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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.
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17 Even if you are creating templates from strings 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
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55 Jinja2 is using Unicode internally which means that you have to pass Unicode
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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
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64 used. With Python 2.6 it is possible to make `unicode` the default on a per
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65 module level and with Python 3 it will be the default.
66
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67 To explicitly use a Unicode string you have to prefix the string literal
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68 with a `u`: ``u'Hänsel und Gretel sagen Hallo'``. That way Python will
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69 store the string as Unicode by decoding the string with the character
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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
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74 second line of the Python module using the Unicode literal::
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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
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83 It is not possible to use Jinja2 to process non-Unicode data. The reason
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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
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87 Unicode string.
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88
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89 For more details about Unicode in Python have a look at the excellent
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90 `Unicode documentation`_.
91
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92 Another important thing is how Jinja2 is handling string literals in
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93 templates. A naive implementation would be using Unicode strings for
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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
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96 `datetime.strftime` does not accept Unicode arguments. To not break it
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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])
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118 :members: from_string, get_template, select_template,
119 get_or_select_template, join_path, extend, compile_expression
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120
121 .. attribute:: shared
122
123 If a template was created by using the :class:`Template` constructor
124 an environment is created automatically. These environments are
125 created as shared environments which means that multiple templates
126 may have the same anonymous environment. For all shared environments
127 this attribute is `True`, else `False`.
128
129 .. attribute:: sandboxed
130
131 If the environment is sandboxed this attribute is `True`. For the
132 sandbox mode have a look at the documentation for the
133 :class:`~jinja2.sandbox.SandboxedEnvironment`.
134
135 .. attribute:: filters
136
137 A dict of filters for this environment. As long as no template was
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138 loaded it's safe to add new filters or remove old. For custom filters
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139 see :ref:`writing-filters`. For valid filter names have a look at
140 :ref:`identifier-naming`.
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141
142 .. attribute:: tests
143
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144 A dict of test functions for this environment. As long as no
145 template was loaded it's safe to modify this dict. For custom tests
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146 see :ref:`writing-tests`. For valid test names have a look at
147 :ref:`identifier-naming`.
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148
149 .. attribute:: globals
150
151 A dict of global variables. These variables are always available
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152 in a template. As long as no template was loaded it's safe
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153 to modify this dict. For more details see :ref:`global-namespace`.
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154 For valid object names have a look at :ref:`identifier-naming`.
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155
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156 .. automethod:: overlay([options])
157
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158 .. method:: undefined([hint, obj, name, exc])
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159
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160 Creates a new :class:`Undefined` object for `name`. This is useful
161 for filters or functions that may return undefined objects for
162 some operations. All parameters except of `hint` should be provided
163 as keyword parameters for better readability. The `hint` is used as
164 error message for the exception if provided, otherwise the error
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165 message will be generated from `obj` and `name` automatically. The exception
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166 provided as `exc` is raised if something with the generated undefined
167 object is done that the undefined object does not allow. The default
168 exception is :exc:`UndefinedError`. If a `hint` is provided the
169 `name` may be ommited.
170
171 The most common way to create an undefined object is by providing
172 a name only::
173
174 return environment.undefined(name='some_name')
175
176 This means that the name `some_name` is not defined. If the name
177 was from an attribute of an object it makes sense to tell the
178 undefined object the holder object to improve the error message::
179
180 if not hasattr(obj, 'attr'):
181 return environment.undefined(obj=obj, name='attr')
182
183 For a more complex example you can provide a hint. For example
184 the :func:`first` filter creates an undefined object that way::
185
186 return environment.undefined('no first item, sequence was empty')
187
188 If it the `name` or `obj` is known (for example because an attribute
189 was accessed) it shold be passed to the undefined object, even if
190 a custom `hint` is provided. This gives undefined objects the
191 possibility to enhance the error message.
192
193 .. autoclass:: Template
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194 :members: module, make_module
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195
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196 .. attribute:: globals
197
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198 The dict with the globals of that template. It's unsafe to modify
199 this dict as it may be shared with other templates or the environment
200 that loaded the template.
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201
202 .. attribute:: name
203
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204 The loading name of the template. If the template was loaded from a
205 string this is `None`.
206
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207 .. attribute:: filename
208
209 The filename of the template on the file system if it was loaded from
210 there. Otherwise this is `None`.
