def METHOD_NAME(self, utype, event):
    value = None
    vtype = self.data.get('value', None)
    if vtype is None:
        return
    if vtype == "userName":
        if utype == "IAMUser":
            value = event['userIdentity'].get('userName', '')
        elif utype == "AssumedRole" or utype == "FederatedUser":
            value = event['userIdentity'].get('sessionContext', {}).get(
                'sessionIssuer', {}).get('userName', '')
    elif vtype == "arn":
        value = event['userIdentity'].get('arn', '')
    elif vtype == "sourceIPAddress":
        value = event.get('sourceIPAddress', '')
    elif vtype == "principalId":
        value = event['userIdentity'].get('principalId', '')
    return value