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1 API
2 ===
3
4 .. module:: jinja2
5 :synopsis: public Jinja2 API
6
7 This document describes the API to Jinja2 and not the template language. It
8 will be most useful as reference to those implementing the template interface
9 to the application and not those who are creating Jinja2 templates.
10
11 Basics
12 ------
13
14 Jinja2 uses a central object called the template :class:`Environment`.
15 Instances of this class are used to store the configuration, global objects
16 and are used to load templates from the file system or other locations.
17 Even if you are creating templates from string by using the constructor of
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18 :class:`Template` class, an environment is created automatically for you,
19 albeit a shared one.
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20
21 Most applications will create one :class:`Environment` object on application
22 initialization and use that to load templates. In some cases it's however
23 useful to have multiple environments side by side, if different configurations
24 are in use.
25
26 The simplest way to configure Jinja2 to load templates for your application
27 looks roughly like this::
28
29 from jinja2 import Environment, PackageLoader
30 env = Environment(loader=PackageLoader('yourapplication', 'templates'))
31
32 This will create a template environment with the default settings and a
33 loader that looks up the templates in the `templates` folder inside the
34 `yourapplication` python package. Different loaders are available
35 and you can also write your own if you want to load templates from a
36 database or other resources.
37
38 To load a template from this environment you just have to call the
39 :meth:`get_template` method which then returns the loaded :class:`Template`::
40
41 template = env.get_template('mytemplate.html')
42
43 To render it with some variables, just call the :meth:`render` method::
44
45 print template.render(the='variables', go='here')
46
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47 Using a template loader rather then passing strings to :class:`Template`
48 or :meth:`Environment.from_string` has multiple advantages. Besides being
49 a lot easier to use it also enables template inheritance.
50
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52 Unicode
53 -------
54
55 Jinja2 is using unicode internally which means that you have to pass unicode
56 objects to the render function or bytestrings that only consist of ASCII
57 characters. Additionally newlines are normalized to one end of line
58 sequence which is per default UNIX style (``\n``).
59
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60 Python 2.x supports two ways of representing string objects. One is the
61 `str` type and the other is the `unicode` type, both of which extend a type
62 called `basestring`. Unfortunately the default is `str` which should not
63 be used to store text based information unless only ASCII characters are
64 used. With Python 2.6 it is possible to my `unicode` the default on a per
65 module level and with Python 3 it will be the default.
66
67 To explicitly use a unicode string you have to prefix the string literal
68 with a `u`: ``u'Hänsel und Gretel sagen Hallo'``. That way Python will
69 store the string as unicode by decoding the string with the character
70 encoding from the current Python module. If no encoding is specified this
71 defaults to 'ASCII' which means that you can't use any non ASCII identifier.
72
73 To set a better module encoding add the following comment to the first or
74 second line of the Python module using the unicode literal::
75
76 # -*- coding: utf-8 -*-
77
78 We recommend utf-8 as Encoding for Python modules and templates as it's
79 possible to represent every Unicode character in utf-8 and because it's
80 backwards compatible to ASCII. For Jinja2 the default encoding of templates
81 is assumed to be utf-8.
82
83 It is not possible to use Jinja2 to process non unicode data. The reason
84 for this is that Jinja2 uses Unicode already on the language level. For
85 example Jinja2 treats the non-breaking space as valid whitespace inside
86 expressions which requires knowledge of the encoding or operating on an
87 unicode string.
88
89 For more details about unicode in Python have a look at the excellent
90 `Unicode documentation`_.
91
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92 Another important thing is how Jinja2 is handling string literals in
93 templates. A naive implementation would be using unicode strings for
94 all string literals but it turned out in the past that this is problematic
95 as some libraries are typechecking against `str` explicitly. For example
96 `datetime.strftime` does not accept unicode arguments. To not break it
97 completely Jinja2 is returning `str` for strings that fit into ASCII and
98 for everything else `unicode`:
99
100 >>> m = Template(u"{% set a, b = 'foo', 'föö' %}").module
101 >>> m.a
102 'foo'
103 >>> m.b
104 u'f\xf6\xf6'
105
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106
107 .. _Unicode documentation: http://docs.python.org/dev/howto/unicode.html
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108
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109 High Level API
110 --------------
111
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112 The high-level API is the API you will use in the application to load and
113 render Jinja2 templates. The :ref:`low-level-api` on the other side is only
114 useful if you want to dig deeper into Jinja2 or :ref:`develop extensions
115 <jinja-extensions>`.