211
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212 .. automethod:: render([context])
213
214 .. automethod:: generate([context])
215
216 .. automethod:: stream([context])
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217
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218
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219 .. autoclass:: jinja2.environment.TemplateStream()
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220 :members: disable_buffering, enable_buffering, dump
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221
222
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223 .. _identifier-naming:
224
225 Notes on Identifiers
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226 --------------------
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227
228 Jinja2 uses the regular Python 2.x naming rules. Valid identifiers have to
229 match ``[a-zA-Z_][a-zA-Z0-9_]*``. As a matter of fact non ASCII characters
230 are currently not allowed. This limitation will probably go away as soon as
231 unicode identifiers are fully specified for Python 3.
232
233 Filters and tests are looked up in separate namespaces and have slightly
234 modified identifier syntax. Filters and tests may contain dots to group
235 filters and tests by topic. For example it's perfectly valid to add a
236 function into the filter dict and call it `to.unicode`. The regular
237 expression for filter and test identifiers is
238 ``[a-zA-Z_][a-zA-Z0-9_]*(\.[a-zA-Z_][a-zA-Z0-9_]*)*```.
239
240
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241 Undefined Types
242 ---------------
243
244 These classes can be used as undefined types. The :class:`Environment`
245 constructor takes an `undefined` parameter that can be one of those classes
246 or a custom subclass of :class:`Undefined`. Whenever the template engine is
247 unable to look up a name or access an attribute one of those objects is
248 created and returned. Some operations on undefined values are then allowed,
249 others fail.
250
251 The closest to regular Python behavior is the `StrictUndefined` which
252 disallows all operations beside testing if it's an undefined object.
253
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254 .. autoclass:: jinja2.Undefined()
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255
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256 .. attribute:: _undefined_hint
257
258 Either `None` or an unicode string with the error message for
259 the undefined object.
260
261 .. attribute:: _undefined_obj
262
263 Either `None` or the owner object that caused the undefined object
264 to be created (for example because an attribute does not exist).
265
266 .. attribute:: _undefined_name
267
268 The name for the undefined variable / attribute or just `None`
269 if no such information exists.
270
271 .. attribute:: _undefined_exception
272
273 The exception that the undefined object wants to raise. This
274 is usually one of :exc:`UndefinedError` or :exc:`SecurityError`.
275
276 .. method:: _fail_with_undefined_error(\*args, \**kwargs)
277
278 When called with any arguments this method raises
279 :attr:`_undefined_exception` with an error message generated
280 from the undefined hints stored on the undefined object.
281
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282 .. autoclass:: jinja2.DebugUndefined()
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283
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284 .. autoclass:: jinja2.StrictUndefined()
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285
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286 Undefined objects are created by calling :attr:`undefined`.
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287
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288 .. admonition:: Implementation
289
290 :class:`Undefined` objects are implemented by overriding the special
291 `__underscore__` methods. For example the default :class:`Undefined`
292 class implements `__unicode__` in a way that it returns an empty
293 string, however `__int__` and others still fail with an exception. To
294 allow conversion to int by returning ``0`` you can implement your own::
295
296 class NullUndefined(Undefined):
297 def __int__(self):
298 return 0
299 def __float__(self):
300 return 0.0
301
302 To disallow a method, just override it and raise
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303 :attr:`~Undefined._undefined_exception`. Because this is a very common
304 idom in undefined objects there is the helper method
305 :meth:`~Undefined._fail_with_undefined_error` that does the error raising
306 automatically. Here a class that works like the regular :class:`Undefined`
307 but chokes on iteration::
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308
309 class NonIterableUndefined(Undefined):
310 __iter__ = Undefined._fail_with_undefined_error
311
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312
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313 The Context
314 -----------
315
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316 .. autoclass:: jinja2.runtime.Context()
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317 :members: resolve, get_exported, get_all
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318
319 .. attribute:: parent
320
321 A dict of read only, global variables the template looks up. These
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322 can either come from another :class:`Context`, from the
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323 :attr:`Environment.globals` or :attr:`Template.globals` or points
324 to a dict created by combining the globals with the variables
325 passed to the render function. It must not be altered.
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326
327 .. attribute:: vars
328
329 The template local variables. This list contains environment and
330 context functions from the :attr:`parent` scope as well as local
331 modifications and exported variables from the template. The template
332 will modify this dict during template evaluation but filters and
333 context functions are not allowed to modify it.
