Create custom predicate in Gremlin Python

[ha...@nugit.co]

Hello,

I looked at the example of creating custom predicates or tests in Kelvin Lawrence's book using Java.

// Create a new BiPredicate that handles regular expression pattern matching
bp = new java.util.function.BiPredicate<String, String>() {
         boolean test(String val, String pattern) {
           return val ==~ pattern  }}

// Create a new closure we can use for regular expression pattern matching.
regex = {new P(bp, it)}

// Use our new closure to find descriptions that start with 'Dal'. As this
// unwinds, the contents of 'desc' are passed to the test method as the first parameter
// and the regex pattern as the second paramter.
g.V().has('desc', regex(/^Dal.*/)).values('desc')

Would anybody be able to guide me on the way to do this in Gremlin Python?

Thanks,

Harsh

[Stephen Mallette]

In python you have to write lambdas as follows:

>>> g.V().out().map(lambda: "lambda x: len(x.get().value('name'))").sum().toList() #(1)

[24]

or if not using gremlin-python on the server then you would write your lambda with groovy:

>>> g.V().out().map(lambda: ("it.get().value('name').length()", "gremlin-groovy")).sum().toList() #(3)

[24]

which is described more here:

http://tinkerpop.apache.org/docs/current/reference/#_the_lambda_solution_2

So, you could basically just extract that lambda, defined that way, to a python function and then re-use it as needed. 

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[Harshvardhan Joshi]

Hey Stephen,

Thank you for taking the time to reply.

I see that it is possible to add custom lambdas to the map / filter step.

But I wanted to know if it’s possible to do so in the has step.

For example,

g.V().has(“name”, lambda: ‘lambda x: x.get().value(“name”).startswith(“abc”)’).toList()

This traversal returns only an empty list. Even if there are nodes whose “name” start with “abc”.

Also, I did encounter issues when trying to use the filter step in the where step with or/and conditions.

For example, the below traversal works

g.V().has("name").where(__.filter(lambda: "lambda x: ‘abc' in x.get().value('name').lower()")).values("name").toList()

But something like,

g.V().has("name").where(__.or_(__.filter(lambda: "lambda x: 'dev' in x.get().value('name').lower()"), __.values("age").is_(P.between(10,40)))).values("name").toList()

Returns this error:-

type object 'org.apache.tinkerpop.gremlin.process.traversal.dsl' has no attribute 'or_' in <script> at line number 1

Your input on this would be helpful.

Thank you,

Harsh

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[Stephen Mallette]

A lambda won't work with has() that way because the has() step doesn't expect a lambda - it expects an instance of the P predicate:

http://tinkerpop.apache.org/javadocs/current/core/org/apache/tinkerpop/gremlin/process/traversal/dsl/graph/GraphTraversal.html#has-java.lang.String-org.apache.tinkerpop.gremlin.process.traversal.P-

So you'd need an instance of P to get that in there. Unfortunately, P is not an abstract class or an interface so all that can really happen is this:

gremlin> g.V().has('name', new P({testValue,value -> testValue.startsWith(value)}, 'm'))

==>v[1]

So, more "unfortunately", i don't think that this will work with our lambda system. If you have to do something like this then I guess you are stuck with scripts in this case.

>  type object 'org.apache.tinkerpop.gremlin.process.traversal.dsl' has no attribute 'or_' in <script> at line number 1

i'm aware of that bug. it will be fixed in 3.2.8/3.3.2 which should release in about a week.

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