[Python-checkins] python/nondist/peps pep-0343.txt,1.19,1.20

[gvanrossum@users.sourceforge.net]

Update of /cvsroot/python/python/nondist/peps
In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv12669

Modified Files:
	pep-0343.txt 
Log Message:
Raise RuntimeError in close().  Add list of open issues.


Index: pep-0343.txt
===================================================================
RCS file: /cvsroot/python/python/nondist/peps/pep-0343.txt,v
retrieving revision 1.19
retrieving revision 1.20
diff -u -d -r1.19 -r1.20
--- pep-0343.txt	1 Jun 2005 21:01:11 -0000	1.19
+++ pep-0343.txt	2 Jun 2005 14:56:36 -0000	1.20
@@ -306,11 +306,9 @@
             except (GeneratorExit, StopIteration):
                 pass
             else:
-                raise TypeError("generator ignored GeneratorExit")
+                raise RuntimeError("generator ignored GeneratorExit")
             # Other exceptions are not caught
 
-    (XXX is TypeError an acceptable exception here?)
-
   New generator method: __del__()
 
     g.__del__() is an alias for g.close().  This will be called when
@@ -417,6 +415,50 @@
 
     OTOH such mistakes are easily diagnosed.
 
+Open Issues
+
+    Discussion on python-dev has revealed some open issues.  I list
+    them here, with my preferred resolution and its motivation.  The
+    PEP as currently written reflects this preferred resolution.
+
+    1. What exception should be raised by close() when the generator
+       yields another value as a response to the GeneratorExit
+       exception?
+
+       I originally chose TypeError because it represents gross
+       misbehavior of the generator function, which should be fixed by
+       changing the code.  But the with_template decorator class uses
+       RuntimeError for similar offenses.  Arguably they should all
+       use the same exception.  I'd rather not introduce a new
+       exception class just for this purpose, since it's not an
+       exception that I want people to catch: I want it to turn into a
+       traceback which is seen by the programmer who then fixes the
+       code.  So now I believe they should both raise RuntimeError.
+       There are some precedents for that: it's raised by the core
+       Python code in situations where endless recursion is detected,
+       and for uninitialized objects (and for a variety of
+       miscellaneous conditions).
+
+    2. Both the generator close() method and the __exit__() method of
+       the with_template decorator class catch StopIteration and
+       consider it equivalent to re-raising the exception passed to
+       throw().  Is allowing StopIteration right here?
+
+       This is so that a generator doing cleanup depending on the
+       exception thrown (like the transactional() example below) can
+       *catch* the exception thrown if it wants to and doesn't have to
+       worry about re-raising it.  I find this more convenient for the
+       generator writer.  Against this was brought in that the
+       generator *appears* to suppress an exception that it cannot
+       suppress: the transactional() example would be more clear
+       according to this view if it re-raised the original exception
+       after the call to db.rollback().  I personally would find the
+       requirement to re-raise the exception an annoyance in a
+       generator used as a with-template, since all the code after
+       yield is used for is cleanup, and it is invoked from a
+       finally-clause (the one implicit in the with-statement) which
+       re-raises the original exception anyway.
+
 Examples
 
     (Note: several of these examples contain "yield None".  If PEP 342