django之block extend标签

class ExtendsNode(Node):
    must_be_first = True
    context_key = 'extends_context'

    def __init__(self, nodelist, parent_name, template_dirs=None):
        self.nodelist = nodelist
        self.parent_name = parent_name
        self.template_dirs = template_dirs
        self.blocks = {n.name: n for n in nodelist.get_nodes_by_type(BlockNode)}

    def __repr__(self):
        return '<ExtendsNode: extends %s>' % self.parent_name.token

    def find_template(self, template_name, context):
        """
        This is a wrapper around engine.find_template(). A history is kept in
        the render_context attribute between successive extends calls and
        passed as the skip argument. This enables extends to work recursively
        without extending the same template twice.
        """
        # RemovedInDjango20Warning: If any non-recursive loaders are installed
        # do a direct template lookup. If the same template name appears twice,
        # raise an exception to avoid system recursion.
        for loader in context.template.engine.template_loaders:
            if not loader.supports_recursion:
                history = context.render_context.setdefault(
                    self.context_key, [context.template.origin.template_name],
                )
                if template_name in history:
                    raise ExtendsError(
                        "Cannot extend templates recursively when using "
                        "non-recursive template loaders",
                    )
                template = context.template.engine.get_template(template_name)
                history.append(template_name)
                return template

        history = context.render_context.setdefault(
            self.context_key, [context.template.origin],
        )
        template, origin = context.template.engine.find_template(
            template_name, skip=history,
        )
        history.append(origin)
        return template

    def get_parent(self, context):
        parent = self.parent_name.resolve(context)
        if not parent:
            error_msg = "Invalid template name in 'extends' tag: %r." % parent
            if self.parent_name.filters or\
                    isinstance(self.parent_name.var, Variable):
                error_msg += " Got this from the '%s' variable." %\
                    self.parent_name.token
            raise TemplateSyntaxError(error_msg)
        if isinstance(parent, Template):
            # parent is a django.template.Template
            return parent
        if isinstance(getattr(parent, 'template', None), Template):
            # parent is a django.template.backends.django.Template
            return parent.template
        return self.find_template(parent, context)

    def render(self, context):
        compiled_parent = self.get_parent(context)#获取父模板对象

        if BLOCK_CONTEXT_KEY not in context.render_context:
            context.render_context[BLOCK_CONTEXT_KEY] = BlockContext()#在context中块上下文，默认词典
        block_context = context.render_context[BLOCK_CONTEXT_KEY]

        # Add the block nodes from this node to the block context
        block_context.add_blocks(self.blocks)#为块上下文添加当前页的块。

        # If this block's parent doesn't have an extends node it is the root,
        # and its block nodes also need to be added to the block context.
        for node in compiled_parent.nodelist:
            # The ExtendsNode has to be the first non-text node.
            if not isinstance(node, TextNode):
                if not isinstance(node, ExtendsNode):
                    blocks = {n.name: n for n in#取得父类中的块节点
                              compiled_parent.nodelist.get_nodes_by_type(BlockNode)}
                    block_context.add_blocks(blocks)#使得块节点插到最前面
                break

        # Call Template._render explicitly so the parser context stays
        # the same.
        return compiled_parent._render(context)#看块节点怎么样render？？？

@register.tag('extends')
def do_extends(parser, token):#从html文件中生成extend node
    """
    Signal that this template extends a parent template.

    This tag may be used in two ways: ``{% extends "base" %}`` (with quotes)
    uses the literal value "base" as the name of the parent template to extend,
    or ``{% extends variable %}`` uses the value of ``variable`` as either the
    name of the parent template to extend (if it evaluates to a string) or as
    the parent template itself (if it evaluates to a Template object).
    """
    bits = token.split_contents()
    if len(bits) != 2:
        raise TemplateSyntaxError("'%s' takes one argument" % bits[0])
    parent_name = parser.compile_filter(bits[1])#父模板名字
    nodelist = parser.parse()#当前html的所有node列表。
    if nodelist.get_nodes_by_type(ExtendsNode):
        raise TemplateSyntaxError("'%s' cannot appear more than once in the same template" % bits[0])
    return ExtendsNode(nodelist, parent_name)

class BlockNode(Node):
    def __init__(self, name, nodelist, parent=None):
        self.name, self.nodelist, self.parent = name, nodelist, parent

    def __repr__(self):
        return "<Block Node: %s. Contents: %r>" % (self.name, self.nodelist)

    def render(self, context):
        block_context = context.render_context.get(BLOCK_CONTEXT_KEY)#从上下文取得块node的信息
        with context.push():
            if block_context is None:
                context['block'] = self
                result = self.nodelist.render(context)
            else:
                push = block = block_context.pop(self.name)#弹出块node，排在后面的先出
                if block is None:
                    block = self
                # Create new block so we can store context without thread-safety issues.
                block = type(self)(block.name, block.nodelist)
                block.context = context
                context['block'] = block
                result = block.nodelist.render(context)
                if push is not None:
                    block_context.push(self.name, push)
        return result