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Fri Dec 11 19:36:52 CET 2009 · https://mail.python.org/pipermail/python-dev/2009-December/094513.html

Roy Hyunjin Han wrote:
> 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
> 

y[1].update(y.pop(1))

is going to be evaluating y[1] before it evaluates y.pop(1).
Which means that it has the original set returned, which is then removed
by y.pop, and updated.

You probably get the same behavior without using a defaultdict:
  y.setdefault(1, set()).update(y.pop(1))
  ^^^^^^^^^^^^^^^^^^^^^^- evaluated first

Oh and I should probably give the standard: "This list is for the
development *of* python, not development *with* python."

John
=:->

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Showing content from https://mail.python.org/pipermail/python-dev/2009-December/094513.html below:

[Python-Dev] Splitting something into two steps produces different behavior from doing it in one fell swoop in Python 2.6.2