On 2005 Jan 12, at 05:11, Guido van Rossum wrote: ...
I had exactly the same metabug in the pep 246 reference implementation,
Armin Rigo showed how to fix it in his only recent post.
Don't recall seeing that, but if you or he can fix this without breaking other stuff, it's clear you should go ahead. (This worked in 2.2, FWIW; it broke in 2.3.)
Armin's fix was to change:
conform = getattr(type(obj), '__conform__', None)
into:
for basecls in type(obj).__mro__:
if '__conform__' in basecls.__dict__:
conform = basecls.__dict__['__conform__']
break
else:
# not foundI have only cursorily examined the rest of the standard library, but it seems to me there may be a few other places where getattr is being used on a type for this purpose, such as pprint.py which has a couple of occurrences of
r = getattr(typ, "__repr__", None)
Since this very same replacement is needed in more than one place for "get the following special attribute from the type of the object', it seems that a function to do it should be introduced in one place and used from where it's needed:
def get_from_first_dict(dicts, name, default=None):
for adict in dicts:
try:
return adict[name]
except KeyError:
pass
return defaultto be called, e.g. in the above example with '__conform__', as:
conform = get_from_first_dict(
(basecls.__dict__ for basecls in type(obj).__mro__),
'__conform__'
)The needed function could of course be made less general, by giving more work to the function and less work to the caller, all the way down to:
def getspecial(obj, name, default=None):
for basecls in type(obj).__mro__:
try:
return basecls.__dict__[name]
except KeyError:
pass
return defaultto be called, e.g. in the above example with '__conform__', as:
conform = getspecial(obj, '__conform__')
This has the advantage of not needing the genexp, so it's usable to implement the fix in 2.3.5 as well as in 2.4.1. Moreover, it can specialcase old-style class instances to provide the backwards compatible behavior, if desired -- that doesn't matter (but doesn't hurt) to fix the bug in copy.py, because in that case old-style instances have been already specialcases previously, and might help to avoid breaking anything in other similar bugfixes, so that's what I would suggest:
def getspecial(obj, name, default=None):
if isinstance(obj, types.InstanceType):
return getattr(obj, name, default)
for basecls in type(obj).__mro__:
try:
return basecls.__dict__[name]
except KeyError:
pass
return defaultThe tradeoff between using type(obj) and obj.__class__ isn't crystal-clear to me, but since the latter might apparently be faked by some proxy to survive isinstance calls type(obj) appears to me to be right.
Where in the standard library to place this function is not clear to me either. Since it's going into bugfix-only releases, I assume it shouldn't be "published". Maybe having it as copy._getspecial (i.e. with a private name) is best, as long as it's OK to introduce some coupling by having (e.g.) pprint call copy._getspecial too.
Performance might be a problem, but the few bugfix locations where a getattr would be replaced by this getspecial don't seem to be hotspots, so maybe we don't need to worry about it for 2.3 and 2.4 (it might be nice to have this functionality "published" in 2.5, of course, and then it should probably be made fast).
Feedback welcome -- the actual patch will doubtless be tiny, but it would be nice to have it right the first time (as it needs to go into both the 2.3 and 2.4 bugfix branches and the 2.5 head).
Alex
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