Please consider the timings below, where a generator expression starts out slower than the equivalent list comprehension, and gets worse:
>python -m timeit -s "orig=range(100000)" "lst=orig[:];lst[:]=(x for x in orig)" 10 loops, best of 3: 6.84e+004 usec per loop >python -m timeit -s "orig=range(200000)" "lst=orig[:];lst[:]=(x for x in orig)" 10 loops, best of 3: 5.22e+005 usec per loop >python -m timeit -s "orig=range(300000)" "lst=orig[:];lst[:]=(x for x in orig)" 10 loops, best of 3: 1.32e+006 usec per loop >python -m timeit -s "orig=range(100000)" "lst=orig[:];lst[:]=[x for x in orig]" 10 loops, best of 3: 6.15e+004 usec per loop >python -m timeit -s "orig=range(200000)" "lst=orig[:];lst[:]=[x for x in orig]" 10 loops, best of 3: 1.43e+005 usec per loop >python -m timeit -s "orig=range(300000)" "lst=orig[:];lst[:]=[x for x in orig]" 10 loops, best of 3: 2.33e+005 usec per loop Specs: Python 2.4, Windows 2000, 1.4GHz Athlon chip, 768Mb of memory. Background: There was/is a very recent thread about ways of removing all instances of x from a list. /F proposed a list comprehension to build the result list. Given a requirement to mutate the original list, this necessitates the assignment to lst[:]. I tried a generator expression as well. However while the listcomp stayed competitive up to a million-element list, the genexp went into outer space, taking about 20 times as long. The above timeit runs show a simpler scenario where the genexp also seems to be going quadratic. Comments, clues, ... please. TIA, John -- http://mail.python.org/mailman/listinfo/python-list
