Why this apparent assymetry in set operations?

[Colin J. Williams]

Neil Cerutti wrote:
> On Jan 15, 2008 10:10 AM,  <skip at pobox.com> wrote:
>> I've noticed that I can update() a set with a list but I can't extend a set
>> with a list using the |= assignment operator.
>>
>>     >>> s = set()
>>     >>> s.update([1,2,3])
>>     >>> s
>>     set([1, 2, 3])
>>     >>> s |= [4,5,6]
>>     Traceback (most recent call last):
>>       File "<stdin>", line 1, in <module>
>>     TypeError: unsupported operand type(s) for |=: 'set' and 'list'
>>     >>> s |= set([4,5,6])
>>     >>> s
>>     set([1, 2, 3, 4, 5, 6])
>>
>> Why is that?  Doesn't the |= operator essentially map to an update() call?
> 
> No, according to 3.7 Set Types, s | t maps to s.union(t).
> 
If the RHS is a set then it works OK:

*** Python 2.5.1 (r251:54863, Apr 18 
2007, 08:51:08) [MSC v.1310 32 bit 
(Intel)] on win32. ***
>>> import sets
>>> s1= sets.Set([2, 4, 5])
Set([2, 4, 5])
>>> s1= sets.Set([2, 4, 5])
>>> s2= sets.Set([4, 5, 6])
>>> s1|s2
Set([2, 4, 5, 6])
>>> s1|=s2
>>> s1
Set([2, 4, 5, 6])
>>> 

It could be modifies to handle any 
iterable on the RHS.

Colin W.