New in version 2.2. 元 API 返回一些关于抽象语法树的信息，这些信息能帮助应用实现更多的高级模板概 念。所有的元 API 函数操作一个 [Environment.parse()](http://docs.jinkan.org/docs/jinja2/api.html#jinja2.Environment.parse "jinja2.Environment.parse") 方法返回的抽象语法 树。 jinja2.meta.find_undeclared_variables(*ast*)[](http://docs.jinkan.org/docs/jinja2/api.html#jinja2.meta.find_undeclared_variables "Permalink to this definition") Returns a set of all variables in the AST that will be looked up from the context at runtime. Because at compile time it’s not known which variables will be used depending on the path the execution takes at runtime, all variables are returned. ~~~ >>> from jinja2 import Environment, meta >>> env = Environment() >>> ast = env.parse('{% set foo = 42 %}{{ bar + foo }}') >>> meta.find_undeclared_variables(ast) set(['bar']) ~~~ Implementation Internally the code generator is used for finding undeclared variables. This is good to know because the code generator might raise a [TemplateAssertionError](http://docs.jinkan.org/docs/jinja2/api.html#jinja2.TemplateAssertionError "jinja2.TemplateAssertionError") during compilation and as a matter of fact this function can currently raise that exception as well. jinja2.meta.find_referenced_templates(*ast*)[](http://docs.jinkan.org/docs/jinja2/api.html#jinja2.meta.find_referenced_templates "Permalink to this definition") Finds all the referenced templates from the AST. This will return an iterator over all the hardcoded template extensions, inclusions and imports. If dynamic inheritance or inclusion is used, None will be yielded. ~~~ >>> from jinja2 import Environment, meta >>> env = Environment() >>> ast = env.parse('{% extends "layout.html" %}{% include helper %}') >>> list(meta.find_referenced_templates(ast)) ['layout.html', None] ~~~ This function is useful for dependency tracking. For example if you want to rebuild parts of the website after a layout template has changed.