Error 'int' object has no attribute 'ufl_id'

Errors
1

Hello everybody,
I try to run the code below:

from fenics import *
T=Constant(273.15)
P= 200000  #pressure in [Pa]
R      = Constant(8.314462176) # [J*mol/K] universal gas constant

xml_file = "02Mesh.xml"
mesh = Mesh(xml_file)
fd = MeshFunction('size_t', mesh, "02Mesh_facet_region.xml");
materials = MeshFunction('size_t', mesh, "02Mesh_physical_region.xml");

V = FunctionSpace(mesh, 'P', 2)
W = VectorFunctionSpace(mesh, 'P', 2)

P1 = FiniteElement('P', triangle, 1)
element = MixedElement([P1, P1, P1])
V = FunctionSpace(mesh, element)
v_1, v_2, v_3 = TestFunctions(V)

w = Function(W)
u = Function(V)
u_n = Function(V)

u_1, u_2, u_3 = split(u)
u_n1, u_n2, u_n3  = split(u_n)

u_o2 = Constant((0.7, 0.7, 0.7))  
u_h = Constant((0.4, 0.4, 0.4)) 

b_1 = DirichletBC(V, u_o2, fd, 101)

Irefrence=Constant(1)
Co2refrence=Constant(1)
alpha = Constant(-1.1 )
F=Constant(2.3)
factor =Constant(1.001)
A=Constant(0.001)
Variable=Constant(alpha*F*factor)
EffectiveDiffO2 = 0.0001
dx = Measure('dx', domain=mesh, subdomain_data=3)

F = EffectiveDiffO2*dot(grad(u_1), grad(v_1))*dx \
  - (u_1*(A*Irefrence/Co2refrence)*exp(Variable/(R*T))/16)*v_1*dx

solve(F == 0, u, b_1)

but unfortunately, I got this error:

'int' object has no attribute 'ufl_id'
  File "/usr/lib/petsc/lib/python3/dist-packages/dolfin/fem/form.py", line 22, in __init__
    sd = form.subdomain_data()
  File "/usr/lib/petsc/lib/python3/dist-packages/dolfin/fem/problem.py", line 157, in __init__
    F = Form(F, form_compiler_parameters=form_compiler_parameters)
  File "/usr/lib/petsc/lib/python3/dist-packages/dolfin/fem/solving.py", line 308, in _solve_varproblem
    problem = NonlinearVariationalProblem(eq.lhs, u, bcs, J,
  File "/usr/lib/petsc/lib/python3/dist-packages/dolfin/fem/solving.py", line 233, in solve
    _solve_varproblem(*args, **kwargs)
  File "/mnt/c/Users/Asus/Desktop/Master thesis/Ubuntu Code/TEstt.py", line 45, in <module>
    solve(F == 0, u, b_1)

It would be great if somebody can suggest that where is the problem.

2

subdomain_data should be a MeshFunction and subdomain_id=3

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