1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
|
# 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.
## 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:
```jinja
{% 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.
```jinja
{% 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](filters.md).
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.
## 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:
```text
{% 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:
```text
<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 `~`:
```jinja
{% 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](configuration.md) 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 (`+`)
## 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
```html
<!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.
### Child template
Here's an example child template:
```html
{% 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.
## 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][owasp]. 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.
[owasp]: https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html#output-encoding-for-html-contexts
```rust
#[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:
```html
<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
```html
<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 is used as you might expect:
```text
{% if users.len() == 0 %}
No users
{% else if users.len() == 1 %}
1 user
{% else %}
{{ users.len() }} users
{% 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`:
```text
{% 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:
```text
{% for i in iter %}
{% include "item.html" %}
{% endfor %}
```
```text
* 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.
```rust
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](https://github.com/djc/askama/blob/main/testing/tests/render_in_place.rs)
using a vector of templates in a for block.
## Comments
Askama supports block comments delimited by `{#` and `#}`.
```jinja
{# A Comment #}
```
Like Rust, Askama also supports nested block comments.
```jinja
{#
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 .
```rust
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 later with `{% call name(args) %}`
```
{% macro heading(arg) %}
<h1>{{arg}}</h1>
{% endmacro %}
{% call heading(s) %}
```
You can place templates in a separate file and use it in your templates by using `{% import %}`
```
{%- import "macro.html" as scope -%}
{% call scope::heading(s) %}
```
|
