Unpickling Exception-type objects

[David McClosky]

Hi everyone,

I'm stumped over what is probably (hopefully?) a very simple problem.
Suppose I have a class that multiply inherits from two classes, call
them A and B. A inherits from a class that takes any number of
arguments in __init__. B does not inherit from anything and requires
one argument. For reasons I don't understand, if A inherits from
Exception, unpickling fails due to a TypeError:

__init__() takes exactly 2 arguments (1 given)

If A in inherits from a class that also takes any number of arguments,
the TypeError vanishes. To illustrate:

# ------------------------------------
import pickle

class Parent:
def __init__(self, *optional):
self.optional = optional

for Base in (Parent, Exception):
print "Base =", Base
print

class A(Base):
pass

class B:
def __init__(self, required):
self.required = required

class Child(A, B):
def __init__(self, *args):
A.__init__(self)
B.__init__(self, *args)

def test_dump(obj):
print 'obj:', type(obj), obj.__dict__
pickle.dump(obj, file('/tmp/pickle.p', 'w'), -1)
try:
loaded = pickle.load(file('/tmp/pickle.p'))
print 'loaded', type(loaded), loaded.__dict__
except TypeError, e:
print 'failed to load', e
print

test_dump(Child(7))
# ------------------------------------

I get the following output in python 2.5.2:

# ------------------------------------
Base = __main__.Parent

obj: <type 'instance'> {'required': 7, 'optional': ()}
loaded <type 'instance'> {'required': 7, 'optional': ()}

Base = <type 'exceptions.Exception'>

obj: <class '__main__.Child'> {'required': 7}
failed to load __init__() takes exactly 2 arguments (1 given)
# ------------------------------------

Any help on this would be greatly appreciated.

Thanks,
David