Error "z array must not contain non-finite values within the triangulation"

2

It is not safe to use Expressions when your division is singular (at x[1]=0). Usage of ufl and projection is safer:

V = FunctionSpace(mesh, "Lagrange", 1)
x = SpatialCoordinate(mesh)
u_D = project(atan(x[0]/x[1]), FunctionSpace(mesh, "Lagrange", 4))
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