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I am using TitanGraphDB + Cassandra.I am starting Titan as follows

cd titan-cassandra-0.3.1
bin/titan.sh config/titan-server-rexster.xml config/titan-server-cassandra.properties

I have a Rexster shell that I can use to communicate to Titan+Cassandra above.

cd rexster-console-2.3.0
bin/rexster-console.sh

I want to program the Titan Graph DB from my python program.I am using bulbs package for that.

I create 3 types of vertices from python using bulbs as given below. The 3 types of vertices are 

- switch
- port
- device

from bulbs.titan import Graph
 vswitch = self.g.vertices.get_or_create('dpid',dpid_str,{'state':'active','dpid':dpid_str,'type':'switch'})
 vport   = self.g.vertices.get_or_create('port_id',port_id,{'desc':desc,'port_id':port_id,'state':state,'port_state':port_state,'number':number,'type':'port'})

If I try to print out the variables vswitch,vport and vdevice I get the following results.

vswitch     <Vertex: http://localhost:8182/graphs/graph/vertices/4>
vport       <Vertex: http://localhost:8182/graphs/graph/vertices/28>

However If I try to retrieve the above vertices using a key as follows.

vswitch = self.g.vertices.index.lookup(dpid=dpid_str)
vport   = self.g.vertices.index.lookup(port_id=port_id_str)

And try to print-out vswitch and vport variables I get the following values

<generator object <genexpr> at 0x26d6370>)
<generator object <genexpr> at 0x26d63c0>

Am I doing something WRONG in trying to retrieve vertices as above using g.vertices.index.lookup(dpid=dpid_str)

liv2hak
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1 Answers1

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The Bulbs g.vertices.index.lookup() method returns a Python generator (which is a type of iterator).

Use next() to get the next value in the generator:

>>> # lookup() returns an generator (can return more than 1 value)

>>> switches = self.g.vertices.index.lookup(dpid=dpid_str)
>>> switch = switches.next()

>>> ports   = self.g.vertices.index.lookup(port_id=port_id_str)
>>> port = ports.next()

Or you can use list() to turn the generator into a Python list:

>>> switches = self.g.vertices.index.lookup(dpid=dpid_str)
>>> list(switches)

>>> ports   = self.g.vertices.index.lookup(port_id=port_id_str)
>>> list(ports)

However, if the indexed item is unique, you can use the get_unique() method to return one value or None:

# returns 1 vertex or None (errors if more than 1)
>>> vertex = g.vertices.index.get_unique( "dpid", dpid_str)

See...

Rexter index documentation:

index.lookup() https://github.com/espeed/bulbs/blob/afa28ccbacd2fb92e0039800090b8aa8bf2c6813/bulbs/titan/index.py#L251

index.get_unique() https://github.com/espeed/bulbs/blob/afa28ccbacd2fb92e0039800090b8aa8bf2c6813/bulbs/titan/index.py#L274

NOTE: Iterators and generators are Python programming basics -- they're used everywhere and are not specific to Bulbs -- if you're new to Python programming, see my answer to How Can I Learn to Program in Python? for a list of good online resources for learning to program in Python.

espeed
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