On a side note, I don't see too much point in having __except__ return something when it is otherwise easy to say:
def __except__(self, typ, val, tb): self.abort_transaction() raise typ, val, tb
It has to do with "Errors should never pass silently, unless explicitly silenced". Consider:
def __except__(self, typ, val, tb):
self.abort_transaction()With __except__ returning a value, the implicit 'return None' means that the exception is propagated by default. Without the 'suppress exception' boolean return value, this naive handler would not only abort the transaction, but swallow each and every exception that occured inside the 'with' block.
Another common error with a manual reraise would involve not including the traceback properly, leading to difficulties with debugging.
IOW, returning a value from __except__ should make the exception handlers cleaner, and easier to 'do right' (since reraising simply means returning a value that evaluates to False, or falling off the end of the function). Suppressing the exception would require actively adding 'return True' to the end of the handler.
But actually i'd like to to mention some other than transaction-use cases for __except__, for example with
class MyObject: def __except__(self, typ, val, tb): if isinstance(val, KeyboardInterrupt): raise # process exception and swallow it
s/raise/return True/ for the return value version.
def __getattr__(self, name): Key2AttributeError: return self._cache[key]
...
with an obvious __except__() implementation for Key2AttributeError.
Seeing this example has convinced me of something. PEP 310 should use the 'with' keyword, and 'expression block' syntax should be used to denote the 'default object' semantics proposed for Python 3K. For example:
class Key2AttributeError(object):
def __init__(self, obj, attr):
self:
.obj_type = type(obj)
.attr = attr
def __except__(self, ex_type, ex_val, ex_tb):
if isinstance(ex_type, KeyError):
self:
raise AttributeError("%s instance has no attribute %s"
% (.obj_type, .attr))
# Somewhere else. . . def __getattr__(self, name): with Key2AttributeError(self, key): self: return ._cache[key]
Cheers, Nick.
--
Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net
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