[New-bugs-announce] [issue7604] delattr __slots__ inconsistancy

Brian Harring report at bugs.python.org
Wed Dec 30 19:46:49 CET 2009

New submission from Brian Harring <ferringb at gmail.com>:

Everything I've read about __slots__, seen w/ them, etc, they're
effectively just a change in the underlying allocation- yes they can
limit the attributes, but that's about it.

Specifically, for general attribute access/mangling, best I can tell,
they're *supposed* to be exactly equivalent when manipulating the
object- the only difference being the backing store.

That said, delattr against slotted objects vs non slotted differs in a
rather buggy way- even worse, the behaviour differs on slotted classes
dependant on if you're delattr'ing a slotted attr or a nonslotted-
consider the following code.

class nonslotted(object):

class slotted(object):
  __slots__ = ("monkeys",)

  ns = nonslotted()
  assert not hasattr(ns, 'monkeys')
  del ns.monkeys
  raise AssertionError("this is unreachable")
except AttributeError:
  s = slotted()
  assert not hasattr(s, 'monkeys')
  del s.monkeys
  print "slotting causes delattr to not throw an AttributeError"
  # and now for the kicker
  del s.some_attr_that_is_not_slotted
except AttributeError:
  print "so... delattr results in AttributeError on a non-slotted attr,
but throws no AttributeError on a slotted attr..."

I'm guessing this isn't intentional/desired?

Confirmed on py2.6/py3.1 also; that said, I'd assume it affects all
versions of python that support __slots__...

components: Interpreter Core
messages: 97051
nosy: ferringb
severity: normal
status: open
title: delattr __slots__ inconsistancy
versions: Python 2.6, Python 3.1

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