How to use the exabgp.protocol.family.AFI function in exabgp

def unpack_message (cls, data, negotiated):
		header_length = len(EOR.NLRI.PREFIX)
		return cls(AFI.unpack(data[header_length:header_length+2]),SAFI.unpack(data[header_length+2]))

def _add (self, tokeniser, afi_name, safi_names):
		self._check_duplicate(tokeniser,RaisedFamily)
		known = self.content.setdefault(AFI(AFI.value(afi_name)),[])

		for (idx_line,idx_column,line,safi_name) in safi_names:
			if safi_name not in AFI.implemented_safi(afi_name):
				raise RaisedFamily(Location(idx_line,idx_column,line),'the family pair afi/safi %s/%s is unimplemented' % (afi_name,safi_name))

			safi = SAFI(SAFI.value(safi_name))
			if safi in known:
				raise RaisedFamily(Location(idx_line,idx_column,line),'afi/safi pair already defined in this family')
			known.append(safi)

rr_announced.append((afi,safi))
			yield Update(RouteRefresh(afi,safi,RouteRefresh.start),Attributes())

		# generating Updates from what is in the RIB

		for attr_index,per_family in attr_af_nlri.items():
			for family, changes in per_family.items():
				if not changes:
					continue

				# only yield once we have a consistent state, otherwise it will go wrong
				# as we will try to modify things we are iterating over and using

				attributes = new_attr[attr_index]

				if family == (AFI.ipv4,SAFI.unicast) and grouped:
					yield Update([change.nlri for change in changes.values()], attributes)
				else:
					for change in changes.values():
						yield Update([change.nlri,], attributes)

		# If we are performing a route-refresh, indicating that the
		# update were all sent

		if rr_announced:
			for afi,safi in rr_announced:
				self._enhanced_refresh_start.remove((afi,safi))
				yield Update(RouteRefresh(afi,safi,RouteRefresh.end),Attributes())

			for change in self._enhanced_refresh_delay:
				self.add_to_rib(change,True)
			self._enhanced_refresh_delay = []

env.log.enable = True

    if LOG.logger.getEffectiveLevel():
        env.log.level = environment.syslog_value(
            python_logging.getLevelName(LOG.logger.getEffectiveLevel())
            )
    else:
        env.log.level = environment.syslog_value('INFO')
    env.log.all = True
    env.log.packets = True

    # monkey patch exabgp to work around exabgp issue #690
    # only relevant for exabgp 4.0.2
    # ( https://github.com/Exa-Networks/exabgp/issues/690 )
    if hasattr(exa_family.Family, 'size'):
        exa_family.Family.size[(exa_family.AFI.l2vpn,
                                exa_family.SAFI.evpn)] = ((4,), 0)

for c in string:
		value = (value << 8) + ord(c)
	return value

# def short (value):
# 	return (ord(value[0]) << 8) + ord(value[1])

# Interface ..................


class IPv4 (object):
	afi = AFI.ipv4


class IPv6 (object):
	afi = AFI.ipv6


class IPrefix (object):
	pass


# Prococol

class IPrefix4 (IPrefix,IComponent,IPv4):
	# Must be defined in subclasses
	CODE = -1
	NAME = ''

	# not used, just present for simplying the nlri generation
	operations = 0x0

	def top (self, negotiated=None, afi=AFI.undefined):
		if not self._ip:
			self._ip = IP.ntop(self._packed)
		return self._ip

if self.debug: raise Exception()  # noqa
			return False

		try:
			safi = tokens.pop(0)
		except IndexError:
			self._error = 'missing family safi'
			if self.debug: raise Exception()  # noqa
			return False

		if safi == 'unicast':
			scope[-1]['families'].append((AFI(AFI.ipv4),SAFI(SAFI.unicast)))
		elif safi == 'multicast':
			scope[-1]['families'].append((AFI(AFI.ipv4),SAFI(SAFI.multicast)))
		elif safi == 'nlri-mpls':
			scope[-1]['families'].append((AFI(AFI.ipv4),SAFI(SAFI.nlri_mpls)))
		elif safi == 'mpls-vpn':
			scope[-1]['families'].append((AFI(AFI.ipv4),SAFI(SAFI.mpls_vpn)))
		elif safi in ('flow'):
			scope[-1]['families'].append((AFI(AFI.ipv4),SAFI(SAFI.flow_ip)))
		elif safi == 'flow-vpn':
			scope[-1]['families'].append((AFI(AFI.ipv4),SAFI(SAFI.flow_vpn)))
		else:
			return False
		return True