Jinja2 supports extensions that can add extra filters, tests, globals or even
extend the parser. The main motivation of extensions is to move often used
code into a reusable class like adding support for internationalization.
Adding Extensions
Extensions are added to the Jinja2 environment at creation time. Once the
environment is created additional extensions cannot be added. To add an
extension pass a list of extension classes or import paths to the
extensions parameter of the Environment constructor. The following
example creates a Jinja2 environment with the i18n extension loaded:
jinja_env = Environment(extensions=['jinja2.ext.i18n'])
i18n Extension
Import name: jinja2.ext.i18n
The i18n extension can be used in combination with gettext or babel. If
the i18n extension is enabled Jinja2 provides a trans statement that marks
the wrapped string as translatable and calls gettext.
After enabling, dummy _ function that forwards calls to gettext is added
to the environment globals. An internationalized application then has to
provide a gettext function and optionally an ngettext function into the
namespace, either globally or for each rendering.
Environment Methods
After enabling the extension, the environment provides the following
additional methods:
-
jinja2.Environment.install_gettext_translations(translations, newstyle=False)
Installs a translation globally for that environment. The translations
object provided must implement at least ugettext and ungettext.
The gettext.NullTranslations and gettext.GNUTranslations classes
as well as Babels Translations class are supported.
Changed in version 2.5: newstyle gettext added
-
jinja2.Environment.install_null_translations(newstyle=False)
Install dummy gettext functions. This is useful if you want to prepare
the application for internationalization but don’t want to implement the
full internationalization system yet.
Changed in version 2.5: newstyle gettext added
-
jinja2.Environment.install_gettext_callables(gettext, ngettext, newstyle=False)
Installs the given gettext and ngettext callables into the
environment as globals. They are supposed to behave exactly like the
standard library’s gettext.ugettext() and
gettext.ungettext() functions.
If newstyle is activated, the callables are wrapped to work like
newstyle callables. See Whitespace Trimming for more information.
-
jinja2.Environment.uninstall_gettext_translations()
Uninstall the translations again.
Extract localizable strings from the given template node or source.
For every string found this function yields a (lineno, function,
message) tuple, where:
- lineno is the number of the line on which the string was found,
- function is the name of the gettext function used (if the
string was extracted from embedded Python code), and
- message is the string itself (a unicode object, or a tuple
of unicode objects for functions with multiple string arguments).
If Babel is installed, the babel integration
can be used to extract strings for babel.
For a web application that is available in multiple languages but gives all
the users the same language (for example a multilingual forum software
installed for a French community) may load the translations once and add the
translation methods to the environment at environment generation time:
translations = get_gettext_translations()
env = Environment(extensions=['jinja2.ext.i18n'])
env.install_gettext_translations(translations)
The get_gettext_translations function would return the translator for the
current configuration. (For example by using gettext.find)
The usage of the i18n extension for template designers is covered as part
of the template documentation.
Whitespace Trimming
Linebreaks and surrounding whitespace can be automatically trimmed by enabling
the ext.i18n.trimmed policy.
Newstyle Gettext
Starting with version 2.5 you can use newstyle gettext calls. These are
inspired by trac’s internal gettext functions and are fully supported by
the babel extraction tool. They might not work as expected by other
extraction tools in case you are not using Babel’s.
What’s the big difference between standard and newstyle gettext calls? In
general they are less to type and less error prone. Also if they are used
in an autoescaping environment they better support automatic escaping.
Here are some common differences between old and new calls:
standard gettext:
{{ gettext('Hello World!') }}
{{ gettext('Hello %(name)s!')|format(name='World') }}
{{ ngettext('%(num)d apple', '%(num)d apples', apples|count)|format(
num=apples|count
)}}
newstyle gettext looks like this instead:
{{ gettext('Hello World!') }}
{{ gettext('Hello %(name)s!', name='World') }}
{{ ngettext('%(num)d apple', '%(num)d apples', apples|count) }}
The advantages of newstyle gettext are that you have less to type and that
named placeholders become mandatory. The latter sounds like a
disadvantage but solves a lot of troubles translators are often facing
when they are unable to switch the positions of two placeholder. With
newstyle gettext, all format strings look the same.
Furthermore with newstyle gettext, string formatting is also used if no
placeholders are used which makes all strings behave exactly the same.
