Push an input source onto the lexer's input source stack.

def push_source(self, newstream, newfile=None):
        "Push an input source onto the lexer's input source stack."
        if isinstance(newstream, basestring):
            newstream = StringIO(newstream)
        self.filestack.appendleft((self.infile, self.instream, self.lineno))
        self.infile = newfile
        self.instream = newstream
        self.lineno = 1