334
335 .. attribute:: environment
336
337 The environment that loaded the template.
338
339 .. attribute:: exported_vars
340
341 This set contains all the names the template exports. The values for
342 the names are in the :attr:`vars` dict. In order to get a copy of the
343 exported variables as dict, :meth:`get_exported` can be used.
344
345 .. attribute:: name
346
347 The load name of the template owning this context.
348
349 .. attribute:: blocks
350
351 A dict with the current mapping of blocks in the template. The keys
352 in this dict are the names of the blocks, and the values a list of
353 blocks registered. The last item in each list is the current active
354 block (latest in the inheritance chain).
355
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356 .. attribute:: eval_ctx
357
358 The current :ref:`eval-context`.
359
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360 .. automethod:: jinja2.runtime.Context.call(callable, \*args, \**kwargs)
361
362
363 .. admonition:: Implementation
364
365 Context is immutable for the same reason Python's frame locals are
366 immutable inside functions. Both Jinja2 and Python are not using the
367 context / frame locals as data storage for variables but only as primary
368 data source.
369
370 When a template accesses a variable the template does not define, Jinja2
371 looks up the variable in the context, after that the variable is treated
372 as if it was defined in the template.
373
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374
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375 .. _loaders:
376
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377 Loaders
378 -------
379
380 Loaders are responsible for loading templates from a resource such as the
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381 file system. The environment will keep the compiled modules in memory like
382 Python's `sys.modules`. Unlike `sys.modules` however this cache is limited in
383 size by default and templates are automatically reloaded.
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384 All loaders are subclasses of :class:`BaseLoader`. If you want to create your
385 own loader, subclass :class:`BaseLoader` and override `get_source`.
386
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387 .. autoclass:: jinja2.BaseLoader
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388 :members: get_source, load
389
390 Here a list of the builtin loaders Jinja2 provides:
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391
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392 .. autoclass:: jinja2.FileSystemLoader
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393
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394 .. autoclass:: jinja2.PackageLoader
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395
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396 .. autoclass:: jinja2.DictLoader
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397
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398 .. autoclass:: jinja2.FunctionLoader
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399
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400 .. autoclass:: jinja2.PrefixLoader
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401
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402 .. autoclass:: jinja2.ChoiceLoader
403
404
405 .. _bytecode-cache:
406
407 Bytecode Cache
408 --------------
409
410 Jinja 2.1 and higher support external bytecode caching. Bytecode caches make
411 it possible to store the generated bytecode on the file system or a different
412 location to avoid parsing the templates on first use.
413
414 This is especially useful if you have a web application that is initialized on
415 the first request and Jinja compiles many templates at once which slows down
416 the application.
417
418 To use a bytecode cache, instanciate it and pass it to the :class:`Environment`.
419
420 .. autoclass:: jinja2.BytecodeCache
421 :members: load_bytecode, dump_bytecode, clear
422
423 .. autoclass:: jinja2.bccache.Bucket
424 :members: write_bytecode, load_bytecode, bytecode_from_string,
425 bytecode_to_string, reset
426
427 .. attribute:: environment
428
429 The :class:`Environment` that created the bucket.
430
431 .. attribute:: key
432
433 The unique cache key for this bucket
434
435 .. attribute:: code
436
437 The bytecode if it's loaded, otherwise `None`.
438
439
440 Builtin bytecode caches:
441
442 .. autoclass:: jinja2.FileSystemBytecodeCache
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443
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444 .. autoclass:: jinja2.MemcachedBytecodeCache
445
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446
447 Utilities
448 ---------
449
450 These helper functions and classes are useful if you add custom filters or
451 functions to a Jinja2 environment.
452
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453 .. autofunction:: jinja2.environmentfilter
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454
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455 .. autofunction:: jinja2.contextfilter
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456
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457 .. autofunction:: jinja2.evalcontextfilter
458
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459 .. autofunction:: jinja2.environmentfunction
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460
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461 .. autofunction:: jinja2.contextfunction
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462
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463 .. autofunction:: jinja2.evalcontextfunction
464
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465 .. function:: escape(s)
466
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467 Convert the characters ``&``, ``<``, ``>``, ``'``, and ``"`` in string `s`
468 to HTML-safe sequences. Use this if you need to display text that might
469 contain such characters in HTML. This function will not escaped objects
470 that do have an HTML representation such as already escaped data.
