On Sun, 16 Jan 2005 12:18:23 GMT, "Raymond Hettinger" <[EMAIL PROTECTED]> wrote:
>"John Machin" <[EMAIL PROTECTED]> wrote in message >news:[EMAIL PROTECTED] >> 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)" > . . . >> >python -m timeit -s "orig=range(200000)" "lst=orig[:];lst[:]=(x for x >> in orig)" > >This has nothing to do with genexps and everything to do with list slice >assignment. > >List slice assignment is an example of a tool with a special case optimization >for inputs that know their own length -- that enables the tool to pre-allocate >its result rather than growing and resizing in spurts. Other such tools >include >tuple(), map() and zip(). > My reading of the source: if the input is not a list or tuple, a (temporary) tuple is built from the input, using PySequence_Tuple() in abstract.c. If the input cannot report its own length, then that function resorts to "growing and resizing in spurts", using the following code: if (j >= n) { if (n < 500) n += 10; else n += 100; if (_PyTuple_Resize(&result, n) != 0) { Perhaps it could be changed to use a proportional increase, like list_resize() in listobject.c, which advertises (amortised) linear time. Alternative: build a temporary list instead? -- http://mail.python.org/mailman/listinfo/python-list
