A function that tags all the newly created instances based on their 'pending' state.Of course it can be used for tagging any other state just by providing different argument.

tag_AWS_instances.py

def tag_instances(connection,tag , instance_state="pending"):
	"""
		-The function must be run immediately after the spawn of an instance!!!-

		1. The function fetches all the instances
		2. Filters them by instance-state-name:pending
		(for a short period the newly created instances are 'pending' before becoming 'running')
		3. Tags them with the user defined tag.
		4. Returns the just tagged instances (ids) for future reference
	"""
	newly_tagged_instances = []

	reserves = connection.get_all_instances(filters={"instance-state-name":instance_state})
	for r in reserves:
		for instance in r.instances:
			print "--Creating instance-- \n", "ID: "+str(instance.id)+"\n", "Status: "+instance.state
			# set a tag
			instance.add_tag("Name", tag ) ; print "Name: "+tag+"\n"
			# save the newly created instance to the list
			newly_tagged_instances.append( instance.id )

	return newly_tagged_instances