116
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117 .. autoclass:: Environment([options])
118 :members: from_string, get_template, join_path, extend
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119
120 .. attribute:: shared
121
122 If a template was created by using the :class:`Template` constructor
123 an environment is created automatically. These environments are
124 created as shared environments which means that multiple templates
125 may have the same anonymous environment. For all shared environments
126 this attribute is `True`, else `False`.
127
128 .. attribute:: sandboxed
129
130 If the environment is sandboxed this attribute is `True`. For the
131 sandbox mode have a look at the documentation for the
132 :class:`~jinja2.sandbox.SandboxedEnvironment`.
133
134 .. attribute:: filters
135
136 A dict of filters for this environment. As long as no template was
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137 loaded it's safe to add new filters or remove old. For custom filters
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138 see :ref:`writing-filters`. For valid filter names have a look at
139 :ref:`identifier-naming`.
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140
141 .. attribute:: tests
142
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143 A dict of test functions for this environment. As long as no
144 template was loaded it's safe to modify this dict. For custom tests
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145 see :ref:`writing-tests`. For valid test names have a look at
146 :ref:`identifier-naming`.
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147
148 .. attribute:: globals
149
150 A dict of global variables. These variables are always available
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151 in a template. As long as no template was loaded it's safe
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152 to modify this dict. For more details see :ref:`global-namespace`.
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153 For valid object names have a look at :ref:`identifier-naming`.
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154
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155 .. automethod:: overlay([options])
156
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157 .. method:: undefined([hint, obj, name, exc])
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159 Creates a new :class:`Undefined` object for `name`. This is useful
160 for filters or functions that may return undefined objects for
161 some operations. All parameters except of `hint` should be provided
162 as keyword parameters for better readability. The `hint` is used as
163 error message for the exception if provided, otherwise the error
164 message generated from `obj` and `name` automatically. The exception
165 provided as `exc` is raised if something with the generated undefined
166 object is done that the undefined object does not allow. The default
167 exception is :exc:`UndefinedError`. If a `hint` is provided the
168 `name` may be ommited.
169
170 The most common way to create an undefined object is by providing
171 a name only::
172
173 return environment.undefined(name='some_name')
174
175 This means that the name `some_name` is not defined. If the name
176 was from an attribute of an object it makes sense to tell the
177 undefined object the holder object to improve the error message::
178
179 if not hasattr(obj, 'attr'):
180 return environment.undefined(obj=obj, name='attr')
181
182 For a more complex example you can provide a hint. For example
183 the :func:`first` filter creates an undefined object that way::
184
185 return environment.undefined('no first item, sequence was empty')
186
187 If it the `name` or `obj` is known (for example because an attribute
188 was accessed) it shold be passed to the undefined object, even if
189 a custom `hint` is provided. This gives undefined objects the
190 possibility to enhance the error message.
191
192 .. autoclass:: Template
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193 :members: module, make_module
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194
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195 .. attribute:: globals
196
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197 The dict with the globals of that template. It's unsafe to modify
198 this dict as it may be shared with other templates or the environment
199 that loaded the template.
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200
201 .. attribute:: name
202
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203 The loading name of the template. If the template was loaded from a
204 string this is `None`.
205
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206 .. attribute:: filename
207
208 The filename of the template on the file system if it was loaded from
209 there. Otherwise this is `None`.
210
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211 .. automethod:: render([context])
212
213 .. automethod:: generate([context])
214
215 .. automethod:: stream([context])
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216
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217
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218 .. autoclass:: jinja2.environment.TemplateStream()
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219 :members: disable_buffering, enable_buffering
220
221
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222 .. _identifier-naming:
223
224 Notes on Identifiers
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225 --------------------
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226
227 Jinja2 uses the regular Python 2.x naming rules. Valid identifiers have to
228 match ``[a-zA-Z_][a-zA-Z0-9_]*``. As a matter of fact non ASCII characters
229 are currently not allowed. This limitation will probably go away as soon as
230 unicode identifiers are fully specified for Python 3.