Last but not least are newstyle gettext calls able to properly mark
strings for autoescaping which solves lots of escaping related issues many
templates are experiencing over time when using autoescaping.
Expression Statement
Import name: jinja2.ext.do
The “do” aka expression-statement extension adds a simple do tag to the
template engine that works like a variable expression but ignores the
return value.
Loop Controls
Import name: jinja2.ext.loopcontrols
This extension adds support for break and continue in loops. After
enabling, Jinja2 provides those two keywords which work exactly like in
Python.
Autoescape Extension
Import name: jinja2.ext.autoescape
This extension was removed and is now built-in. Enabling the extension
no longer does anything.
Writing Extensions
By writing extensions you can add custom tags to Jinja2. This is a non-trivial
task and usually not needed as the default tags and expressions cover all
common use cases. The i18n extension is a good example of why extensions are
useful. Another one would be fragment caching.
When writing extensions you have to keep in mind that you are working with the
Jinja2 template compiler which does not validate the node tree you are passing
to it. If the AST is malformed you will get all kinds of compiler or runtime
errors that are horrible to debug. Always make sure you are using the nodes
you create correctly. The API documentation below shows which nodes exist and
how to use them.
Example Extension
The following example implements a cache tag for Jinja2 by using the
Werkzeug caching contrib module:
from jinja2 import nodes
from jinja2.ext import Extension
class FragmentCacheExtension(Extension):
# a set of names that trigger the extension.
tags = set(['cache'])
def __init__(self, environment):
super(FragmentCacheExtension, self).__init__(environment)
# add the defaults to the environment
environment.extend(
fragment_cache_prefix='',
fragment_cache=None
)
def parse(self, parser):
# the first token is the token that started the tag. In our case
# we only listen to ``'cache'`` so this will be a name token with
# `cache` as value. We get the line number so that we can give
# that line number to the nodes we create by hand.
lineno = next(parser.stream).lineno
# now we parse a single expression that is used as cache key.
args = [parser.parse_expression()]
# if there is a comma, the user provided a timeout. If not use
# None as second parameter.
if parser.stream.skip_if('comma'):
args.append(parser.parse_expression())
else:
args.append(nodes.Const(None))
# now we parse the body of the cache block up to `endcache` and
# drop the needle (which would always be `endcache` in that case)
body = parser.parse_statements(['name:endcache'], drop_needle=True)
# now return a `CallBlock` node that calls our _cache_support
# helper method on this extension.
return nodes.CallBlock(self.call_method('_cache_support', args),
[], [], body).set_lineno(lineno)
def _cache_support(self, name, timeout, caller):
"""Helper callback."""
key = self.environment.fragment_cache_prefix + name
# try to load the block from the cache
# if there is no fragment in the cache, render it and store
# it in the cache.
rv = self.environment.fragment_cache.get(key)
if rv is not None:
return rv
rv = caller()
self.environment.fragment_cache.add(key, rv, timeout)
return rv
And here is how you use it in an environment:
from jinja2 import Environment
from werkzeug.contrib.cache import SimpleCache
env = Environment(extensions=[FragmentCacheExtension])
env.fragment_cache = SimpleCache()
Inside the template it’s then possible to mark blocks as cacheable. The
following example caches a sidebar for 300 seconds:
{% cache 'sidebar', 300 %}
<div class="sidebar">
...
</div>
{% endcache %}
Extension API
Extensions always have to extend the jinja2.ext.Extension class:
-
class
jinja2.ext.Extension(environment)
Extensions can be used to add extra functionality to the Jinja template
system at the parser level. Custom extensions are bound to an environment
but may not store environment specific data on self. The reason for
this is that an extension can be bound to another environment (for
overlays) by creating a copy and reassigning the environment attribute.
As extensions are created by the environment they cannot accept any
arguments for configuration. One may want to work around that by using
a factory function, but that is not possible as extensions are identified
by their import name. The correct way to configure the extension is
storing the configuration values on the environment. Because this way the
environment ends up acting as central configuration storage the
attributes may clash which is why extensions have to ensure that the names
they choose for configuration are not too generic. prefix for example
is a terrible name, fragment_cache_prefix on the other hand is a good
name as includes the name of the extension (fragment cache).
