Describe an old but good rule for consistent return statements.

pep-0008.txt

@@ -1151,6 +1151,37 @@ Programming Recommendations
   closing the connection after a transaction.  Being explicit is
   important in this case.
 
+- Be consistent in return statements.  Either all return statements in
+  a function should return an expression, or none of them should.  If
+  any return statement returns an expression, any return statements
+  where no value is returned should explicitly state this as ``return
+  None``, and an explicit return statement should be present at the
+  end of the function (if reachable).
+
+  Yes::
+
+      def foo(x):
+          if x >= 0:
+              return math.sqrt(x)
+          else:
+              return None
+
+      def bar(x):
+          if x < 0:
+              return None
+          return math.sqrt(x)
+
+  No::
+
+      def foo(x):
+          if x >= 0:
+              return math.sqrt(x)
+
+      def bar(x):
+          if x < 0:
+              return
+          return math.sqrt(x)
+
 - Use string methods instead of the string module.
 
   String methods are always much faster and share the same API with