編寫擴(kuò)展

你可以編寫擴(kuò)展來向 Jinja2 中添加自定義標(biāo)簽。這是一個(gè)不平凡的任務(wù)，而且通常不需 要，因?yàn)槟J(rèn)的標(biāo)簽和表達(dá)式涵蓋了所有常用情況。如 i18n 擴(kuò)展是一個(gè)擴(kuò)展有用的好例 子，而另一個(gè)會(huì)是碎片緩存。

當(dāng)你編寫擴(kuò)展時(shí)，你需要記住你在與 Jinja2 模板編譯器一同工作，而它并不驗(yàn)證你傳遞 到它的節(jié)點(diǎn)樹。如果 AST 是畸形的，你會(huì)得到各種各樣的編譯器或運(yùn)行時(shí)錯(cuò)誤，這調(diào)試起 來極其可怕。始終確保你在使用創(chuàng)建正確的節(jié)點(diǎn)。下面的 API 文檔展示了有什么節(jié)點(diǎn)和如 何使用它們。

示例擴(kuò)展

下面的例子用?Werkzeug?的緩存 contrib 模塊為 Jinja2 實(shí)現(xiàn)了一個(gè)?cache?標(biāo)簽:

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

而這是你在環(huán)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，在模板中可以標(biāo)記塊為可緩存的。下面的例子緩存一個(gè)邊欄 300 秒:

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

擴(kuò)展 API

擴(kuò)展總是繼承?jinja2.ext.Extension?類:

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

擴(kuò)展的標(biāo)識(shí)符。這始終是擴(kuò)展類的真實(shí)導(dǎo)入名，不能被修改。

tags

如果擴(kuò)展實(shí)現(xiàn)自定義標(biāo)簽，這是擴(kuò)展監(jiān)聽的標(biāo)簽名的集合。

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

傳遞到?Extension.parse()?的解析器提供解析不同類型表達(dá)式的方式。下 面的方法可能會(huì)在擴(kuò)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

解析器處理的模板文件名。這?不是?模板的加載名。加載名見?name?。對(duì)于不是從文件系統(tǒng)中加載的模板，這個(gè)值為?None?。

name

模板的加載名。

stream

當(dāng)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

當(dāng)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 的行號(hào)。

type

token 的類型。這個(gè)值是被禁錮的，所以你可以用?is?運(yùn)算符同任意字符 串比較。

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.

同樣，在詞法分析模塊中也有一個(gè)實(shí)用函數(shù)可以計(jì)算字符串中的換行符數(shù)目:

.. autofunction:: jinja2.lexer.count_newlines

AST

AST（抽象語法樹: Abstract Syntax Tree）用于表示解析后的模板。它有編譯器之后 轉(zhuǎn)換到可執(zhí)行的 Python 代碼對(duì)象的節(jié)點(diǎn)構(gòu)建。提供自定義語句的擴(kuò)展可以返回執(zhí)行自 定義 Python 代碼的節(jié)點(diǎn)。

下面的清單展示了所有當(dāng)前可用的節(jié)點(diǎn)。 AST 在 Jinja2 的各個(gè)版本中有差異，但會(huì)向 后兼容。

更多信息請(qǐng)見?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:

• Stmt: statements
• Expr: expressions
• Helper: helper nodes
• Template: the outermost wrapper node

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