<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def has_tag(self, p_key, p_value=""):
 """ Returns true when there is at least one tag with the given key. If a value is passed, it will only return true when there exists a tag with the given key-value combination. """

        tags = self.fields['tags']
        return p_key in tags and (p_value == "" or p_value in tags[p_key])