Raise TypeError when composing a Morphism with a non-Morphism.

Invoking Morphism.compose on something that is not a Morphism or a
derived class now raises TypeError.  __mul__ will in this case raise a
NotImplementedError instead.

sympy/categories/baseclasses.py

@@ -256,6 +256,9 @@ def compose(self, g, new_name=""):
         'h'
 
         """
+        if not isinstance(g, Morphism):
+            raise TypeError("Morphisms can only be composed with morphisms.")
+
         if g.codomain != self.domain:
             raise ValueError("Uncomponsable morphisms.")
 
@@ -281,7 +284,10 @@ def __mul__(self, g):
         =======
         compose
         """
-        return self.compose(g)
+        try:
+            return self.compose(g)
+        except TypeError:
+            return NotImplemented
 
     def flatten(self, new_name=""):
         """
@@ -478,6 +484,9 @@ def compose(self, g, new_name=""):
         True
 
         """
+        if not isinstance(g, Morphism):
+            raise TypeError("Morphisms can only be composed with morphisms.")
+
         if g.codomain != self.domain:
             raise ValueError("Uncomponsable morphisms.")
 

sympy/categories/tests/test_baseclasses.py

@@ -107,6 +107,11 @@ def test_morphism():
     assert f != Morphism(A, B)
     assert f == f
 
+    raises(TypeError, lambda: f.compose(None))
+    raises(TypeError, lambda: id_A.compose(None))
+    raises(TypeError, lambda: f * None)
+    raises(TypeError, lambda: id_A * None)
+
 def test_diagram():
     A = Object("A")
     B = Object("B")