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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