Spawning child traversals anonymously using Gremlin-python and graphexp

Question

I'm trying to create a graph from scratch that i'm visualising in graphexp, but i'm struggling to undertand the anonymized traversals concept and how to create them

I'm using python 3.9 and gremlinpython 3.5.1

• Creating the connection and graph:

from gremlin_python.process.anonymous_traversal import traversal

self.g = traversal().withRemote(DriverRemoteConnection('ws://localhost:8182/gremlin', 'g'))

• I imported the statics so I could use the steps without the __ class:

statics.load_statics(globals())

• Creating a vertex if it doesnt exists:

def _add_vertex(self, name):
        return self.g.V().has('name', name).fold().coalesce(unfold(), addV().property('name',name)).next()

• Creating an edge if it doesnt exist between two vertices:

def _add_edge(self, v1, v2, weight, label):
        return self.g.V(v1).as_("fromVertex").V(v2).coalesce(inE(label).where(outV().as_(
            "fromVertex")), addE(label).property("weight", weight).from_("fromVertex")).next()

But i'm getting this error in graphexp when clicking on a vertex

Error retrieving data
The child traversal of [GraphStep(vertex,[696560]), PropertyMapStep(value)] was not spawned anonymously - use the __ class rather than a TraversalSource to construct the child traversal

The documentation is generally good but not very helpful for the anonymized traversals part. So how can I spawn anonymous child traversals using this method? and what does it really mean ?

Answer 1

Starting with TinkerPop 3.5.x, using a g anywhere but at the start of a traversal will raise an error. It used to be possible to write something like:

g.V('1').addE('test').from(g.V('2'))

This actually can have some bad side effects, so in 3.5.x the parser now enforces that you must do one of:

g.V('1').addE('test').from(V('2'))

or

g.V('1').addE('test').from(__.V('2'))

That "double underscore" class __. is the so called anonymous traversal source. It is called that as it is not connected to a prior step by a dot, rather it is inside parens as a child traversal.

I would check your code to see if you are actually injecting a second g into your traversal.