Takes care of (literal) corner cases

game.py

@@ -1,7 +1,19 @@
 class Grid:
     def __init__(self, size):
-        self.diagram = [[] for i in range(0, size)]
+        self.length = size
+        self.diagram = [[Cell() for i in range(0, size)] for i in range(0, size)]
+
+    def neighbour_coordinates(self, y_coordinate, x_coordinate):
+        coords = [
+            (y_coordinate + y, x_coordinate + x)
+            for y in range(-1, 2)
+            for x in range(-1, 2)
+            if (self.length > y_coordinate + y >= 0) and (self.length > x_coordinate + x >= 0)
+        ]
+        coords.remove((y_coordinate, x_coordinate))
+        return coords
+
 
 class Cell:
     def __init__(self):
-        pass
+        pass

test_game.py

@@ -1,9 +1,21 @@
 import pytest
 from game import Grid, Cell
 
+@pytest.fixture
+def ten_grid():
+    return Grid(10)
 
 class TestGrid:
-    def test_give_size_builds_array_of_length_size(self):
+    def test_give_size_builds_array_of_length_size(self, ten_grid):
         test_size = 10
-        test_grid = Grid(test_size)
-        assert len(test_grid.diagram) == test_size
+        assert len(ten_grid.diagram) == test_size
+
+    def test_given_size_builds_array_of_cells(self, ten_grid):
+        assert isinstance(ten_grid.diagram[2][4], Cell)
+
+    def test_given_position_returns_correct_count_of_neighbours(self, ten_grid):
+        assert len(ten_grid.neighbour_coordinates(1, 1)) == 8
+        assert len(ten_grid.neighbour_coordinates(0, 0)) == 3
+        assert len(ten_grid.neighbour_coordinates(9, 9)) == 3
+        assert len(ten_grid.neighbour_coordinates(0, 9)) == 3
+        assert len(ten_grid.neighbour_coordinates(9, 0)) == 3