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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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51
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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 .. automethod:: jinja2.runtime.Context.call(callable, \*args, \**kwargs)
357
358
359 .. admonition:: Implementation
360
361 Context is immutable for the same reason Python's frame locals are
362 immutable inside functions. Both Jinja2 and Python are not using the
363 context / frame locals as data storage for variables but only as primary
364 data source.
365
366 When a template accesses a variable the template does not define, Jinja2
367 looks up the variable in the context, after that the variable is treated
368 as if it was defined in the template.
369
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370
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371 .. _loaders:
372
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373 Loaders
374 -------
375
376 Loaders are responsible for loading templates from a resource such as the
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377 file system. The environment will keep the compiled modules in memory like
378 Python's `sys.modules`. Unlike `sys.modules` however this cache is limited in
379 size by default and templates are automatically reloaded.
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380 All loaders are subclasses of :class:`BaseLoader`. If you want to create your
381 own loader, subclass :class:`BaseLoader` and override `get_source`.
382
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383 .. autoclass:: jinja2.BaseLoader
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384 :members: get_source, load
385
386 Here a list of the builtin loaders Jinja2 provides:
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387
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388 .. autoclass:: jinja2.FileSystemLoader
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389
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390 .. autoclass:: jinja2.PackageLoader
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391
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392 .. autoclass:: jinja2.DictLoader
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393
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394 .. autoclass:: jinja2.FunctionLoader
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395
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396 .. autoclass:: jinja2.PrefixLoader
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397
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398 .. autoclass:: jinja2.ChoiceLoader
399
400
401 .. _bytecode-cache:
402
403 Bytecode Cache
404 --------------
405
406 Jinja 2.1 and higher support external bytecode caching. Bytecode caches make
407 it possible to store the generated bytecode on the file system or a different
408 location to avoid parsing the templates on first use.
409
410 This is especially useful if you have a web application that is initialized on
411 the first request and Jinja compiles many templates at once which slows down
412 the application.
413
414 To use a bytecode cache, instanciate it and pass it to the :class:`Environment`.
415
416 .. autoclass:: jinja2.BytecodeCache
417 :members: load_bytecode, dump_bytecode, clear
418
419 .. autoclass:: jinja2.bccache.Bucket
420 :members: write_bytecode, load_bytecode, bytecode_from_string,
421 bytecode_to_string, reset
422
423 .. attribute:: environment
424
425 The :class:`Environment` that created the bucket.
426
427 .. attribute:: key
428
429 The unique cache key for this bucket
430
431 .. attribute:: code
432
433 The bytecode if it's loaded, otherwise `None`.
434
435
436 Builtin bytecode caches:
437
438 .. autoclass:: jinja2.FileSystemBytecodeCache
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439
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440 .. autoclass:: jinja2.MemcachedBytecodeCache
441
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442
443 Utilities
444 ---------
445
446 These helper functions and classes are useful if you add custom filters or
447 functions to a Jinja2 environment.
448
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449 .. autofunction:: jinja2.environmentfilter
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450
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451 .. autofunction:: jinja2.contextfilter
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452
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453 .. autofunction:: jinja2.environmentfunction
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454
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455 .. autofunction:: jinja2.contextfunction
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456
457 .. function:: escape(s)
458
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459 Convert the characters ``&``, ``<``, ``>``, ``'``, and ``"`` in string `s`
460 to HTML-safe sequences. Use this if you need to display text that might
461 contain such characters in HTML. This function will not escaped objects
462 that do have an HTML representation such as already escaped data.
463
464 The return value is a :class:`Markup` string.
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465
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466 .. autofunction:: jinja2.clear_caches
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467
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468 .. autofunction:: jinja2.is_undefined
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469
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470 .. autoclass:: jinja2.Markup([string])
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471 :members: escape, unescape, striptags
472
473 .. admonition:: Note
474
475 The Jinja2 :class:`Markup` class is compatible with at least Pylons and
476 Genshi. It's expected that more template engines and framework will pick
477 up the `__html__` concept soon.
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478
479
480 Exceptions
481 ----------
482
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483 .. autoexception:: jinja2.TemplateError
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484
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485 .. autoexception:: jinja2.UndefinedError
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486
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487 .. autoexception:: jinja2.TemplateNotFound
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488
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489 .. autoexception:: jinja2.TemplatesNotFound
490
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491 .. autoexception:: jinja2.TemplateSyntaxError
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492
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493 .. attribute:: message
494
495 The error message as utf-8 bytestring.
496
497 .. attribute:: lineno
498
499 The line number where the error occurred
500
501 .. attribute:: name
502
503 The load name for the template as unicode string.
504
505 .. attribute:: filename
506
507 The filename that loaded the template as bytestring in the encoding
508 of the file system (most likely utf-8 or mbcs on Windows systems).
509
510 The reason why the filename and error message are bytestrings and not
511 unicode strings is that Python 2.x is not using unicode for exceptions
512 and tracebacks as well as the compiler. This will change with Python 3.
513
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514 .. autoexception:: jinja2.TemplateAssertionError
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515
516
517 .. _writing-filters:
518
519 Custom Filters
520 --------------
521
522 Custom filters are just regular Python functions that take the left side of
523 the filter as first argument and the the arguments passed to the filter as
524 extra arguments or keyword arguments.
