DiagramGrid: Don't use "loop" morphisms in laying out objects.

"Loop" morphisms are morphisms which have the same domains and
codomains.  They will be eventually drawn as loops and should not
influence the layout of the diagram.

sympy/categories/diagram_drawing.py

@@ -996,11 +996,25 @@ def place_objects(pt, placed_objects):
 
         return grid
 
+    @staticmethod
+    def _drop_inessential_morphisms(merged_morphisms):
+        r"""
+        Removes those morphisms which should appear in the diagram,
+        but which have no relevance to object layout.
+
+        Currently this removes "loop" morphisms: the non-identity
+        morphisms with the same domains and codomains.
+        """
+        morphisms = [m for m in merged_morphisms if m.domain != m.codomain]
+        return morphisms
+
     def __init__(self, diagram, groups=None, **hints):
         premises = DiagramGrid._simplify_morphisms(diagram.premises)
         conclusions = DiagramGrid._simplify_morphisms(diagram.conclusions)
-        merged_morphisms = DiagramGrid._merge_premises_conclusions(
+        all_merged_morphisms = DiagramGrid._merge_premises_conclusions(
             premises, conclusions)
+        merged_morphisms = DiagramGrid._drop_inessential_morphisms(
+            all_merged_morphisms)
 
         if groups and (groups != diagram.objects):
             # Lay out the diagram according to the groups.
@@ -1023,7 +1037,7 @@ def __init__(self, diagram, groups=None, **hints):
                 self._grid = grid
 
         # Store the merged morphisms for later use.
-        self._morphisms = merged_morphisms
+        self._morphisms = all_merged_morphisms
 
     @property
     def width(self):

sympy/categories/tests/test_drawing.py

@@ -105,6 +105,19 @@ def test_DiagramGrid():
     assert grid.morphisms == {f:FiniteSet(), g:FiniteSet(),
                               g * f:FiniteSet("unique")}
 
+    # A triangle with a "loop" morphism.
+    l_A = NamedMorphism(A, A, "l_A")
+    d = Diagram([f, g, l_A])
+    grid = DiagramGrid(d)
+
+    assert grid.width == 2
+    assert grid.height == 2
+    assert grid[0, 0] == A
+    assert grid[0, 1] == B
+    assert grid[1, 0] is None
+    assert grid[1, 1] == C
+    assert grid.morphisms == {f:FiniteSet(), g:FiniteSet(), l_A:FiniteSet()}
+
     # A simple diagram.
     d = Diagram([f, g, h, k])
     grid = DiagramGrid(d)