Performance comment (oddity with set.update())

[John Arbash Meinel]

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I was just debugging some performance issues, and I came across something funny.

Specifically, calling 'set.update()' with a generator that returns nothing is
slower than building it into a list and then updating.

With python 2.4.3:

$ python -m timeit -s 'x = set(range(10000))' 'x.update([])'
1000000 loops, best of 3: 0.296 usec per loop
$ python -m timeit -s 'x = set(range(10000))' 'x.update(y for y in [])'
1000000 loops, best of 3: 0.837 usec per loop
$ python -m timeit -s 'x = set(range(10000))' 'x.update([y for y in []])'
1000000 loops, best of 3: 0.462 usec per loop



With 2.5.1 (on a different machine)
$ python -m timeit -s 'x = set(range(10000))' 'x.update([])'
1000000 loops, best of 3: 0.265 usec per loop
$ python -m timeit -s 'x = set(range(10000))' 'x.update(y for y in [])'
1000000 loops, best of 3: 0.717 usec per loop
$ python -m timeit -s 'x = set(range(10000))' 'x.update([y for y in []])'
1000000 loops, best of 3: 0.39 usec per loop


Anyway, might be something to keep in mind for some of our inner functions.

John
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