525
526 For example in the filter ``{{ 42|myfilter(23) }}`` the function would be
527 called with ``myfilter(42, 23)``. Here for example a simple filter that can
528 be applied to datetime objects to format them::
529
530 def datetimeformat(value, format='%H:%M / %d-%m-%Y'):
531 return value.strftime(format)
532
533 You can register it on the template environment by updating the
534 :attr:`~Environment.filters` dict on the environment::
535
536 environment.filters['datetimeformat'] = datetimeformat
537
538 Inside the template it can then be used as follows:
539
540 .. sourcecode:: jinja
541
542 written on: {{ article.pub_date|datetimeformat }}
543 publication date: {{ article.pub_date|datetimeformat('%d-%m-%Y') }}
544
545 Filters can also be passed the current template context or environment. This
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546 is useful if a filter wants to return an undefined value or check the current
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547 :attr:`~Environment.autoescape` setting. For this purpose two decorators
548 exist: :func:`environmentfilter` and :func:`contextfilter`.
549
550 Here a small example filter that breaks a text into HTML line breaks and
551 paragraphs and marks the return value as safe HTML string if autoescaping is
552 enabled::
553
554 import re
555 from jinja2 import environmentfilter, Markup, escape
556
557 _paragraph_re = re.compile(r'(?:\r\n|\r|\n){2,}')
558
559 @environmentfilter
560 def nl2br(environment, value):
561 result = u'\n\n'.join(u'<p>%s</p>' % p.replace('\n', '<br>\n')
562 for p in _paragraph_re.split(escape(value)))
563 if environment.autoescape:
564 result = Markup(result)
565 return result
566
567 Context filters work the same just that the first argument is the current
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568 active :class:`Context` rather then the environment.
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569
570
571 .. _writing-tests:
572
573 Custom Tests
574 ------------
575
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576 Tests work like filters just that there is no way for a test to get access
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577 to the environment or context and that they can't be chained. The return
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578 value of a test should be `True` or `False`. The purpose of a test is to
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579 give the template designers the possibility to perform type and conformability
580 checks.
581
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582 Here a simple test that checks if a variable is a prime number::
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583
584 import math
585
586 def is_prime(n):
587 if n == 2:
588 return True
589 for i in xrange(2, int(math.ceil(math.sqrt(n))) + 1):
590 if n % i == 0:
591 return False
592 return True
593
594
595 You can register it on the template environment by updating the
596 :attr:`~Environment.tests` dict on the environment::
597
598 environment.tests['prime'] = is_prime
599
600 A template designer can then use the test like this:
601
602 .. sourcecode:: jinja
603
604 {% if 42 is prime %}
605 42 is a prime number
606 {% else %}
607 42 is not a prime number
608 {% endif %}
609
610
611 .. _global-namespace:
612
613 The Global Namespace
614 --------------------
615
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616 Variables stored in the :attr:`Environment.globals` dict are special as they
617 are available for imported templates too, even if they are imported without
618 context. This is the place where you can put variables and functions
619 that should be available all the time. Additionally :attr:`Template.globals`
620 exist that are variables available to a specific template that are available
621 to all :meth:`~Template.render` calls.
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622
623
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624 .. _low-level-api:
625
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626 Low Level API
627 -------------
628
629 The low level API exposes functionality that can be useful to understand some
630 implementation details, debugging purposes or advanced :ref:`extension
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631 <jinja-extensions>` techniques. Unless you know exactly what you are doing we
632 don't recommend using any of those.
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633
634 .. automethod:: Environment.lex
635
636 .. automethod:: Environment.parse
637
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638 .. automethod:: Environment.preprocess
639
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640 .. automethod:: Template.new_context
641
642 .. method:: Template.root_render_func(context)
643
644 This is the low level render function. It's passed a :class:`Context`
645 that has to be created by :meth:`new_context` of the same template or
646 a compatible template. This render function is generated by the
647 compiler from the template code and returns a generator that yields
648 unicode strings.
649
650 If an exception in the template code happens the template engine will
651 not rewrite the exception but pass through the original one. As a
652 matter of fact this function should only be called from within a
653 :meth:`render` / :meth:`generate` / :meth:`stream` call.
654
655 .. attribute:: Template.blocks
656
657 A dict of block render functions. Each of these functions works exactly
658 like the :meth:`root_render_func` with the same limitations.
659
660 .. attribute:: Template.is_up_to_date
661
662 This attribute is `False` if there is a newer version of the template
663 available, otherwise `True`.
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664
665 .. admonition:: Note
666
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667 The low-level API is fragile. Future Jinja2 versions will try not to
668 change it in a backwards incompatible way but modifications in the Jinja2
669 core may shine through. For example if Jinja2 introduces a new AST node
670 in later versions that may be returned by :meth:`~Environment.parse`.
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671
672 The Meta API
673 ------------
674
675 .. versionadded:: 2.2
676
677 The meta API returns some information about abstract syntax trees that
678 could help applications to implement more advanced template concepts. All
679 the functions of the meta API operate on an abstract syntax tree as
680 returned by the :meth:`Environment.parse` method.
681
682 .. autofunction:: jinja2.meta.find_undeclared_variables
683
684 .. autofunction:: jinja2.meta.find_referenced_templates
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