• python-igraph
• hy
• python 3.5

pip3 install parvusdb

ParvusDB is a small python3 library for handling graph operations. It acts as an in-memory graph database. It is meant to be used for small graphs (a few hundred nodes)

Graphs are written as a collection of edges and vertices. A vertex is written as

{}(a)

where a is the name of the vertex. This name is used for the operations on the vertex iself. A vertex can have properties written inside the brackets

{'tag': 'PERSON', 'text': 'john'}(a)

The text inside the brackets is in the JSON format. Each of these properties is associated to the node and stored inside the graph

The edges are written as

{}(a,b)

where a is the source node name and b is the target node name. As for the vertex, properties can be added inside the brackets

{'relation': 'LIVES_AT'}(a,b)

A vertex can have a name too, though it must be given as a property

{'relation': 'LIVES_AT', 'name': 'r1'}(a,b)

A triplet can therefore be written in the form.

{'tag': 'PERSON', 'text': 'john'}(a), {'relation': 'LIVES_AT'}(a,b), {'tag': 'PLACE', 'text': 'London'}(b)

There are 6 commands (they must be typed in upper case)

• CREATE
• DELETE
• MATCH
• RETURN
• SET
• WHERE

This command creates the graph on the right hand side

CREATE {'tag': 'PERSON', 'text': 'john'}(a), {'relation': 'LIVES_AT'}(a,b), {'tag': 'PLACE', 'text': 'London'}(b);

This command matches a graph with the topology and properties specified in the right hand side

MATCH {'tag': 'PERSON'}(a), {'relation': 'LIVES_AT', 'name': 'r1'}(a,b), {'tag': 'PLACE'}(b);

This keyword deletes the vertex or edge with the names on the right hand side

DELETE a, b, r1

This keyword let the user specify a LISP code as a condition for the match. For example, if we want the "text" parameter of the node a to be in a list of names

MATCH {'tag': 'PERSON'}(a), {'relation': 'LIVES_AT', 'name': 'r1'}(a,b), {'tag': 'PLACE'}(b)
  WHERE (in (get a "text") ["john" "joseph" "joachim"]);

This command let us modify the content of a graph. For example, if we want to change the text of the node a

MATCH {'tag': 'PERSON'}(a), {'relation': 'LIVES_AT', 'name': 'r1'}(a,b), {'tag': 'PLACE'}(b)
SET (assoc a "text" "not john anymore");

This command let returns the properties of a specific node or edge

MATCH {'tag': 'PERSON'}(a), {'relation': 'LIVES_AT', 'name': 'r1'}(a,b), {'tag': 'PLACE'}(b)
RETURN a, r1;

The return value would be a list of the form

[{'a': {'tag': 'PERSON', 'text': 'john'}, 'r1': {'relation': 'LIVES_AT'}}]

If no vertex or edge name is specified, the system return the whole graph (in the 'parvusdb format' ).

Let's add a triplet to a graph

from igraph import Graph
from parvusdb import GraphDatabase


if __name__ == '__main__':
    g = Graph(directed=True)
    db = GraphDatabase(g)

    creation_string = """
    CREATE {'tag': 'PERSON', 'text': 'john'}(a), {'relation': 'LIVES_AT'}(a,b), 
           {'tag': 'PLACE', 'text': 'London'}(b) 
    RETURN;
    """
    lst = db.query(creation_string)
    print(lst)

which brings the output

[{'GRAPH': "{'tag': 'PERSON', 'text': 'john'}(a), {'tag': 'PLACE', 'text': 'London'}(b), {'relation': 'LIVES_AT'}(a,b)"}]

Then we can try to match elements of the triplet

from igraph import Graph, plot
from parvusdb import GraphDatabase


if __name__ == '__main__':
    g = Graph(directed=True)
    db = GraphDatabase(g)

    creation_string = """
    CREATE {'tag': 'PERSON', 'text': 'john'}(a), {'relation': 'LIVES_AT'}(a,b),
           {'tag': 'PLACE', 'text': 'London'}(b);
    """
    
    match_string = """
    MATCH {}(a), {'relation': 'LIVES_AT'}(a,b), {}(b) 
    RETURN a,b;
    """
    
    lst = db.query(creation_string)
    lst = db.query(match_string)
    print(lst)

with output

[{'a': {'name': 'a', 'tag': 'PERSON', 'text': 'john'}, 'b': {'name': 'b', 'tag': 'PLACE', 'text': 'London'}}]

We can limit the matching process by using WHERE

from igraph import Graph, plot
from parvusdb import GraphDatabase


if __name__ == '__main__':
    g = Graph(directed=True)
    db = GraphDatabase(g)

    
    creation_string = """
    CREATE {'tag': 'PERSON', 'text': 'john'}(a), {'relation': 'LIVES_AT'}(a,b),
           {'tag': 'PLACE', 'text': 'London'}(b)
    CREATE {'tag': 'PERSON', 'text': 'joseph'}(v1), {'relation': 'LIVES_AT'}(v1,v2),
           {'tag': 'PLACE', 'text': 'London'}(v2)
    """
    
    match_string = """
    MATCH {}(_a), {'relation': 'LIVES_AT'}(_a,_b), {}(_b)
      WHERE (= (get _a "text") "joseph")
    RETURN _a,_b;
    """
    
    lst = db.query(creation_string)
    lst = db.query(match_string)
    print(lst)

with output

[{'_b': {'text': 'London', 'tag': 'PLACE', 'name': 'v2'}, '_a': {'text': 'joseph', 'tag': 'PERSON', 'name': 'v1'}}]

• Handling of errors in GraphDatabase.query_lines()
• Ability to add LISP code outside WHERE and SET statements
• Create a stand-alone command line for the database

• The igraph library does not like multiple edges on the same nodes, therefore the MATCH function would not work correctly in those cases