Some review changes.

src/sage/modular/abvar/homspace.py

@@ -229,12 +229,36 @@ def __init__(self, domain, codomain, cat):
 
     @lazy_attribute
     def _matrix_space(self):
+        """
+        Return the matrix space of ``self``.
+
+        .. WARNING::
+
+            During unpickling, the domain and codomain may be unable to
+            provide the necessary information. This is why this is a lazy
+            attribute. See :trac:`14793`.
+
+        EXAMPLES::
+
+            sage: Hom(J0(11), J0(22))._matrix_space
+            Full MatrixSpace of 2 by 4 dense matrices over Integer Ring
+        """
         return MatrixSpace(ZZ,2*self.domain().dimension(), 2*self.codomain().dimension())
 
-    # The following method is used in the coercion framework. Unfortunately, the default
-    # method would get the order of parent and data different from what is expected
-    # in MatrixMorphism.__init__
     def _element_constructor_from_element_class(self, *args, **keywords):
+        """
+        Used in the coercion framework. Unfortunately, the default method
+        would get the order of parent and data different from what is expected
+        in ``MatrixMorphism.__init__``.
+
+        EXAMPLES::
+
+            sage: H = Hom(J0(11), J0(22))
+            sage: phi = H(matrix(ZZ,2,4,[5..12])); phi # indirect doctest
+            Abelian variety morphism:
+              From: Abelian variety J0(11) of dimension 1
+              To:   Abelian variety J0(22) of dimension 2
+        """
         return self.element_class(self, *args, **keywords)
 
     def __call__(self, M):
@@ -753,6 +777,15 @@ def __init__(self, A, gens=None, category=None):
         self._is_full_ring = gens is None
 
     def __reduce__(self):
+        """
+        Used in pickling.
+
+        EXAMPLES::
+
+            sage: E = J0(31).endomorphism_ring()
+            sage: loads(dumps(E)) == E
+            True
+        """
         return End, (self.domain(),self.domain().category())
 
     def _repr_(self):

src/sage/modules/fg_pid/fgp_morphism.py

@@ -488,7 +488,7 @@ class FGP_Homset_class(Homset):
         Set of Morphisms from Finitely generated module V/W over Integer Ring with invariants (4, 12) to Finitely generated module V/W over Integer Ring with invariants (4, 12) in Category of modules over Integer Ring
         sage: type(H)
         <class 'sage.modules.fg_pid.fgp_morphism.FGP_Homset_class_with_category'>
-        
+
     """
     Element = FGP_Morphism
     def __init__(self, X, Y, category=None):

src/sage/modules/matrix_morphism.py

@@ -1164,16 +1164,16 @@ def restrict(self, sub):
 class MatrixMorphism(MatrixMorphism_abstract):
     """
     A morphism defined by a matrix.
-    """
-    def __init__(self, parent, A):
-        """
-        INPUT:
 
+    INPUT:
 
-        -  ``parent`` - a homspace
-
-        -  ``A`` - matrix or a :class:`MatrixMorphism_abstract` instance
+    -  ``parent`` - a homspace
 
+    -  ``A`` - matrix or a :class:`MatrixMorphism_abstract` instance
+    """
+    def __init__(self, parent, A):
+        """
+        Initialize ``self``.
 
         EXAMPLES::
 
@@ -1186,14 +1186,14 @@ def __init__(self, parent, A):
             True
         """
         if parent is None:
-            raise ValueError, "no parent given when creating this matrix morphism"
+            raise ValueError("no parent given when creating this matrix morphism")
         if isinstance(A, MatrixMorphism_abstract):
             A = A.matrix()
         R = A.base_ring()
         if A.nrows() != parent.domain().rank():
-            raise ArithmeticError, "number of rows of matrix (=%s) must equal rank of domain (=%s)"%(A.nrows(), parent.domain().rank())
+            raise ArithmeticError("number of rows of matrix (={}) must equal rank of domain (={})".format(A.nrows(), parent.domain().rank()))
         if A.ncols() != parent.codomain().rank():
-                raise ArithmeticError, "number of columns of matrix (=%s) must equal rank of codomain (=%s)"%(A.ncols(), parent.codomain().rank())
+                raise ArithmeticError("number of columns of matrix (={}) must equal rank of codomain (={})".format(A.ncols(), parent.codomain().rank()))
         self._matrix = A
         MatrixMorphism_abstract.__init__(self, parent)
 

src/sage/schemes/generic/homset.py

@@ -255,6 +255,17 @@ class SchemeHomset_generic(HomsetWithBase):
     Element = SchemeMorphism
 
     def __reduce__(self):
+        """
+        Used in pickling.
+
+        EXAMPLES::
+
+            sage: A2 = AffineSpace(QQ,2)
+            sage: A3 = AffineSpace(QQ,3)
+            sage: Hom = A3.Hom(A2)
+            sage: loads(Hom.dumps()) == Hom
+            True
+        """
         #return SchemeHomset.reduce_data(self)
         return SchemeHomset, (self.domain(), self.codomain(), self.homset_category())