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The problem:

P2 = VectorFunctionSpace(mesh, 'Lagrange', 2) # its dofmap not needed anywhere
P1 = FunctionSpace(mesh, 'Lagrange', 1) # its dofmap not needed anywhere
TH = P2 * P1

Proper way to do it:

P2 = VectorElement('Lagrange', mesh.ufl_cell(), 2)
P1 = FiniteElement('Lagrange', mesh.ufl_cell(), 1)
TH = FunctionSpaceBase(mesh, P2 * P1)

Possible solutions:

deprecate current interface, see also https://bitbucket.org/fenics-project/dolfin/issues/576/function-space-model-in-python-layer-of
or lazy construction of dofmap