[Python-checkins] commit of r41343 - peps/trunk

[brett.cannon@python.org]

Author: brett.cannon
Date: Fri Oct 28 21:35:57 2005
New Revision: 41343

Modified:
   peps/trunk/pep-0352.txt
Log:
Merge in suggestions from Neal Norwitz.

- Simplified __init__ and fixed __getitem__
- Added another way to catch all exceptions sans KeyboardInterrupt and
  SystemExit as Python stands now (does not invalidate argument that it it
  needlessly explicit)
- Added a diagram of how the exception hierarchy will look
- Small grammar and clarification fixes


Modified: peps/trunk/pep-0352.txt
==============================================================================
--- peps/trunk/pep-0352.txt	(original)
+++ peps/trunk/pep-0352.txt	Fri Oct 28 21:35:57 2005
@@ -22,10 +22,10 @@
 inherit from.  Imposing the restriction will allow a standard
 interface for exceptions to exist that can be relied upon.
 
-On might counter that requiring a specific base class for a particular
-interface is unPythonic.  However, in the specific case of exceptions
-there's a good reason (which has generally been agreed to on
-python-dev): requiring hierarchy helps code that wants to *catch*
+One might counter that requiring a specific base class for a
+particular interface is unPythonic.  However, in the specific case of
+exceptions there's a good reason (which has generally been agreed to
+on python-dev): requiring hierarchy helps code that wants to *catch*
 exceptions by making it possible to catch *all* exceptions explicitly
 by writing ``except BaseException:`` instead of
 ``except *:``. [#hierarchy-good]_
@@ -48,7 +48,8 @@
 =============================
 
 This PEP proposes introducing a new exception named BaseException that
-is a new-style class and has a single attribute, ``message``::
+is a new-style class and has a single attribute, ``message`` (that
+will cause the deprecation of the existing ``args`` attribute)::
 
   class BaseException(object):
 
@@ -65,9 +66,7 @@
       def __init__(self, message='', *args):
           """Set 'message' and 'args' attribute"""
           self.message = message
-	  self.args = (message,)
-	  if args:
-	      self.args += args
+	  self.args = (message,) + args
 
       def __str__(self):
 	  """Return the str of 'message'"""
@@ -97,10 +96,7 @@
 	  deprecated.
 
 	  """
-          if index == 0:
-	      return self.message
-	  else:
-	      return self.args[index-1]
+	  return self.args[index]
 
 
 The ``message`` attribute will contain either the argument passed in
@@ -114,13 +110,13 @@
 This provides backwards-compatibility with how the argument passed
 into Exception has no restrictions.
 
-The ``args`` attribute is to be deprecated.  While allowing multiple
+The ``args`` attribute is deprecated.  While allowing multiple
 arguments to be passed can be helpful, it is in no way essential.  It
-also does not make it necessarily clear which argument is going to be
-represented by the ``__str__`` method.  Restricting to a single
-argument keeps the API simple and clear.  This also means providing a
-``__getitem__`` method is unneeded for exceptions and thus will be
-deprecated as well.
+also does not make it clear which argument is going to be represented
+by the ``__str__`` method.  Restricting initialization to accepting a
+single argument keeps the API simple and clear.  This also means
+providing a ``__getitem__`` method is unneeded for exceptions and thus
+will be deprecated as well.
 
 The ``raise`` statement will be changed to require that any object
 passed to it must inherit from BaseException.  This will make sure
@@ -141,11 +137,27 @@
 basic root, a change to the existing hierarchy is called for.  As it
 stands now, if one wants to catch all exceptions that signal an error
 *and* do not mean the interpreter should be allowed to exit, you must
-specify all but two exceptions specifically in an ``except`` clause.
+specify all but two exceptions specifically in an ``except`` clause
+or catch the two exceptions separately and then re-raise them and
+have all other exceptions fall through to a bare ``except`` clause::
+
+ except (KeyboardInterrupt, SystemExit):
+     raise
+ except:
+     ...
+
 That is needlessly explicit.  This PEP proposes moving
 KeyboardInterrupt and SystemExit to inherit directly from
 BaseException.
 
+::
+
+  - BaseException
+    |- KeyboardInterrupt
+    |- SystemExit
+    |- Exception
+       |- (all other current built-in exceptions)
+
 Doing this makes catching Exception more reasonable.  It would catch
 only exceptions that signify errors.  Exceptions that signal that the
 intepreter should exit will not be caught and thus be allowed to