

Tera uses the same kind of inheritance as Jinja2 and Django templates: you define a base template and extend it in child templates through blocks. There can be multiple levels of inheritance (i.e. A extends B that extends C).
Base template

A base template typically contains the basic document structure as well as several blocks that can have content.

For example, here's a base.html almost copied from the Jinja2 documentation:

<!DOCTYPE html>
<html lang="en">
<head>
    {% block head %}
    <link rel="stylesheet" href="style.css" />
    <title>{% block title %}{% endblock title %} - My Webpage</title>
    {% endblock head %}
</head>
<body>
    <div id="content">{% block content %}{% endblock content %}</div>
    <div id="footer">
        {% block footer %}
        © Copyright 2008 by <a href="http://domain.invalid/">you</a>.
        {% endblock footer %}
    </div>
</body>
</html>

This base.html template defines 4 block tags that child templates can override. The head and footer block have some content already which will be rendered if they are not overridden.
Child template

Again, straight from Jinja2 docs:

{% extends "base.html" %}
{% block title %}Index{% endblock title %}
{% block head %}
    {{ super() }}
    <style type="text/css">
        .important { color: #336699; }
    </style>
{% endblock head %}
{% block content %}
    <h1>Index</h1>
    <p class="important">
      Welcome to my awesome homepage.
    </p>
{% endblock content %}

To indicate inheritance, you have to use the extends tag as the first thing in the file followed by the name of the template you want to extend. The {{ super() }} variable call tells Tera to render the parent block there.

Nested blocks also work in Tera. Consider the following templates:

// grandparent
{% block hey %}hello{% endblock hey %}

// parent
{% extends "grandparent" %}
{% block hey %}hi and grandma says {{ super() }} {% block ending %}sincerely{% endblock ending %}{% endblock hey %}

// child
{% extends "parent" %}
{% block hey %}dad says {{ super() }}{% endblock hey %}
{% block ending %}{{ super() }} with love{% endblock ending %}

The block ending is nested in the hey block. Rendering the child template will do the following:

Find the first base template: grandparent
    See hey block in it and check if it is in child and parent template
    It is in child so we render it, it contains a super() call so we render the hey block from parent, which also contains a super() so we render the hey block of the grandparent template as well
    See ending block in child, render it and also render the ending block of parent as there is a super()

The end result of that rendering (not counting whitespace) will be: "dad says hi and grandma says hello sincerely with love".

This example explicitly terminates named blocks with {% endblock hey %}. It's not required to give the name of the block being terminated {% endblock %}, though it may add some clarity.

See the note in the Include section regarding mixing inheritance and includes.
Built-ins
Built-in filters

Tera has the following filters built-in:
lower

Converts a string to lowercase.
upper

Converts a string to uppercase.
wordcount

Returns the number of words in a string.
capitalize

Returns the string with all its characters lowercased apart from the first char which is uppercased.
replace

Takes 2 mandatory string named arguments: from and to. It will return a string with all instances of the from string replaced with the to string.

Example: {{ name | replace(from="Robert", to="Bob")}}
addslashes

Adds slashes before quotes.

Example: {{ value | addslashes }}

If value is "I'm using Tera", the output will be "I\'m using Tera".
slugify

Only available if the builtins feature is enabled.

Transforms a string into ASCII, lowercases it, trims it, converts spaces to hyphens and removes all characters that are not numbers, lowercase letters or hyphens.

Example: {{ value | slugify }}

If value is "-Hello world! ", the output will be "hello-world".
title

Capitalizes each word inside a sentence.

Example: {{ value | title }}

If value is "foo bar", the output will be "Foo Bar".
trim

Removes leading and trailing whitespace if the variable is a string.
trim_start

Removes leading whitespace if the variable is a string.
trim_end

Removes trailing whitespace if the variable is a string.
trim_start_matches

Removes leading characters that match the given pattern if the variable is a string.

Example: {{ value | trim_start_matches(pat="//") }}

If value is "//a/b/c//", the output will be "a/b/c//".
trim_end_matches

Removes trailing characters that match the given pattern if the variable is a string.

Example: {{ value | trim_end_matches(pat="//") }}

If value is "//a/b/c//", the output will be "//a/b/c".
truncate

Only available if the builtins feature is enabled.

Truncates a string to the indicated length. If the string has a smaller length than the length argument, the string is returned as is.

