Incorrect hash for bound nodes with identity=0

[atsysin]

Expected behavior

For bound nodes with identity=0, hash computation should be based on the graph.service, graph.name and identity attributes.

Actual behavior

If identity=0 for a bound node, then the if statement on line 691 is equivalent to the snippet below and always evaluates to False:

if self.graph and 0:
    ...

which means that the hash is computed based on the Python object's identity.

Steps to reproduce the problem

You would need a fresh Neo4j instance, or to query a node with <id>: 0, or to set node's identity to 0 manually:

from py2neo import Graph, Node

g = Graph()
n = Node()
g.create(n)  # node is bound now
n.identity = 0  # re-write identity for demonstration
h = hash(n)

h_unbound = hash(id(n))
print(h == h_unbound)

h_bound = hash(n.graph.service) ^ hash(n.graph.name) ^ hash(n.identity)
print(h == h_bound)

Proposed solution

Replace the line 691 with

if self.graph and (self.identity is not None):
    ...