Get the template source, filename and reload helper for a template.
        It's passed the environment and template name and has to return a
        tuple in the form ``(source, filename, uptodate)`` or raise a
        `TemplateNotFound` error if it can't locate the template.

        The source part of the returned tuple must be the source of the
        template as unicode string or a ASCII bytestring.  The filename should
        be the name of the file on the filesystem if it was loaded from there,
        otherwise `None`.  The filename is used by python for the tracebacks
        if no loader extension is used.

        The last item in the tuple is the `uptodate` function.  If auto
        reloading is enabled it's always called to check if the template
        changed.  No arguments are passed so the function must store the
        old state somewhere (for example in a closure).  If it returns `False`
        the template will be reloaded.

def get_source(self, environment, template):
        """Get the template source, filename and reload helper for a template.
        It's passed the environment and template name and has to return a
        tuple in the form ``(source, filename, uptodate)`` or raise a
        `TemplateNotFound` error if it can't locate the template.

        The source part of the returned tuple must be the source of the
        template as unicode string or a ASCII bytestring.  The filename should
        be the name of the file on the filesystem if it was loaded from there,
        otherwise `None`.  The filename is used by python for the tracebacks
        if no loader extension is used.

        The last item in the tuple is the `uptodate` function.  If auto
        reloading is enabled it's always called to check if the template
        changed.  No arguments are passed so the function must store the
        old state somewhere (for example in a closure).  If it returns `False`
        the template will be reloaded.
        """
        if not self.has_source_access:
            raise RuntimeError('%s cannot provide access to the source' %
                               self.__class__.__name__)
        raise TemplateNotFound(template)