231
232 Filters and tests are looked up in separate namespaces and have slightly
233 modified identifier syntax. Filters and tests may contain dots to group
234 filters and tests by topic. For example it's perfectly valid to add a
235 function into the filter dict and call it `to.unicode`. The regular
236 expression for filter and test identifiers is
237 ``[a-zA-Z_][a-zA-Z0-9_]*(\.[a-zA-Z_][a-zA-Z0-9_]*)*```.
238
239
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240 Undefined Types
241 ---------------
242
243 These classes can be used as undefined types. The :class:`Environment`
244 constructor takes an `undefined` parameter that can be one of those classes
245 or a custom subclass of :class:`Undefined`. Whenever the template engine is
246 unable to look up a name or access an attribute one of those objects is
247 created and returned. Some operations on undefined values are then allowed,
248 others fail.
249
250 The closest to regular Python behavior is the `StrictUndefined` which
251 disallows all operations beside testing if it's an undefined object.
252
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253 .. autoclass:: jinja2.runtime.Undefined()
254
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255 .. attribute:: _undefined_hint
256
257 Either `None` or an unicode string with the error message for
258 the undefined object.
259
260 .. attribute:: _undefined_obj
261
262 Either `None` or the owner object that caused the undefined object
263 to be created (for example because an attribute does not exist).
264
265 .. attribute:: _undefined_name
266
267 The name for the undefined variable / attribute or just `None`
268 if no such information exists.
269
270 .. attribute:: _undefined_exception
271
272 The exception that the undefined object wants to raise. This
273 is usually one of :exc:`UndefinedError` or :exc:`SecurityError`.
274
275 .. method:: _fail_with_undefined_error(\*args, \**kwargs)
276
277 When called with any arguments this method raises
278 :attr:`_undefined_exception` with an error message generated
279 from the undefined hints stored on the undefined object.
280
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281 .. autoclass:: jinja2.runtime.DebugUndefined()
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282
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283 .. autoclass:: jinja2.runtime.StrictUndefined()
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284
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285 Undefined objects are created by calling :attr:`undefined`.
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286
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287 .. admonition:: Implementation
288
289 :class:`Undefined` objects are implemented by overriding the special
290 `__underscore__` methods. For example the default :class:`Undefined`
291 class implements `__unicode__` in a way that it returns an empty
292 string, however `__int__` and others still fail with an exception. To
293 allow conversion to int by returning ``0`` you can implement your own::
294
295 class NullUndefined(Undefined):
296 def __int__(self):
297 return 0
298 def __float__(self):
299 return 0.0
300
301 To disallow a method, just override it and raise
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302 :attr:`~Undefined._undefined_exception`. Because this is a very common
303 idom in undefined objects there is the helper method
304 :meth:`~Undefined._fail_with_undefined_error` that does the error raising
305 automatically. Here a class that works like the regular :class:`Undefined`
306 but chokes on iteration::
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307
308 class NonIterableUndefined(Undefined):
309 __iter__ = Undefined._fail_with_undefined_error
310
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312 The Context
313 -----------
314
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315 .. autoclass:: jinja2.runtime.Context()
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316 :members: resolve, get_exported, get_all
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317
318 .. attribute:: parent
319
320 A dict of read only, global variables the template looks up. These
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321 can either come from another :class:`Context`, from the
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322 :attr:`Environment.globals` or :attr:`Template.globals` or points
323 to a dict created by combining the globals with the variables
324 passed to the render function. It must not be altered.
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325
326 .. attribute:: vars
327
328 The template local variables. This list contains environment and
329 context functions from the :attr:`parent` scope as well as local
330 modifications and exported variables from the template. The template
331 will modify this dict during template evaluation but filters and
332 context functions are not allowed to modify it.
333
334 .. attribute:: environment
335
336 The environment that loaded the template.
337
338 .. attribute:: exported_vars
339
340 This set contains all the names the template exports. The values for
341 the names are in the :attr:`vars` dict. In order to get a copy of the
342 exported variables as dict, :meth:`get_exported` can be used.
343
344 .. attribute:: name
345
346 The load name of the template owning this context.
347
348 .. attribute:: blocks
349
350 A dict with the current mapping of blocks in the template. The keys
351 in this dict are the names of the blocks, and the values a list of
352 blocks registered. The last item in each list is the current active
353 block (latest in the inheritance chain).
354
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355 .. automethod:: jinja2.runtime.Context.call(callable, \*args, \**kwargs)
356
357
358 .. admonition:: Implementation
359
360 Context is immutable for the same reason Python's frame locals are
361 immutable inside functions. Both Jinja2 and Python are not using the
362 context / frame locals as data storage for variables but only as primary
363 data source.
