[Python-checkins] CVS: python/nondist/peps pep-0255.txt,1.7,1.8

[Tim Peters]

Update of /cvsroot/python/python/nondist/peps
In directory usw-pr-cvs1:/tmp/cvs-serv20081/peps

Modified Files:
	pep-0255.txt 
Log Message:
Add section explaining that yield *isn't* special wrt try/except/finally.


Index: pep-0255.txt
===================================================================
RCS file: /cvsroot/python/python/nondist/peps/pep-0255.txt,v
retrieving revision 1.7
retrieving revision 1.8
diff -C2 -r1.7 -r1.8
*** pep-0255.txt	2001/06/20 19:08:20	1.7
--- pep-0255.txt	2001/06/20 20:32:56	1.8
***************
*** 200,203 ****
--- 200,237 ----
  
  
+ Yield and Try/Except/Finally
+ 
+     While "yield" is a control-flow statement, and in most respects acts
+     like a "return" statement from the caller's point of view, within a
+     generator it acts more like a callback function.  In particular, it has
+     no special semantics with respect to try/except/finally.  This is best
+     illustrated by a contrived example; the primary lesson to take from
+     this is that using yield in a finally block is a dubious idea!
+ 
+     >>> def g():
+     ...     try:
+     ...         yield 1
+     ...         1/0      # raises exception
+     ...         yield 2  # we never get here
+     ...     finally:
+     ...         yield 3  # yields, and we raise the exception *next* time
+     ...     yield 4      # we never get here
+     >>> k = g()
+     >>> k.next()
+     1
+     >>> k.next()
+     3
+     >>> k.next() # as if "yield 3" were a callback, exception raised now
+     Traceback (most recent call last):
+       File "<stdin>", line 1, in ?
+       File "<stdin>", line 4, in g
+     ZeroDivisionError: integer division or modulo by zero
+     >>> k.next() # unhandled exception terminated the generator
+     Traceback (most recent call last):
+       File "<stdin>", line 1, in ?
+     StopIteration
+     >>>
+ 
+ 
  Example