[issue32945] sorted(generator) is slower than sorted(list-comprehension)

[Raymond Hettinger]

Raymond Hettinger <raymond.hettinger at gmail.com> added the comment:

FYI, here is the disassembly of the inner code objects.  It shows where the difference in speed arises:

>>> from dis import dis
>>> def f():
	lc = [i for i in [1, 2]]
	ge = (i for i in [1, 2])
	return lc, ge

>>> for obj in f.__code__.co_consts:
	if type(obj) == type(f.__code__):
		print(obj)
		dis(obj)

		
<code object <listcomp> at 0x113d881e0, file "<pyshell#26>", line 2>
  2           0 BUILD_LIST               0
              2 LOAD_FAST                0 (.0)
        >>    4 FOR_ITER                 8 (to 14)
              6 STORE_FAST               1 (i)
              8 LOAD_FAST                1 (i)
             10 LIST_APPEND              2           <-- Append directly
             12 JUMP_ABSOLUTE            4
        >>   14 RETURN_VALUE
<code object <genexpr> at 0x1035afe40, file "<pyshell#26>", line 3>
  3           0 LOAD_FAST                0 (.0)
        >>    2 FOR_ITER                10 (to 14)
              4 STORE_FAST               1 (i)
              6 LOAD_FAST                1 (i)
              8 YIELD_VALUE                          <-- Pass data through iterator
             10 POP_TOP                              <-- Dispose of None from send()
             12 JUMP_ABSOLUTE            2
        >>   14 LOAD_CONST               0 (None)
             16 RETURN_VALUE

The list comprehension builds the list directly with the high-speed specialized, LIST_APPEND opcode.

The generator runs YIELD_VALUE and POP_TOP but the work isn't done.  There needs to be a context switch to list_extend() which then extract the value from the iterator and finally appends it to the list.

Executive summary: there is overhead when passing data through the iterator protocol.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32945>
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