How to create lists of cell neighbours?

2

Instead of using the filter function, you can simply unwrap it in multiple loops:

from dolfin import *

mesh = UnitSquareMesh(2, 2)

# Init facet-cell connectivity
tdim = mesh.topology().dim()
mesh.init(tdim - 1, tdim)

# For every cell, build a list of cells that are connected to its facets
# but are not the iterated cell
cell_neighbours = {}
for cell in cells(mesh):
    index = cell.index()
    cell_neighbours[index] = []
    for facet in facets(cell):
        facet_cells = facet.entities(tdim)
        for facet_cell in facet_cells:
            if (index!=facet_cell):
                cell_neighbours[index].append(facet_cell)
print(cell_neighbours)
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