https://github.com/cl0ne/euro-diffusion/blob/master/euro_diffusion/simulator.py#L93

[IanaKhitsko]

            for x, y in self._get_neighbours(city.x, city.y):
                neighbour: City = self._field[x, y]

==

How many times _get_neighbours(city.x, city.y) will be called in this case ? only once or 1..4 times?

[cl0ne]

Method _get_neighbours implemented as a generator function so here it'll be called only once and it returns iterator-like generator object.