-
identifier
The identifier of the extension. This is always the true import name
of the extension class and must not be changed.
-
tags
If the extension implements custom tags this is a set of tag names
the extension is listening for.
-
attr(name, lineno=None)
Return an attribute node for the current extension. This is useful
to pass constants on extensions to generated template code.
self.attr('_my_attribute', lineno=lineno)
-
call_method(name, args=None, kwargs=None, dyn_args=None, dyn_kwargs=None, lineno=None)
Call a method of the extension. This is a shortcut for
attr() + jinja2.nodes.Call.
-
filter_stream(stream)
It’s passed a TokenStream that can be used
to filter tokens returned. This method has to return an iterable of
Tokens, but it doesn’t have to return a
TokenStream.
In the ext folder of the Jinja2 source distribution there is a file
called inlinegettext.py which implements a filter that utilizes this
method.
-
parse(parser)
If any of the tags matched this method is called with the
parser as first argument. The token the parser stream is pointing at
is the name token that matched. This method has to return one or a
list of multiple nodes.
-
preprocess(source, name, filename=None)
This method is called before the actual lexing and can be used to
preprocess the source. The filename is optional. The return value
must be the preprocessed source.
Parser API
The parser passed to Extension.parse() provides ways to parse
expressions of different types. The following methods may be used by
extensions:
-
class
jinja2.parser.Parser(environment, source, name=None, filename=None, state=None)
This is the central parsing class Jinja2 uses. It’s passed to
extensions and can be used to parse expressions or statements.
-
filename
The filename of the template the parser processes. This is not
the load name of the template. For the load name see name.
For templates that were not loaded form the file system this is
None.
-
name
The load name of the template.
-
stream
The current TokenStream
-
fail(msg, lineno=None, exc=<class 'jinja2.exceptions.TemplateSyntaxError'>)
Convenience method that raises exc with the message, passed
line number or last line number as well as the current name and
filename.
-
free_identifier(lineno=None)
Return a new free identifier as InternalName.
-
parse_assign_target(with_tuple=True, name_only=False, extra_end_rules=None, with_namespace=False)
Parse an assignment target. As Jinja2 allows assignments to
tuples, this function can parse all allowed assignment targets. Per
default assignments to tuples are parsed, that can be disable however
by setting with_tuple to False. If only assignments to names are
wanted name_only can be set to True. The extra_end_rules
parameter is forwarded to the tuple parsing function. If
with_namespace is enabled, a namespace assignment may be parsed.
-
parse_expression(with_condexpr=True)
Parse an expression. Per default all expressions are parsed, if
the optional with_condexpr parameter is set to False conditional
expressions are not parsed.
-
parse_statements(end_tokens, drop_needle=False)
Parse multiple statements into a list until one of the end tokens
is reached. This is used to parse the body of statements as it also
parses template data if appropriate. The parser checks first if the
current token is a colon and skips it if there is one. Then it checks
for the block end and parses until if one of the end_tokens is
reached. Per default the active token in the stream at the end of
the call is the matched end token. If this is not wanted drop_needle
can be set to True and the end token is removed.
-
parse_tuple(simplified=False, with_condexpr=True, extra_end_rules=None, explicit_parentheses=False)
Works like parse_expression but if multiple expressions are
delimited by a comma a Tuple node is created.
This method could also return a regular expression instead of a tuple
if no commas where found.
The default parsing mode is a full tuple. If simplified is True
only names and literals are parsed. The no_condexpr parameter is
forwarded to parse_expression().
Because tuples do not require delimiters and may end in a bogus comma
an extra hint is needed that marks the end of a tuple. For example
for loops support tuples between for and in. In that case the
extra_end_rules is set to ['name:in'].
explicit_parentheses is true if the parsing was triggered by an
expression in parentheses. This is used to figure out if an empty
tuple is a valid expression or not.
-
class
jinja2.lexer.TokenStream(generator, name, filename)
A token stream is an iterable that yields Tokens. The
parser however does not iterate over it but calls next() to go
one token ahead. The current active token is stored as current.
-
current
The current Token.
-
__next__()
Go one token ahead and return the old one.
Use the built-in next() instead of calling this directly.
-
eos
Are we at the end of the stream?
