How to create an "empty" UFL form in Python

Question

Is it possible to create an "empty" UFL form in Python and then add terms to it as required?

The code would look something like this:

form = EmptyForm()
for term in terms:
    form += term

I have tried the following which seems to work:

form = 0
for term in terms:
    form += term

Are there any potential issues with this implementation?

Answer 1

The problem with this is that EmptyForm() would not know about rank (and other details) required for consistency checks.

You could mimic this by form = Constant(0.0)*TestFunction(V)*dx. Disadvantage is that actual code is generated (and executed by assembler) for this term.

Alternatively you could maybe take advantage of list/generator comprehension: form = sum(term for term in terms).

Comments

Haven't read properly that 0 + term works in your post. If it works it should be fine.

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Hi, the constructs below seem equivalent. I agree with Jan that there should be no issues with 0. I you insist on ''type stability'' then perhaps f3 and f4 are the way to go.

from dolfin import *
from ufl import zero

mesh = UnitSquareMesh(10, 10)
V = FunctionSpace(mesh, 'CG', 1)
u = TrialFunction(V)
v = TestFunction(V)

cs = map(Constant, range(4))
forms = [c*inner(u, v)*dx for c in cs]

f0 = sum(forms)
f1 = sum(forms, 0)
f2 = sum(forms, zero())
f3 = sum(forms[1:], forms[0])

iforms = iter(forms)
f4 = sum(iforms, next(iforms))

print len(set([f.signature() for f in (f0, f1, f2, f3, f4)]))