def find_referenced_templates(ast):
    """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.
    """
    for node in ast.find_all((nodes.Extends, nodes.FromImport, nodes.Import,
                              nodes.Include)):
        if not isinstance(node.template, nodes.Const):
            # a tuple with some non consts in there
            if isinstance(node.template, (nodes.Tuple, nodes.List)):
                for template_name in node.template.items:
                    # something const, only yield the strings and ignore
                    # non-string consts that really just make no sense
                    if isinstance(template_name, nodes.Const):
                        if isinstance(template_name.value, string_types):
                            yield template_name.value
                    # something dynamic in there
                    else:
                        yield None
            # something dynamic we don't know about here
            else:
                yield None
            continue
        # constant is a basestring, direct template name
        if isinstance(node.template.value, string_types):
            yield node.template.value
        # a tuple or list (latter *should* not happen) made of consts,
        # yield the consts that are strings.  We could warn here for
        # non string values
        elif isinstance(node, nodes.Include) and \
             isinstance(node.template.value, (tuple, list)):
            for template_name in node.template.value:
                if isinstance(template_name, string_types):
                    yield template_name
        # something else we don't care about, we could warn here
        else:
            yield None
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.