-
expect(expr)
Expect a given token type and return it. This accepts the same
argument as jinja2.lexer.Token.test().
-
look()
Look at the next token.
-
next_if(expr)
Perform the token test and return the token if it matched.
Otherwise the return value is None.
-
push(token)
Push a token back to the stream.
-
skip(n=1)
Got n tokens ahead.
-
skip_if(expr)
Like next_if() but only returns True or False.
-
class
jinja2.lexer.Token
Token class.
-
lineno
The line number of the token
-
type
The type of the token. This string is interned so you may compare
it with arbitrary strings using the is operator.
-
value
The value of the token.
-
test(expr)
Test a token against a token expression. This can either be a
token type or 'token_type:token_value'. This can only test
against string values and types.
-
test_any(*iterable)
Test against multiple token expressions.
There is also a utility function in the lexer module that can count newline
characters in strings:
-
jinja2.lexer.count_newlines(value)
Count the number of newline characters in the string. This is
useful for extensions that filter a stream.
AST
The AST (Abstract Syntax Tree) is used to represent a template after parsing.
It’s build of nodes that the compiler then converts into executable Python
code objects. Extensions that provide custom statements can return nodes to
execute custom Python code.
The list below describes all nodes that are currently available. The AST may
change between Jinja2 versions but will stay backwards compatible.
For more information have a look at the repr of jinja2.Environment.parse().
-
class
jinja2.nodes.Node
Baseclass for all Jinja2 nodes. There are a number of nodes available
of different types. There are four major types:
All nodes have fields and attributes. Fields may be other nodes, lists,
or arbitrary values. Fields are passed to the constructor as regular
positional arguments, attributes as keyword arguments. Each node has
two attributes: lineno (the line number of the node) and environment.
The environment attribute is set at the end of the parsing process for
all nodes automatically.
-
find(node_type)
Find the first node of a given type. If no such node exists the
return value is None.
-
find_all(node_type)
Find all the nodes of a given type. If the type is a tuple,
the check is performed for any of the tuple items.
-
iter_child_nodes(exclude=None, only=None)
Iterates over all direct child nodes of the node. This iterates
over all fields and yields the values of they are nodes. If the value
of a field is a list all the nodes in that list are returned.
-
iter_fields(exclude=None, only=None)
This method iterates over all fields that are defined and yields
(key, value) tuples. Per default all fields are returned, but
it’s possible to limit that to some fields by providing the only
parameter or to exclude some using the exclude parameter. Both
should be sets or tuples of field names.
-
set_ctx(ctx)
Reset the context of a node and all child nodes. Per default the
parser will all generate nodes that have a ‘load’ context as it’s the
most common one. This method is used in the parser to set assignment
targets and other nodes to a store context.
-
set_environment(environment)
Set the environment for all nodes.
-
set_lineno(lineno, override=False)
Set the line numbers of the node and children.
-
class
jinja2.nodes.Expr
Baseclass for all expressions.
-
as_const(eval_ctx=None)
Return the value of the expression as constant or raise
Impossible if this was not possible.
An EvalContext can be provided, if none is given
a default context is created which requires the nodes to have
an attached environment.
Changed in version 2.4: the eval_ctx parameter was added.
-
can_assign()
Check if it’s possible to assign something to this node.
-
class
jinja2.nodes.BinExpr(left, right)
Baseclass for all binary expressions.
-
class
jinja2.nodes.Add(left, right)
Add the left to the right node.
-
class
jinja2.nodes.And(left, right)
Short circuited AND.
-
class
jinja2.nodes.Div(left, right)
Divides the left by the right node.
-
class
jinja2.nodes.FloorDiv(left, right)
Divides the left by the right node and truncates conver the
result into an integer by truncating.
-
class
jinja2.nodes.Mod(left, right)
Left modulo right.
-
class
jinja2.nodes.Mul(left, right)
Multiplies the left with the right node.
-
class
jinja2.nodes.Or(left, right)
Short circuited OR.
-
class
jinja2.nodes.Pow(left, right)
Left to the power of right.
-
class
jinja2.nodes.Sub(left, right)
Subtract the right from the left node.
