Why this apparent assymetry in set operations?

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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):

>>> 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?

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