Note that 'yield' w/o an EXPR is currently illegal.

pep-0340.txt

@@ -383,13 +383,16 @@ Comparison to Thunks
 
 Examples
 
+    (Several of these examples contain "yield None".  If PEP 342 is
+    accepted, these can be changed to just "yield" of course.)
+
     1. A template for ensuring that a lock, acquired at the start of a
        block, is released when the block is left:
 
         def locking(lock):
             lock.acquire()
             try:
-                yield
+                yield None
             finally:
                 lock.release()
 
@@ -421,7 +424,7 @@ Examples
 
         def transactional(db):
             try:
-                yield
+                yield None
             except:
                 db.rollback()
                 raise
@@ -433,7 +436,7 @@ Examples
         def auto_retry(n=3, exc=Exception):
             for i in range(n):
                 try:
-                    yield
+                    yield None
                     return
                 except exc, err:
                     # perhaps log exception here
@@ -498,7 +501,7 @@ Examples
             save_stdout = sys.stdout
             try:
                 sys.stdout = new_stdout
-                yield
+                yield None
             finally:
                 sys.stdout = save_stdout
 

pep-0342.txt

@@ -140,6 +140,12 @@ Specification: Generators and Yield-Expressions
     are all illegal.  (Some of the edge cases are motivated by the
     current legality of "yield 12, 42".)
 
+    Note that a yield-statement or yield-expression without an
+    expression is now legal.  This makes sense: when the information
+    flow in the next() call is reversed, it should be possible to
+    yield without passing an explicit value ("yield" is of course
+    equivalent to "yield None").
+
     When __next__() is called with an argument that is not None, the
     yield-expression that it resumes will return the argument.  If it
     resumes a yield-statement, the value is ignored (this is similar