-
class
jinja2.nodes.Call(node, args, kwargs, dyn_args, dyn_kwargs)
Calls an expression. args is a list of arguments, kwargs a list
of keyword arguments (list of Keyword nodes), and dyn_args
and dyn_kwargs has to be either None or a node that is used as
node for dynamic positional (*args) or keyword (**kwargs)
arguments.
-
class
jinja2.nodes.Compare(expr, ops)
Compares an expression with some other expressions. ops must be a
list of Operands.
-
class
jinja2.nodes.Concat(nodes)
Concatenates the list of expressions provided after converting them to
unicode.
-
class
jinja2.nodes.CondExpr(test, expr1, expr2)
A conditional expression (inline if expression). ({{
foo if bar else baz }})
-
class
jinja2.nodes.ContextReference
Returns the current template context. It can be used like a
Name node, with a 'load' ctx and will return the
current Context object.
Here an example that assigns the current template name to a
variable named foo:
Assign(Name('foo', ctx='store'),
Getattr(ContextReference(), 'name'))
-
class
jinja2.nodes.EnvironmentAttribute(name)
Loads an attribute from the environment object. This is useful for
extensions that want to call a callback stored on the environment.
-
class
jinja2.nodes.ExtensionAttribute(identifier, name)
Returns the attribute of an extension bound to the environment.
The identifier is the identifier of the Extension.
This node is usually constructed by calling the
attr() method on an extension.
-
class
jinja2.nodes.Filter(node, name, args, kwargs, dyn_args, dyn_kwargs)
This node applies a filter on an expression. name is the name of
the filter, the rest of the fields are the same as for Call.
If the node of a filter is None the contents of the last buffer are
filtered. Buffers are created by macros and filter blocks.
-
class
jinja2.nodes.Getattr(node, attr, ctx)
Get an attribute or item from an expression that is a ascii-only
bytestring and prefer the attribute.
-
class
jinja2.nodes.Getitem(node, arg, ctx)
Get an attribute or item from an expression and prefer the item.
-
class
jinja2.nodes.ImportedName(importname)
If created with an import name the import name is returned on node
access. For example ImportedName('cgi.escape') returns the escape
function from the cgi module on evaluation. Imports are optimized by the
compiler so there is no need to assign them to local variables.
-
class
jinja2.nodes.InternalName(name)
An internal name in the compiler. You cannot create these nodes
yourself but the parser provides a
free_identifier() method that creates
a new identifier for you. This identifier is not available from the
template and is not threated specially by the compiler.
-
class
jinja2.nodes.Literal
Baseclass for literals.
-
class
jinja2.nodes.Const(value)
All constant values. The parser will return this node for simple
constants such as 42 or "foo" but it can be used to store more
complex values such as lists too. Only constants with a safe
representation (objects where eval(repr(x)) == x is true).
-
class
jinja2.nodes.Dict(items)
Any dict literal such as {1: 2, 3: 4}. The items must be a list of
Pair nodes.
-
class
jinja2.nodes.List(items)
Any list literal such as [1, 2, 3]
-
class
jinja2.nodes.TemplateData(data)
A constant template string.
-
class
jinja2.nodes.Tuple(items, ctx)
For loop unpacking and some other things like multiple arguments
for subscripts. Like for Name ctx specifies if the tuple
is used for loading the names or storing.
-
class
jinja2.nodes.MarkSafe(expr)
Mark the wrapped expression as safe (wrap it as Markup).
-
class
jinja2.nodes.MarkSafeIfAutoescape(expr)
Mark the wrapped expression as safe (wrap it as Markup) but
only if autoescaping is active.
-
class
jinja2.nodes.Name(name, ctx)
Looks up a name or stores a value in a name.
The ctx of the node can be one of the following values:
- store: store a value in the name
- load: load that name
- param: like store but if the name was defined as function parameter.
-
class
jinja2.nodes.NSRef(name, attr)
Reference to a namespace value assignment
-
class
jinja2.nodes.Slice(start, stop, step)
Represents a slice object. This must only be used as argument for
Subscript.
-
class
jinja2.nodes.Test(node, name, args, kwargs, dyn_args, dyn_kwargs)
Applies a test on an expression. name is the name of the test, the
rest of the fields are the same as for Call.
-
class
jinja2.nodes.UnaryExpr(node)
Baseclass for all unary expressions.
-
class
jinja2.nodes.Neg(node)
Make the expression negative.
