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Fri Dec 11 19:31:02 CET 2009 · https://mail.python.org/pipermail/python-dev/2009-December/094512.html

While debugging a network algorithm in Python 2.6.2, I encountered
some strange behavior and was wondering whether it has to do with some
sort of code optimization that Python does behind the scenes.

************
After initialization: defaultdict(<type 'set'>, {1: set([1])})
Popping and updating in two steps: defaultdict(<type 'set'>, {1: set([1])})
************
After initialization: defaultdict(<type 'set'>, {1: set([1])})
Popping and updating in one step: defaultdict(<type 'set'>, {})

import collections
print '************'
x = collections.defaultdict(set)
x[1].update([1])
print 'After initialization: %s' % x
items = x.pop(1)
x[1].update(items)
print 'Popping and updating in two steps: %s' % x
print '************'
y = collections.defaultdict(set)
y[1].update([1])
print 'After initialization: %s' % y
y[1].update(y.pop(1))
print 'Popping and updating in one step: %s' % y
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[Python-Dev] Splitting something into two steps produces different behavior from doing it in one fell swoop in Python 2.6.2