[New-bugs-announce] [issue8743] set() operators don't work with collections.Set instances
report at bugs.python.org
Mon May 17 20:50:46 CEST 2010
New submission from Daniel Stutzbach <daniel at stutzbachenterprises.com>:
The set() operators (__or__, __and__, __sub__, __xor__, and their in-place counterparts) require that the parameter also be an instance of set().
They're documented that way: "This precludes error-prone constructions like set('abc') & 'cbs' in favor of the more readable set('abc').intersection('cbs')."
However, an unintended consequence of this behavior is that they don't inter-operate with user-created types that derive from collections.Set.
That leads to oddities like this:
MySimpleSet() | set() # This works
set() | MySimpleSet() # Raises TypeError
(MySimpleSet is a minimal class derived from collections.Set for illustrative purposes -- set attached file)
collections.Set's operators accept any iterable.
I'm not 100% certain what the correct behavior should be. Perhaps set's operators should be a bit more liberal and accept any collections.Set instance, while collections.Set's operators should be a bit more conservative. Perhaps not. It's a little subjective.
It seems to me that at minimum set() and collections.Set() should inter-operate and have the same behavior.
components: Interpreter Core
stage: unit test needed
title: set() operators don't work with collections.Set instances
versions: Python 2.7, Python 3.2
Added file: http://bugs.python.org/file17383/set-vs-set.py
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