def expression(self, string):
<EOL>if not isinstance(string, six.string_types):<EOL><INDENT>return lambda ctxt: string<EOL><DEDENT>expr = self._jinja.compile_expression(string)<EOL>return lambda ctxt: expr(ctxt.variables)<EOL>
Interpret an expression string.  This returns a callable taking
one argument--this context--and returning the result of
evaluating the expression.

:param string: The expression.

:returns: A callable of one argument that will return the
          desired expression.