_count attribute duplication by Function objects

[James R. Maddison]

Function objects pass a count keyword to the Coefficient constructor. This can conflict with the counts of existing Coefficients, which can then lead to Coefficient.__eq__ eroneously returning True. This leads to obscure bugs with Python collections.

Example:

from dolfin import *
from ufl import Coefficient

space = FunctionSpace(UnitSquareMesh(1, 1), "CG", 1)
e = space.ufl_element()

c = [Coefficient(e) for i in xrange(10)]
F = [Function(space) for i in xrange(10)]
for i in xrange(len(c)):
  for j in xrange(len(F)):
    if c[i] == F[j]:
      print("Coefficient %i count = %i, Function %i count = %i" % (i, c[i]._count, j, F[j]._count))

[Martin Sandve Alnæs]

This is by design. Don't mix Functions and Coefficients.

[James R. Maddison]

Functions are Coefficients?

[James R. Maddison]

To check correct usage: When using dolfin, classes which inherit from Coefficient should pass their id to the optional count keyword argument

[Martin Sandve Alnæs]

Yes that is the correct behaviour. Each Coefficient needs a unique count which must be preserved when a Function moves from python to C++ and back, and using the id() is the perfect solution. Since it's inherited from Variable it's globally unique in a dolfin program across classes and monotonously increasing, which gives deterministic behaviour (a random number would not allow stable signature computation the same way).

[James R. Maddison]

OK, understood