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

MRAB python at mrabarnett.plus.com
Fri Dec 11 20:43:35 CET 2009

John Arbash Meinel wrote:

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." To me the question was whether Python was behaving correctly; the behaviour was more subtle than the legendary mutable default argument.

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