DiagramGrid: Factor out the generic laying out functionality.

In the future, the constructor of DiagramGrid will live through further
enhancements, like hints, which makes it important to push what it does
to a higher level of abstraction.

sympy/categories/diagram_drawing.py

@@ -661,17 +661,11 @@ def group_size(group):
 
         return grid
 
-    def __init__(self, diagram, groups=None):
-        premises = DiagramGrid._simplify_morphisms(diagram.premises)
-        conclusions = DiagramGrid._simplify_morphisms(diagram.conclusions)
-        merged_morphisms = DiagramGrid._merge_premises_conclusions(
-            premises, conclusions)
-
-        if groups and (groups != diagram.objects):
-            # Lay out the diagram according to the groups.
-            self._grid = DiagramGrid._handle_groups(diagram, groups, merged_morphisms)
-            return
-
+    @staticmethod
+    def _generic_layout(diagram, merged_morphisms):
+        """
+        Produces the generic layout for the supplied diagram.
+        """
         skeleton = DiagramGrid._build_skeleton(merged_morphisms)
 
         all_objects = diagram.objects
@@ -684,9 +678,7 @@ def __init__(self, diagram, groups=None):
             grid[0, 0] = objects[0]
             grid[0, 1] = objects[1]
 
-            self._grid = grid
-
-            return
+            return grid
 
         triangles = DiagramGrid._list_triangles(skeleton)
         triangles = DiagramGrid._drop_redundant_triangles(triangles, skeleton)
@@ -729,7 +721,20 @@ def __init__(self, diagram, groups=None):
             triangles = DiagramGrid._drop_irrelevant_triangles(
                 triangles, placed_objects)
 
-        self._grid = grid
+        return grid
+
+    def __init__(self, diagram, groups=None):
+        premises = DiagramGrid._simplify_morphisms(diagram.premises)
+        conclusions = DiagramGrid._simplify_morphisms(diagram.conclusions)
+        merged_morphisms = DiagramGrid._merge_premises_conclusions(
+            premises, conclusions)
+
+        if groups and (groups != diagram.objects):
+            # Lay out the diagram according to the groups.
+            self._grid = DiagramGrid._handle_groups(diagram, groups, merged_morphisms)
+            return
+
+        self._grid = DiagramGrid._generic_layout(diagram, merged_morphisms)
 
     @property
     def width(self):