Gremlin Snippets are typically short and fun dissections of some aspect of the Gremlin language. For a full list of all steps in the Gremlin language see the Reference Documentation of Apache TinkerPop™. This snippet is based on Gremlin 3.4.6.This snippet demonstrates its lesson using the data of the "modern" toy graph (image).Please consider bringing any discussion or questions about this snippet to Discord or the Gremlin Users Mailing List.


Gremlin can generate Map instances that have keys that are themselves of type Map. This situation is often encountered when doing a groupCount() using a graph element like Vertex and then converting that Vertex to a Map:

gremlin> g.V().both().groupCount()
==>[v[1]:3,v[2]:1,v[3]:3,v[4]:3,v[5]:1,v[6]:1]
gremlin> g.V().both().groupCount().by(elementMap()).unfold()
==>{id=5, label=software, name=ripple, lang=java}=1
==>{id=2, label=person, name=vadas, age=27}=1
==>{id=4, label=person, name=josh, age=32}=3
==>{id=3, label=software, name=lop, lang=java}=3
==>{id=1, label=person, name=marko, age=29}=3
==>{id=6, label=person, name=peter, age=35}=1

Unfortunately, not every programming language allows a Map to have such a key and, as a result, a traversal that works perfectly well in the Gremlin Console (using Groovy) ends up not working in another programming language despite the Gremlin being identical. Python is one such language that has this limitation. In Python a dict can only have a key that is a hashable type and dict itself is not. As a result a TypeError occurs in Python when trying to execute that previous traversal.

Luckily, Gremlin is a flexible language with many steps that can help you manipulate collections. It merely takes a bit of creativity to solve the problem. For Python and this particular case, we just need to transform the result to something that Python can handle - a list of list:

gremlin> g.V().both().elementMap().groupCount().
......1>   unfold().
......2>   map(union(select(keys),select(values)).fold())
==>[[id:5,label:software,name:ripple,lang:java],1]
==>[[id:2,label:person,name:vadas,age:27],1]
==>[[id:4,label:person,name:josh,age:32],3]
==>[[id:3,label:software,name:lop,lang:java],3]
==>[[id:1,label:person,name:marko,age:29],3]
==>[[id:6,label:person,name:peter,age:35],1]

In the above case, we convert each Map into a List by way of union() where we grab the keys for the first item in the List and the values for the second item in the List, basically creating a list of pairs. This case was only meant to be an example and other solutions abound. The point is that there are times where you may need to reformulate your result to meet the needs of your host language. Gremlin makes doing that quite straightforward.