471
472 The return value is a :class:`Markup` string.
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473
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474 .. autofunction:: jinja2.clear_caches
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475
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476 .. autofunction:: jinja2.is_undefined
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477
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478 .. autoclass:: jinja2.Markup([string])
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479 :members: escape, unescape, striptags
480
481 .. admonition:: Note
482
483 The Jinja2 :class:`Markup` class is compatible with at least Pylons and
484 Genshi. It's expected that more template engines and framework will pick
485 up the `__html__` concept soon.
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486
487
488 Exceptions
489 ----------
490
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491 .. autoexception:: jinja2.TemplateError
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492
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493 .. autoexception:: jinja2.UndefinedError
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494
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495 .. autoexception:: jinja2.TemplateNotFound
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496
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497 .. autoexception:: jinja2.TemplatesNotFound
498
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499 .. autoexception:: jinja2.TemplateSyntaxError
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500
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501 .. attribute:: message
502
503 The error message as utf-8 bytestring.
504
505 .. attribute:: lineno
506
507 The line number where the error occurred
508
509 .. attribute:: name
510
511 The load name for the template as unicode string.
512
513 .. attribute:: filename
514
515 The filename that loaded the template as bytestring in the encoding
516 of the file system (most likely utf-8 or mbcs on Windows systems).
517
518 The reason why the filename and error message are bytestrings and not
519 unicode strings is that Python 2.x is not using unicode for exceptions
520 and tracebacks as well as the compiler. This will change with Python 3.
521
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522 .. autoexception:: jinja2.TemplateAssertionError
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523
524
525 .. _writing-filters:
526
527 Custom Filters
528 --------------
529
530 Custom filters are just regular Python functions that take the left side of
531 the filter as first argument and the the arguments passed to the filter as
532 extra arguments or keyword arguments.
533
534 For example in the filter ``{{ 42|myfilter(23) }}`` the function would be
535 called with ``myfilter(42, 23)``. Here for example a simple filter that can
536 be applied to datetime objects to format them::
537
538 def datetimeformat(value, format='%H:%M / %d-%m-%Y'):
539 return value.strftime(format)
540
541 You can register it on the template environment by updating the
542 :attr:`~Environment.filters` dict on the environment::
543
544 environment.filters['datetimeformat'] = datetimeformat
545
546 Inside the template it can then be used as follows:
547
548 .. sourcecode:: jinja
549
550 written on: {{ article.pub_date|datetimeformat }}
551 publication date: {{ article.pub_date|datetimeformat('%d-%m-%Y') }}
552
553 Filters can also be passed the current template context or environment. This
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554 is useful if a filter wants to return an undefined value or check the current
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555 :attr:`~Environment.autoescape` setting. For this purpose two decorators
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556 exist: :func:`environmentfilter`, :func:`contextfilter` and
557 :func:`evalcontextfilter`.
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558
559 Here a small example filter that breaks a text into HTML line breaks and
560 paragraphs and marks the return value as safe HTML string if autoescaping is
561 enabled::
562
563 import re
564 from jinja2 import environmentfilter, Markup, escape
565
566 _paragraph_re = re.compile(r'(?:\r\n|\r|\n){2,}')
567
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568 @evalcontextfilter
569 def nl2br(eval_ctx, value):
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570 result = u'\n\n'.join(u'<p>%s</p>' % p.replace('\n', '<br>\n')
571 for p in _paragraph_re.split(escape(value)))
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572 if eval_ctx.autoescape:
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573 result = Markup(result)
574 return result
575
576 Context filters work the same just that the first argument is the current
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577 active :class:`Context` rather then the environment.
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578
579
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580 .. _eval-context:
581
582 Evaluation Context
583 ------------------
584
585 The evaluation context (short eval context or eval ctx) is a new object
586 introducted in Jinja 2.4 that makes it possible to activate and deactivate
587 compiled features at runtime.
588
589 Currently it is only used to enable and disable the automatic escaping but
590 can be used for extensions as well.
591
592 In previous Jinja versions filters and functions were marked as
593 environment callables in order to check for the autoescape status from the
594 environment. In new versions it's encouraged to check the setting from the
595 evaluation context instead.
