Time difference between two alike methods to get length of iterable-CodePudding

I have come up with something rather odd. The goal is to evaluate the length of an iterable.

python -m timeit --setup="x = range(1000)" "x=list(x);len(x)"

1000000 loops, best of 3: 1.82 usec per loop

python -m timeit --setup="x = range(1000)" "len(list(x))"

100000 loops, best of 3: 9.92 usec per loop

Can anyone explain the reason the first method is quicker ?

I tried to look at assembly instructions, but it does not help understanding this behavior.

With: x=list(x);len(x)

>>> dis.dis(meth1)
  2           0 LOAD_GLOBAL              0 (list)
              2 LOAD_FAST                0 (it)
              4 CALL_FUNCTION            1
              6 STORE_FAST               1 (x)

  3           8 LOAD_GLOBAL              1 (len)
             10 LOAD_FAST                1 (x)
             12 CALL_FUNCTION            1
             14 RETURN_VALUE

With len(list(x)):

>>> dis.dis(meth2)
  2           0 LOAD_GLOBAL              0 (len)
              2 LOAD_GLOBAL              1 (list)
              4 LOAD_FAST                0 (it)
              6 CALL_FUNCTION            1
              8 CALL_FUNCTION            1
             10 RETURN_VALUE

CodePudding user response：

You have to consider the fact that setup is executed only once.

The test

--setup="x = range(1000)" "x=list(x);len(x)"

converts the range into a list and stores it back to x. Next iteration, it is already a list, so there's nothing more to do.

If you change that to a different variable name, the difference should be gone:

--setup="x = range(1000)" "something=list(x);len(something)"

In the other test

--setup="x = range(1000)" "len(list(x))"

x remains a range and needs to be converted into a list 100000 times.