364
365 When a template accesses a variable the template does not define, Jinja2
366 looks up the variable in the context, after that the variable is treated
367 as if it was defined in the template.
368
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369
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370 .. _loaders:
371
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372 Loaders
373 -------
374
375 Loaders are responsible for loading templates from a resource such as the
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376 file system. The environment will keep the compiled modules in memory like
377 Python's `sys.modules`. Unlike `sys.modules` however this cache is limited in
378 size by default and templates are automatically reloaded.
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379 All loaders are subclasses of :class:`BaseLoader`. If you want to create your
380 own loader, subclass :class:`BaseLoader` and override `get_source`.
381
382 .. autoclass:: jinja2.loaders.BaseLoader
383 :members: get_source, load
384
385 Here a list of the builtin loaders Jinja2 provides:
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386
387 .. autoclass:: jinja2.loaders.FileSystemLoader
388
389 .. autoclass:: jinja2.loaders.PackageLoader
390
391 .. autoclass:: jinja2.loaders.DictLoader
392
393 .. autoclass:: jinja2.loaders.FunctionLoader
394
395 .. autoclass:: jinja2.loaders.PrefixLoader
396
397 .. autoclass:: jinja2.loaders.ChoiceLoader
398
399
400 Utilities
401 ---------
402
403 These helper functions and classes are useful if you add custom filters or
404 functions to a Jinja2 environment.
405
406 .. autofunction:: jinja2.filters.environmentfilter
407
408 .. autofunction:: jinja2.filters.contextfilter
409
410 .. autofunction:: jinja2.utils.environmentfunction
411
412 .. autofunction:: jinja2.utils.contextfunction
413
414 .. function:: escape(s)
415
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416 Convert the characters ``&``, ``<``, ``>``, ``'``, and ``"`` in string `s`
417 to HTML-safe sequences. Use this if you need to display text that might
418 contain such characters in HTML. This function will not escaped objects
419 that do have an HTML representation such as already escaped data.
420
421 The return value is a :class:`Markup` string.
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423 .. autofunction:: jinja2.utils.clear_caches
424
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425 .. autofunction:: jinja2.utils.is_undefined
426
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427 .. autoclass:: jinja2.utils.Markup([string])
428 :members: escape, unescape, striptags
429
430 .. admonition:: Note
431
432 The Jinja2 :class:`Markup` class is compatible with at least Pylons and
433 Genshi. It's expected that more template engines and framework will pick
434 up the `__html__` concept soon.
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435
436
437 Exceptions
438 ----------
439
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440 .. autoexception:: jinja2.exceptions.TemplateError
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442 .. autoexception:: jinja2.exceptions.UndefinedError
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444 .. autoexception:: jinja2.exceptions.TemplateNotFound
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446 .. autoexception:: jinja2.exceptions.TemplateSyntaxError
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447
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448 .. attribute:: message
449
450 The error message as utf-8 bytestring.
451
452 .. attribute:: lineno
453
454 The line number where the error occurred
455
456 .. attribute:: name
457
458 The load name for the template as unicode string.
459
460 .. attribute:: filename
461
462 The filename that loaded the template as bytestring in the encoding
463 of the file system (most likely utf-8 or mbcs on Windows systems).
464
465 The reason why the filename and error message are bytestrings and not
466 unicode strings is that Python 2.x is not using unicode for exceptions
467 and tracebacks as well as the compiler. This will change with Python 3.
468
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469 .. autoexception:: jinja2.exceptions.TemplateAssertionError
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470
471
472 .. _writing-filters:
473
474 Custom Filters
475 --------------
476
477 Custom filters are just regular Python functions that take the left side of
478 the filter as first argument and the the arguments passed to the filter as
479 extra arguments or keyword arguments.
