deepcopy raises TypeError for method/function?
aleaxit at yahoo.com
Fri Sep 10 08:17:44 CEST 2004
OKB (not okblacke) <BrenBarn at aol.com> wrote:
> Alex Martelli wrote:
> >> Does anyone have any pointers on this? It seems that
> >> certain types
> >> are safe to deepcopy (even if they're not actually copied) whereas
> >> others are not. Do I just have to manually typecheck for
> >> functions and methods? Are there any other types that might raise
> >> such an error?
> > You can use copy_reg to register an identity function as the way to
> > 'copy' function and methods, if that floats your boat. And sure,
> > there are other types that raise errors, imagine copying a file
> > object, a socket, ...!
> Wasn't someone just saying on another thread, though, that copy
> doesn't "officially" use copy_reg?
Right, it was me -- I was carping about the inconsistency between [a]
claiming that if something ain't in the docs it don't count, [b] telling
people to look in the sources as the ultimate reference. We're doing
both things, holding both contradictory stances at once, and I don't
like that:-(. This one bug in the docs we'll fix, of course, but the
overall strategy remains a problem.
> Anyway, I guess my real question is, is there any safe way to do
> something like
> for a in someList:
> b = copy.deepcopy(a)
> . . . without having to know ahead of time what kinds of objects
> are in someList? I could maybe use try/except to catch failure, but if
Failure is surely possible, since not all types are copyable. Yes, you
can use a try/except -- but if you enter the except clause the copy
won't have happened, so if you need it you're in trouble.
> deepcopy fails like it did with methods, will it always throw a
> TypeError, or might it raise some other kind of error? Alternatively,
TypeError would be normal, but I don't think it can be guaranteed.
In : class X(object):
...: def __init__(self): self.xxx = 44
...: def __getstate__(self): return self.xx # oops
Here, a copy.deepcopy(X()) will end with AttributeError, because of the
oops... for different kind of oopses you could surely have all kinds of
errors. 'except Exception:" thus seems sounder.
> is there a way to tell without actually attempting the deepcopy whether
> it will be able to copy the object or not?
None I can think of, sorry.
More information about the Python-list