Example: {{ value | truncate(length=10) }}

By default, the filter will add an ellipsis at the end if the text was truncated. You can change the string appended by setting the end argument. For example, {{ value | truncate(length=10, end="") }} will not append anything.
linebreaksbr

Replaces line breaks (\n or \r\n) with HTML line breaks (<br>).

Example: {{ value | linebreaksbr }}

If value is "Hello\r\nworld\n", the output will be "Hello<br>world<br>".

Note that if the template you are using it in is automatically escaped, you will need to call the safe filter after linebreaksbr.
spaceless

Remove space ( ) and line breaks (\n or \r\n) between HTML tags

Example: {{ value | spaceless }}

If the value is "<p>\n<a> </a>\r\n </p>", the output will be "<p><a></a></p>".

Note that only whitespace between successive opening tags and successive closing tags is removed.

Also note that if the template you are using it in is automatically escaped, you will need to call the safe filter after spaceless.
indent

Indents a string by injecting a prefix at the start of each line. The prefix argument (default 4 spaces) specifies the prefix to insert per line. If the first argument (default false) is set true spaces are inserted for the first line. If the blank argument (default false) is set true spaces are inserted for blank/whitespace lines.
striptags

Tries to remove HTML tags from input. Does not guarantee well formed output if input is not valid HTML.

Example: {{ value | striptags }}

If value is "<b>Joel</b>", the output will be "Joel".

Note that if the template you are using it in is automatically escaped, you will need to call the safe filter after striptags.
first

Returns the first element of an array. If the array is empty, returns empty string.
last

Returns the last element of an array. If the array is empty, returns empty string.
nth

Returns the nth element of an array.§ If the array is empty, returns empty string. It takes a required n argument, corresponding to the 0-based index you want to get.

Example: {{ value | nth(n=2) }}
join

Joins an array with a string.

Example: {{ value | join(sep=" // ") }}

If value is the array ['a', 'b', 'c'], the output will be the string "a // b // c".
length

Returns the length of an array, an object, or a string.
reverse

Returns a reversed string or array.
sort

Sorts an array into ascending order.

The values in the array must be a sortable type:

numbers are sorted by their numerical value.
    strings are sorted in alphabetical order.
    arrays are sorted by their length.
    bools are sorted as if false=0 and true=1

If you need to sort a list of structs or tuples, use the attribute argument to specify which field to sort by.

Example:

Given people is an array of Person

struct Name(String, String);

struct Person {
    name: Name,
    age: u32,
}

The attribute argument can be used to sort by last name:

or by age:

unique

Removes duplicate items from an array. The attribute argument can be used to select items based on the values of an inner attribute. For strings, the case_sensitive argument (default is false) can be used to control the comparison.

Example:

Given people is an array of Person

struct Name(String, String);

struct Person {
    name: Name,
    age: u32,
}

The attribute argument can be used to select one Person for each age:

or by last name:

slice

Slices an array by the given start and end parameter. Both parameters are optional and omitting them will return the same array. Use the start argument to define where to start (inclusive, default to 0) and end argument to define where to stop (exclusive, default to the length of the array). start and end are 0-indexed.

{% for i in my_arr | slice(end=5) %}
{% for i in my_arr | slice(start=1) %}
{% for i in my_arr | slice(start=1, end=5) %}

You can also use negative index values to refer the array from the last element. -1 refers to the last index, -2 refers to the second last index and so on.

For example, let's look at the following template:

{% for i in my_arr | slice(end=-2) %}

will produce the follow output for my_array = [1, 2, 3, 4, 5]: [1, 2, 3]
group_by

Groups an array using the required attribute argument. The filter takes an array and returns a map where the keys are the values of the attribute stringified and the values are all elements of the initial array having that attribute. Values with missing attribute or where attribute is null will be discarded.

Example:

Given posts is an array of Post

struct Author {
    name: String,
};

struct Post {
    content: String,
    year: u32,
    author: Author,
}

The attribute argument can be used to group posts by year:

or by author name:

{% for name, author_posts in posts | group_by(attribute="author.name") %}
    {{ name }}
    {% for post in author_posts %}
        {{ post.year }}: {{ post.content }}
    {% endfor %}
{% endfor %}

Manipulating the hashmap produced by group_by in an arbitrary order requires additional steps to extract the keys into a separate array.

Example:

{% set map = section.pages | group_by(attribute="year") %}
{% set_global years = [] %}
{% for year, ignored in map %}
    {% set_global years = years | concat(with=year) %}
{% endfor %}
{% for year in years | reverse %}
    {% set posts = map[year] %}
{% endfor %}

filter

Filters the array values, returning only the values where the attribute is equal to the value. Values with missing attribute or where attribute is null will be discarded.

attribute is mandatory.

