Added is_identity check for morphisms induced from a base ring morphism.

src/sage/rings/morphism.pyx

@@ -1557,6 +1557,21 @@ cdef class RingHomomorphism_from_base(RingHomomorphism):
         except Exception:
             raise TypeError, "invalid argument %s"%repr(x)
 
+    def is_identity(self):
+        """
+        Return ``True`` if this morphism is the identity morphism.
+
+        EXAMPLES::
+
+            sage: K.<z> = GF(4)
+            sage: phi = End(K)([z^2])
+            sage: R.<t> = K[]
+            sage: psi = End(R)(phi)
+            sage: psi.is_identity()
+            False
+        """
+        return self.__underlying.is_identity() and RingHomomorphism.is_identity(self)
+
 cdef class RingHomomorphism_cover(RingHomomorphism):
     r"""
     A homomorphism induced by quotienting a ring out by an ideal.