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编写扩展
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编写扩展

2016-02-24 15:39:53 更新

你可以编写扩展来向 Jinja2 中添加自定义标签。这是一个不平凡的任务，而且通常不需 要，因为默认的标签和表达式涵盖了所有常用情况。如 i18n 扩展是一个扩展有用的好例 子，而另一个会是碎片缓存。

当你编写扩展时，你需要记住你在与 Jinja2 模板编译器一同工作，而它并不验证你传递 到它的节点树。如果 AST 是畸形的，你会得到各种各样的编译器或运行时错误，这调试起 来极其可怕。始终确保你在使用创建正确的节点。下面的 API 文档展示了有什么节点和如 何使用它们。

示例扩展

下面的例子用 Werkzeug 的缓存 contrib 模块为 Jinja2 实现了一个 cache 标签:

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 = parser.stream.next().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

而这是你在环境中使用它的方式:

from jinja2 import Environment
from werkzeug.contrib.cache import SimpleCache

env = Environment(extensions=[FragmentCacheExtension])
env.fragment_cache = SimpleCache()

之后，在模板中可以标记块为可缓存的。下面的例子缓存一个边栏 300 秒:

{% cache 'sidebar', 300 %}
<div class="sidebar">
    ...
</div>
{% endcache %}

扩展 API

扩展总是继承 jinja2.ext.Extension 类:

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 theenvironment 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

扩展的标识符。这始终是扩展类的真实导入名，不能被修改。

tags

如果扩展实现自定义标签，这是扩展监听的标签名的集合。

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 calledinlinegettext.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.

解析器 API

传递到 Extension.parse() 的解析器提供解析不同类型表达式的方式。下 面的方法可能会在扩展中使用:

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

解析器处理的模板文件名。这 不是 模板的加载名。加载名见 name 。对于不是从文件系统中加载的模板，这个值为 None 。

name

模板的加载名。

stream

当前的 TokenStream 。

fail(msg, lineno=None, exc=)

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)

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. Theextra_end_rules parameter is forwarded to the tuple parsing function.

parse_expression(with_condexpr=True)

Parse an expression. Per default all expressions are parsed, if the optionalwith_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 toparse_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 ascurrent.

current

当前的 Token 。

eos

Are we at the end of the stream?

expect(expr)

Expect a given token type and return it. This accepts the same argument asjinja2.lexer.Token.test().

look()

Look at the next token.

next()

Go one token ahead and return the old one

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

token 的行号。

type

token 的类型。这个值是被禁锢的，所以你可以用 is 运算符同任意字符 串比较。

value

token 的值。

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.

同样，在词法分析模块中也有一个实用函数可以计算字符串中的换行符数目:

.. autofunction:: jinja2.lexer.count_newlines

AST

AST（抽象语法树: Abstract Syntax Tree）用于表示解析后的模板。它有编译器之后 转换到可执行的 Python 代码对象的节点构建。提供自定义语句的扩展可以返回执行自 定义 Python 代码的节点。

下面的清单展示了所有当前可用的节点。 AST 在 Jinja2 的各个版本中有差异，但会向 后兼容。

更多信息请见 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 isNone.

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.

Node type: Node

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.

Node type: Expr

class jinja2.nodes.Add(left, right)

Add the left to the right node.

Node type: BinExpr

class jinja2.nodes.And(left, right)

Short circuited AND.

Node type: BinExpr

class jinja2.nodes.Div(left, right)

Divides the left by the right node.

Node type: BinExpr

class jinja2.nodes.FloorDiv(left, right)

Divides the left by the right node and truncates conver the result into an integer by truncating.

Node type: BinExpr

class jinja2.nodes.Mod(left, right)

Left modulo right.

Node type: BinExpr

class jinja2.nodes.Mul(left, right)

Multiplies the left with the right node.

Node type: BinExpr

class jinja2.nodes.Or(left, right)

Short circuited OR.

Node type: BinExpr

class jinja2.nodes.Pow(left, right)

Left to the power of right.

Node type: BinExpr

class jinja2.nodes.Sub(left, right)

Substract the right from the left node.

Node type: BinExpr

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.

Node type: Expr

class jinja2.nodes.Compare(expr, ops)

Compares an expression with some other expressions. ops must be a list of Operands.

Node type: Expr

class jinja2.nodes.Concat(nodes)

Concatenates the list of expressions provided after converting them to unicode.

Node type: Expr

class jinja2.nodes.CondExpr(test, expr1, expr2)

A conditional expression (inline if expression). ({{ foo if bar else baz }})

Node type: Expr

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'))

Node type: Expr

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.

Node type: Expr

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.

Node type: Expr

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.

Node type: Expr

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.

Node type: Expr

class jinja2.nodes.Getitem(node, arg, ctx)

Get an attribute or item from an expression and prefer the item.
Node type: Expr

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.

Node type: Expr

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.

Node type: Expr

class jinja2.nodes.Literal

Baseclass for literals.

Node type: Expr

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).

Node type: Literal

class jinja2.nodes.Dict(items)

Any dict literal such as {1: 2, 3: 4}. The items must be a list of Pair nodes.

Node type: Literal

class jinja2.nodes.List(items)

Any list literal such as [1, 2, 3]

Node type: Literal

class jinja2.nodes.TemplateData(data)

A constant template string.

Node type: Literal

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.

Node type: Literal

class jinja2.nodes.MarkSafe(expr)

Mark the wrapped expression as safe (wrap it as Markup).

Node type: Expr

class jinja2.nodes.MarkSafeIfAutoescape(expr)

Mark the wrapped expression as safe (wrap it as Markup) but only if autoescaping is active.

New in version 2.5.

Node type: Expr

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.

    Node type: Expr

class jinja2.nodes.Slice(start, stop, step)

Represents a slice object. This must only be used as argument for Subscript.

Node type: Expr

class jinja2.no

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