596
597 Previous versions::
598
599 @environmentfilter
600 def filter(env, value):
601 result = do_something(value)
602 if env.autoescape:
603 result = Markup(result)
604 return result
605
606 In new versions you can either use a :func:`contextfilter` and access the
607 evaluation context from the actual context, or use a
608 :func:`evalcontextfilter` which directly passes the evaluation context to
609 the function::
610
611 @contextfilter
612 def filter(context, value):
613 result = do_something(value)
614 if context.eval_ctx.autoescape:
615 result = Markup(result)
616 return result
617
618 @evalcontextfilter
619 def filter(eval_ctx, value):
620 result = do_something(value)
621 if eval_ctx.autoescape:
622 result = Markup(result)
623 return result
624
625 The evaluation context must not be modified at runtime. Modifications
626 must only happen with a :class:`nodes.EvalContextModifier` and
627 :class:`nodes.ScopedEvalContextModifier` from an extension, not on the
628 eval context object itself.
629
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630 .. _writing-tests:
631
632 Custom Tests
633 ------------
634
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635 Tests work like filters just that there is no way for a test to get access
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636 to the environment or context and that they can't be chained. The return
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637 value of a test should be `True` or `False`. The purpose of a test is to
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638 give the template designers the possibility to perform type and conformability
639 checks.
640
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641 Here a simple test that checks if a variable is a prime number::
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642
643 import math
644
645 def is_prime(n):
646 if n == 2:
647 return True
648 for i in xrange(2, int(math.ceil(math.sqrt(n))) + 1):
649 if n % i == 0:
650 return False
651 return True
652
653
654 You can register it on the template environment by updating the
655 :attr:`~Environment.tests` dict on the environment::
656
657 environment.tests['prime'] = is_prime
658
659 A template designer can then use the test like this:
660
661 .. sourcecode:: jinja
662
663 {% if 42 is prime %}
664 42 is a prime number
665 {% else %}
666 42 is not a prime number
667 {% endif %}
668
669
670 .. _global-namespace:
671
672 The Global Namespace
673 --------------------
674
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675 Variables stored in the :attr:`Environment.globals` dict are special as they
676 are available for imported templates too, even if they are imported without
677 context. This is the place where you can put variables and functions
678 that should be available all the time. Additionally :attr:`Template.globals`
679 exist that are variables available to a specific template that are available
680 to all :meth:`~Template.render` calls.
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681
682
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683 .. _low-level-api:
684
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685 Low Level API
686 -------------
687
688 The low level API exposes functionality that can be useful to understand some
689 implementation details, debugging purposes or advanced :ref:`extension
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690 <jinja-extensions>` techniques. Unless you know exactly what you are doing we
691 don't recommend using any of those.
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692
693 .. automethod:: Environment.lex
694
695 .. automethod:: Environment.parse
696
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697 .. automethod:: Environment.preprocess
698
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699 .. automethod:: Template.new_context
700
701 .. method:: Template.root_render_func(context)
702
703 This is the low level render function. It's passed a :class:`Context`
704 that has to be created by :meth:`new_context` of the same template or
705 a compatible template. This render function is generated by the
706 compiler from the template code and returns a generator that yields
707 unicode strings.
708
709 If an exception in the template code happens the template engine will
710 not rewrite the exception but pass through the original one. As a
711 matter of fact this function should only be called from within a
712 :meth:`render` / :meth:`generate` / :meth:`stream` call.
713
714 .. attribute:: Template.blocks
715
716 A dict of block render functions. Each of these functions works exactly
717 like the :meth:`root_render_func` with the same limitations.
718
719 .. attribute:: Template.is_up_to_date
720
721 This attribute is `False` if there is a newer version of the template
722 available, otherwise `True`.
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723
724 .. admonition:: Note
725
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726 The low-level API is fragile. Future Jinja2 versions will try not to
727 change it in a backwards incompatible way but modifications in the Jinja2
728 core may shine through. For example if Jinja2 introduces a new AST node
729 in later versions that may be returned by :meth:`~Environment.parse`.
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730
731 The Meta API
732 ------------
733
734 .. versionadded:: 2.2
735
736 The meta API returns some information about abstract syntax trees that
737 could help applications to implement more advanced template concepts. All
738 the functions of the meta API operate on an abstract syntax tree as
739 returned by the :meth:`Environment.parse` method.
740
741 .. autofunction:: jinja2.meta.find_undeclared_variables
742
743 .. autofunction:: jinja2.meta.find_referenced_templates
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