-
class
jinja2.nodes.Not(node)
Negate the expression.
-
class
jinja2.nodes.Pos(node)
Make the expression positive (noop for most expressions)
-
class
jinja2.nodes.Helper
Nodes that exist in a specific context only.
-
class
jinja2.nodes.Keyword(key, value)
A key, value pair for keyword arguments where key is a string.
-
class
jinja2.nodes.Operand(op, expr)
Holds an operator and an expression.
The following operators are available: %, **, *, +, -, //, /, eq, gt, gteq, in, lt, lteq, ne, not, notin
-
class
jinja2.nodes.Pair(key, value)
A key, value pair for dicts.
-
class
jinja2.nodes.Stmt
Base node for all statements.
-
class
jinja2.nodes.Assign(target, node)
Assigns an expression to a target.
-
class
jinja2.nodes.AssignBlock(target, filter, body)
Assigns a block to a target.
-
class
jinja2.nodes.Block(name, body, scoped)
A node that represents a block.
-
class
jinja2.nodes.Break
Break a loop.
-
class
jinja2.nodes.CallBlock(call, args, defaults, body)
Like a macro without a name but a call instead. call is called with
the unnamed macro as caller argument this node holds.
-
class
jinja2.nodes.Continue
Continue a loop.
-
class
jinja2.nodes.EvalContextModifier(options)
Modifies the eval context. For each option that should be modified,
a Keyword has to be added to the options list.
Example to change the autoescape setting:
EvalContextModifier(options=[Keyword('autoescape', Const(True))])
-
class
jinja2.nodes.ScopedEvalContextModifier(options, body)
Modifies the eval context and reverts it later. Works exactly like
EvalContextModifier but will only modify the
EvalContext for nodes in the body.
-
class
jinja2.nodes.ExprStmt(node)
A statement that evaluates an expression and discards the result.
-
class
jinja2.nodes.Extends(template)
Represents an extends statement.
-
class
jinja2.nodes.FilterBlock(body, filter)
Node for filter sections.
-
class
jinja2.nodes.For(target, iter, body, else_, test, recursive)
The for loop. target is the target for the iteration (usually a
Name or Tuple), iter the iterable. body is a list
of nodes that are used as loop-body, and else_ a list of nodes for the
else block. If no else node exists it has to be an empty list.
For filtered nodes an expression can be stored as test, otherwise None.
-
class
jinja2.nodes.FromImport(template, names, with_context)
A node that represents the from import tag. It’s important to not
pass unsafe names to the name attribute. The compiler translates the
attribute lookups directly into getattr calls and does not use the
subscript callback of the interface. As exported variables may not
start with double underscores (which the parser asserts) this is not a
problem for regular Jinja code, but if this node is used in an extension
extra care must be taken.
The list of names may contain tuples if aliases are wanted.
-
class
jinja2.nodes.If(test, body, elif_, else_)
If test is true, body is rendered, else else_.
-
class
jinja2.nodes.Import(template, target, with_context)
A node that represents the import tag.
-
class
jinja2.nodes.Include(template, with_context, ignore_missing)
A node that represents the include tag.
-
class
jinja2.nodes.Macro(name, args, defaults, body)
A macro definition. name is the name of the macro, args a list of
arguments and defaults a list of defaults if there are any. body is
a list of nodes for the macro body.
-
class
jinja2.nodes.Output(nodes)
A node that holds multiple expressions which are then printed out.
This is used both for the print statement and the regular template data.
-
class
jinja2.nodes.OverlayScope(context, body)
An overlay scope for extensions. This is a largely unoptimized scope
that however can be used to introduce completely arbitrary variables into
a sub scope from a dictionary or dictionary like object. The context
field has to evaluate to a dictionary object.
Example usage:
OverlayScope(context=self.call_method('get_context'),
body=[...])
-
class
jinja2.nodes.Scope(body)
An artificial scope.
-
class
jinja2.nodes.With(targets, values, body)
Specific node for with statements. In older versions of Jinja the
with statement was implemented on the base of the Scope node instead.
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class
jinja2.nodes.Template(body)
Node that represents a template. This must be the outermost node that
is passed to the compiler.
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exception
jinja2.nodes.Impossible
Raised if the node could not perform a requested action.