Example:

Given posts is an array of Post

struct Author {
    name: String,
};

struct Post {
    content: String,
    year: u32,
    author: Author,
    draft: bool,
}

The attribute argument can be used to filter posts by draft value:

or by author name:

If value is not passed, it will drop any elements where the attribute is null.
map

Retrieves an attribute from each object in an array. The attribute argument is mandatory and specifies what to extract.

Example:

Given people is an array of Person

struct Name(String, String);

struct Person {
    name: Name,
    age: u32,
}

The attribute argument is used to retrieve their ages.

concat

Appends values to an array.

The filter takes an array and returns a new array with the value(s) from the with parameter added. If the with parameter is an array, all of its values will be appended one by one to the new array and not as an array.

This filter can also be used to append a single value to an array if the value passed to with is not an array:

{% set pages_id = pages_id | concat(with=id) %}

The with attribute is mandatory.
urlencode

Only available if the builtins feature is enabled.

Percent-encodes all the characters in a string which are not included in unreserved chars (according to RFC3986) with the exception of forward slash (/).

Example: {{ value | urlencode }}

If value is /foo?a=b&c=d, the output will be /foo%3Fa%3Db%26c%3Dd. / is not escaped.
urlencode_strict

Only available if the builtins feature is enabled.

Similar to urlencode filter but encodes all non-alphanumeric characters in a string including forward slashes (/).

Example: {{ value | urlencode_strict }}

If value is /foo?a=b&c=d, the output will be %2Ffoo%3Fa%3Db%26c%3Dd. / is also encoded.
abs

Returns the absolute value

Example: {{ negative_number | abs }}

If negative_number is -1, the output will be 1. If num_messages is -2.0 the output will be 2.
pluralize

Returns a plural suffix if the value is not equal to ±1, or a singular suffix otherwise. The plural suffix defaults to s and the singular suffix defaults to the empty string (i.e. nothing).

Example: You have {{ num_messages }} message{{ num_messages | pluralize }}

If num_messages is 1, the output will be You have 1 message. If num_messages is 2 the output will be You have 2 messages. You can also customize the singular and plural suffixes with the singular and plural arguments to the filter:

Example: {{ num_categories }} categor{{ num_categories | pluralize(singular="y", plural="ies") }}
round

Returns a number rounded following the method given. Default method is common which will round to the nearest integer. ceil and floor are available as alternative methods. Another optional argument, precision, is available to select the precision of the rounding. It defaults to 0, which will round to the nearest integer for the given method.

Example: {{ num | round }} {{ num | round(method="ceil", precision=2) }}
filesizeformat

Only available if the builtins feature is enabled.

Returns a human-readable file size (i.e. '110 MB') from an integer.

Example: {{ num | filesizeformat }}
date

Only available if the builtins feature is enabled.

Parses a timestamp into a date(time) string. Defaults to YYYY-MM-DD format. Time formatting syntax is inspired from strftime and a full reference is available on chrono docs.

Example: {{ ts | date }} {{ ts | date(format="%Y-%m-%d %H:%M") }}

If you are using ISO 8601 date strings or a UTC timestamp, you can optionally supply a timezone for the date to be rendered in.

Example:

{{ "2019-09-19T13:18:48.731Z" | date(timezone="America/New_York") }}

{{ "2019-09-19T13:18:48.731Z" | date(format="%Y-%m-%d %H:%M", timezone="Asia/Shanghai") }}

Locale can be specified (excepted when the input is a timestamp without timezone argument), default being POSIX. (only available if the date-locale feature is enabled)

Example: {{ 1648252203 | date(format="%A %-d %B", timezone="Europe/Paris", locale="fr_FR") }}
escape

Escapes a string's HTML. Specifically, it makes these replacements:

& is converted to &
    < is converted to <
    > is converted to >
    " (double quote) is converted to "
    ' (single quote) is converted to '
    / is converted to /

escape_xml

Escapes XML special characters. Specifically, it makes these replacements:

& is converted to &
    < is converted to <
    > is converted to >
    " (double quote) is converted to "
    ' (single quote) is converted to '

safe

Marks a variable as safe: HTML will not be escaped anymore. safe only works if it is the last filter of the expression:

{{ content | replace(from="Robert", to="Bob") | safe }} will not be escaped
    {{ content | safe | replace(from="Robert", to="Bob") }} will be escaped

get

Accesses a value from an object when the key is not a Tera identifier. Example: {{ sections | get(key="posts/content") }}

