[Mark Nottingham]
I have to define a fairly large number (~40) of exceptions, and
need to keep a dictionary keyed on an error code for each, so that
the appropriate exception can be looked up. While I can manage
it manually, it would be much cleaner and more reliable to use
a metaclass to automatically create the dictionary from class
attributes.
Yes, it is a bit annoying that exceptions are not types. However, what
you could do with metaclasses for building a registry of exceptions,
you could achieve with module introspection. Here is some example code:
---------------------------------------------------------------------->
class Erreur(Exception):
diagnostic = "Erreur Pyam"
def __str__(self):
if self.args:
return self.diagnostic + ': ' + ', '.join(map(str, self.args))
return self.diagnostic
class ErreurDenotation(Erreur):
diagnostic = "Dénotation invalide"
# [etc.]
def fabriquer_erreur_selon_nom():
# J'aurais bien voulu donner une méta-classe à Erreur pour fabriquer le
# registre au vol, mais une classe dérivée de Exception _doit_ être une
# ancienne classe, pour des raisons techniques internes à Python.
from types import ClassType
for nom, valeur in globals().iteritems():
if isinstance(valeur, ClassType) and issubclass(valeur, Erreur):
erreur_selon_nom[nom] = valeur
erreur_selon_nom = {}
fabriquer_erreur_selon_nom()
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--
François Pinard
http://www.iro.umontreal.ca/~pinard 