480
481 For example in the filter ``{{ 42|myfilter(23) }}`` the function would be
482 called with ``myfilter(42, 23)``. Here for example a simple filter that can
483 be applied to datetime objects to format them::
484
485 def datetimeformat(value, format='%H:%M / %d-%m-%Y'):
486 return value.strftime(format)
487
488 You can register it on the template environment by updating the
489 :attr:`~Environment.filters` dict on the environment::
490
491 environment.filters['datetimeformat'] = datetimeformat
492
493 Inside the template it can then be used as follows:
494
495 .. sourcecode:: jinja
496
497 written on: {{ article.pub_date|datetimeformat }}
498 publication date: {{ article.pub_date|datetimeformat('%d-%m-%Y') }}
499
500 Filters can also be passed the current template context or environment. This
501 is useful if a filters wants to return an undefined value or check the current
502 :attr:`~Environment.autoescape` setting. For this purpose two decorators
503 exist: :func:`environmentfilter` and :func:`contextfilter`.
504
505 Here a small example filter that breaks a text into HTML line breaks and
506 paragraphs and marks the return value as safe HTML string if autoescaping is
507 enabled::
508
509 import re
510 from jinja2 import environmentfilter, Markup, escape
511
512 _paragraph_re = re.compile(r'(?:\r\n|\r|\n){2,}')
513
514 @environmentfilter
515 def nl2br(environment, value):
516 result = u'\n\n'.join(u'<p>%s</p>' % p.replace('\n', '<br>\n')
517 for p in _paragraph_re.split(escape(value)))
518 if environment.autoescape:
519 result = Markup(result)
520 return result
521
522 Context filters work the same just that the first argument is the current
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523 active :class:`Context` rather then the environment.
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524
525
526 .. _writing-tests:
527
528 Custom Tests
529 ------------
530
531 Tests work like filters just that there is no way for a filter to get access
532 to the environment or context and that they can't be chained. The return
533 value of a filter should be `True` or `False`. The purpose of a filter is to
534 give the template designers the possibility to perform type and conformability
535 checks.
536
537 Here a simple filter that checks if a variable is a prime number::
538
539 import math
540
541 def is_prime(n):
542 if n == 2:
543 return True
544 for i in xrange(2, int(math.ceil(math.sqrt(n))) + 1):
545 if n % i == 0:
546 return False
547 return True
548
549
550 You can register it on the template environment by updating the
551 :attr:`~Environment.tests` dict on the environment::
552
553 environment.tests['prime'] = is_prime
554
555 A template designer can then use the test like this:
556
557 .. sourcecode:: jinja
558
559 {% if 42 is prime %}
560 42 is a prime number
561 {% else %}
562 42 is not a prime number
563 {% endif %}
564
565
566 .. _global-namespace:
567
568 The Global Namespace
569 --------------------
570
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571 Variables stored in the :attr:`Environment.globals` dict are special as they
572 are available for imported templates too, even if they are imported without
573 context. This is the place where you can put variables and functions
574 that should be available all the time. Additionally :attr:`Template.globals`
575 exist that are variables available to a specific template that are available
576 to all :meth:`~Template.render` calls.
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577
578
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579 .. _low-level-api:
580
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581 Low Level API
582 -------------
583
584 The low level API exposes functionality that can be useful to understand some
585 implementation details, debugging purposes or advanced :ref:`extension
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586 <jinja-extensions>` techniques. Unless you know exactly what you are doing we
587 don't recommend using any of those.
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588
589 .. automethod:: Environment.lex
590
591 .. automethod:: Environment.parse
592
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593 .. automethod:: Environment.preprocess
594
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595 .. automethod:: Template.new_context
596
597 .. method:: Template.root_render_func(context)
598
599 This is the low level render function. It's passed a :class:`Context`
600 that has to be created by :meth:`new_context` of the same template or
601 a compatible template. This render function is generated by the
602 compiler from the template code and returns a generator that yields
603 unicode strings.
604
605 If an exception in the template code happens the template engine will
606 not rewrite the exception but pass through the original one. As a
607 matter of fact this function should only be called from within a
608 :meth:`render` / :meth:`generate` / :meth:`stream` call.
609
610 .. attribute:: Template.blocks
611
612 A dict of block render functions. Each of these functions works exactly
613 like the :meth:`root_render_func` with the same limitations.
614
615 .. attribute:: Template.is_up_to_date
616
617 This attribute is `False` if there is a newer version of the template
618 available, otherwise `True`.
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619
620 .. admonition:: Note
621
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622 The low-level API is fragile. Future Jinja2 versions will try not to
623 change it in a backwards incompatible way but modifications in the Jinja2
624 core may shine through. For example if Jinja2 introduces a new AST node
625 in later versions that may be returned by :meth:`~Environment.parse`.
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