On Sat, 20 Jun 2009 21:26:56 +0100, Lorenzo Di Gregorio
<lorenzo.digrego...@gmail.com> wrote:
Thank you for your help: I'm working on a rather large source, but I
think I have isolated the problem now.
This listing generates an error:
-----------------------------------------------
class BaseA(object):
def __init__(self):
return
class DebugA(BaseA):
def __init__(self):
return
class B(object):
def __init__(self,test=A()):
self.obj = A()
return
if __name__ == "__main__":
# class A(BaseA): # Uncomment this for using BaseA objects
# pass
class A(DebugA): # Uncomment this for using DebugA objects
pass
-----------------------------------------------
The error happens because Python apparently evaluates the named
arguments before running the script.
I think I have read something about this some (long) time ago but I
can't find it anymore.
I could have sworn this was in the FAQ, but apparently not. You're
right, Python evaluates the default arguments when it executes the
`def` instruction. (Yes, `def` is an executable instruction. It's
got to create the function/method object after all!)
The usual fix is not to make the default an instance of A (which
will have all sorts of other side effects as well, since the same
instance of A will be shared by all the Bs that don't supply a
`test` parameter), but to use `None` as a marker.
class B(object):
def __init__(self, test=None):
if test is None:
test = A()
self.obj = A()
and so on.
--
Rhodri James *-* Wildebeest Herder to the Masses
--
http://mail.python.org/mailman/listinfo/python-list
