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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def get_username(self, userid):
        '''Perform a lookup of users to resolve a userid to a username

        Args:
            userid (string): Slack userid to lookup.

        Returns:
            string: Human-friendly name of the user
        '''

        username = self.user_map.get(userid)
        if not username:
            users = self.get_users()
            if users:
                members = {
                    m['id']: m['name']
                    for m in users.get('members', [{}])
                    if m.get('id')
                    and m.get('name')
                }
                if members:
                    self.user_map.update(members)

                username = self.user_map.get(userid, userid)

        return username