The get filter also has a default parameter which can be used to provide a return value when the key parameter is missing from the set being filtered. Example: {{ sections | get(key="posts/content", default="default") }}
split

Splits a string into an array of strings, separated by a pattern given. Example: {{ path | split(pat="/") }}
int

Converts a value into an integer. The default argument can be used to specify the value to return on error, and the base argument can be used to specify how to interpret the number. Bases of 2, 8, and 16 understand the prefix 0b, 0o, 0x, respectively.
float

Converts a value into a float. The default argument can be used to specify the value to return on error.
json_encode

Transforms any value into a JSON representation. This filter is better used together with safe or when automatic escape is disabled.

Example: {{ value | json_encode() | safe }}

It accepts a parameter pretty (boolean) to print a formatted JSON instead of a one-liner.

Example: {{ value | json_encode(pretty=true) | safe }}
as_str

Returns a string representation of the given value.

Example: {{ value | as_str }}
default

Returns the default value given only if the variable evaluated is not present in the context and is therefore meant to be at the beginning of a filter chain if there are several filters.

Example: {{ value | default(value=1) }}

This is in most cases a shortcut for:

{% if value %}{{ value }}{% else %}1{% endif %}

However, only the existence of the value in the context is checked. With a value that if would evaluate to false (such as an empty string, or the number 0), the default filter will not attempt replace it with the alternate value provided. For example, the following will produce "I would like to read more !":

I would like to read more {{ "" | default (value="Louise Michel") }}!

If you intend to use the default filter to deal with optional values, you should make sure those values aren't set! Otherwise, use a full if block. This is especially relevant for dealing with optional arguments passed to a macro.
Built-in tests

Here are the currently built-in tests:
defined

Returns true if the given variable is defined.
undefined

Returns true if the given variable is undefined.
odd

Returns true if the given variable is an odd number.
even

Returns true if the given variable is an even number.
string

Returns true if the given variable is a string.
number

Returns true if the given variable is a number.
divisibleby

Returns true if the given expression is divisible by the arg given.

Example:

{% if rating is divisibleby(2) %}
    Divisible
{% endif %}

iterable

Returns true if the given variable can be iterated over in Tera (i.e. is an array/tuple or an object).
object

Returns true if the given variable is an object (i.e. can be iterated over key, value).
starting_with

Returns true if the given variable is a string and starts with the arg given.

Example:

{% if path is starting_with("x/") %}
    In section x
{% endif %}

ending_with

Returns true if the given variable is a string and ends with the arg given.
containing

Returns true if the given variable contains the arg given.

The test works on:

strings: is the arg a substring?
    arrays: is the arg given one of the members of the array?
    maps: is the arg given a key of the map?

Example:

{% if username is containing("xXx") %}
    Bad
{% endif %}

matching

Returns true if the given variable is a string and matches the regex in the argument.

Example:

{% if name is matching("^[Qq]ueen") %}
    Her Royal Highness, {{ name }}
{% elif name is matching("^[Kk]ing") %}
    His Royal Highness, {{ name }}
{% else %}
    {{ name }}
{% endif %}

A comprehensive syntax description can be found in the regex crate documentation.
Built-in functions

Tera comes with some built-in global functions.
range

Returns an array of integers created using the arguments given. There are 3 arguments, all integers:

end: stop before end, mandatory
    start: where to start from, defaults to 0
    step_by: with what number do we increment, defaults to 1

now

Only available if the builtins feature is enabled.

Returns the local datetime as string or the timestamp as integer if requested.

There are 2 arguments, both booleans:

timestamp: whether to return the timestamp instead of the datetime
    utc: whether to return the UTC datetime instead of the local one

Formatting is not built-in the global function but you can use the date filter like so now() | date(format="%Y") if you wanted to get the current year.
throw

The template rendering will error with the given message when encountered.

There is only one string argument:

message: the message to display as the error

get_random

Only available if the builtins feature is enabled.

Returns a random integer in the given range. There are 2 arguments, both integers:

start: defaults to 0 if not present
    end: required

start is inclusive (i.e. can be returned) and end is exclusive.
get_env

Returns the environment variable value for the name given. It will error if the environment variable is not found but the call can also take a default value instead.

name: the name of the environment variable to look for, required
    default: a default value in case the environment variable is not found

If the environment variable is found, it will always be a string while your default could be of any type.

https://keats.github.io/tera/docs/

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Tera, a rust template system11.01.24

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