Python's [<generator expression>] at least 3x faster than list(<generator expression>)?

Question

It appears that using [] around a generator expression (test1) behaves substantially better than putting it inside of list() (test2). The slowdown isn't there when I simply pass a list into list() for shallow copy (test3). Why is this?

Evidence:

from timeit import Timer

t1 = Timer("test1()", "from __main__ import test1")
t2 = Timer("test2()", "from __main__ import test2")
t3 = Timer("test3()", "from __main__ import test3")

x = [34534534, 23423523, 77645645, 345346]

def test1():
    [e for e in x]

print t1.timeit()
#0.552290201187


def test2():
    list(e for e in x)

print t2.timeit()
#2.38739395142

def test3():
    list(x)

print t3.timeit()
#0.515818119049

Machine: 64 bit AMD, Ubuntu 8.04, Python 2.7 (r27:82500)

Answer 1

Well, my first step was to set the two tests up independently to ensure that this is not a result of e.g. the order in which the functions are defined.

>python -mtimeit "x=[34534534, 23423523, 77645645, 345346]" "[e for e in x]"
1000000 loops, best of 3: 0.638 usec per loop

>python -mtimeit "x=[34534534, 23423523, 77645645, 345346]" "list(e for e in x)"
1000000 loops, best of 3: 1.72 usec per loop

Sure enough, I can replicate this. OK, next step is to have a look at the bytecode to see what's actually going on:

>>> import dis
>>> x=[34534534, 23423523, 77645645, 345346]
>>> dis.dis(lambda: [e for e in x])
  1           0 LOAD_CONST               0 (<code object <listcomp> at 0x0000000001F8B330, file "<stdin>", line 1>)
              3 MAKE_FUNCTION            0
              6 LOAD_GLOBAL              0 (x)
              9 GET_ITER
             10 CALL_FUNCTION            1
             13 RETURN_VALUE
>>> dis.dis(lambda: list(e for e in x))
  1           0 LOAD_GLOBAL              0 (list)
              3 LOAD_CONST               0 (<code object <genexpr> at 0x0000000001F8B9B0, file "<stdin>", line 1>)
              6 MAKE_FUNCTION            0
              9 LOAD_GLOBAL              1 (x)
             12 GET_ITER
             13 CALL_FUNCTION            1
             16 CALL_FUNCTION            1
             19 RETURN_VALUE

Notice that the first method creates the list directly, whereas the second method creates a genexpr object and passes that to the global list. This is probably where the overhead lies.

Note also that the difference is approximately a microsecond i.e. utterly trivial.

---

Other interesting data

This still holds for non-trivial lists

>python -mtimeit "x=range(100000)" "[e for e in x]"
100 loops, best of 3: 8.51 msec per loop

>python -mtimeit "x=range(100000)" "list(e for e in x)"
100 loops, best of 3: 11.8 msec per loop

and for less trivial map functions:

>python -mtimeit "x=range(100000)" "[2*e for e in x]"
100 loops, best of 3: 12.8 msec per loop

>python -mtimeit "x=range(100000)" "list(2*e for e in x)"
100 loops, best of 3: 16.8 msec per loop

and (though less strongly) if we filter the list:

>python -mtimeit "x=range(100000)" "[e for e in x if e%2]"
100 loops, best of 3: 14 msec per loop

>python -mtimeit "x=range(100000)" "list(e for e in x if e%2)"
100 loops, best of 3: 16.5 msec per loop

Answer 2

list(e for e in x) isn't a list comprehension, it's a genexpr object (e for e in x) being created and passed to the list factory function. Presumably the object creation and method calls create overhead.

Answer 3

In python list name must be looked up in the module and then in builtins. While you cannot change what a list comprehension means a list call must just be a standard lookup + function call as it could be redefined to be something else.

Looking at the vm code generated for a comprehension it can be seen that it is inlined while a call to list is a normal call.

>>> import dis
>>> def foo():
...     [x for x in xrange(4)]
... 
>>> dis.dis(foo)
  2           0 BUILD_LIST               0
              3 DUP_TOP             
              4 STORE_FAST               0 (_[1])
              7 LOAD_GLOBAL              0 (xrange)
             10 LOAD_CONST               1 (4)
             13 CALL_FUNCTION            1
             16 GET_ITER            
        >>   17 FOR_ITER                13 (to 33)
             20 STORE_FAST               1 (x)
             23 LOAD_FAST                0 (_[1])
             26 LOAD_FAST                1 (x)
             29 LIST_APPEND         
             30 JUMP_ABSOLUTE           17
        >>   33 DELETE_FAST              0 (_[1])
             36 POP_TOP             
             37 LOAD_CONST               0 (None)
             40 RETURN_VALUE        

>>> def bar():
...     list(x for x in xrange(4))
... 
>>> dis.dis(bar)
  2           0 LOAD_GLOBAL              0 (list)
              3 LOAD_CONST               1 (<code object <genexpr> at 0x7fd1230cf468, file "<stdin>", line 2>)
              6 MAKE_FUNCTION            0
              9 LOAD_GLOBAL              1 (xrange)
             12 LOAD_CONST               2 (4)
             15 CALL_FUNCTION            1
             18 GET_ITER            
             19 CALL_FUNCTION            1
             22 CALL_FUNCTION            1
             25 POP_TOP             
             26 LOAD_CONST               0 (None)
             29 RETURN_VALUE  

Answer 4

Your test2 is roughly equivalent to:

def test2():
    def local():
        for i in x:
            yield i
    return list(local())

The call overhead explains the increased processing time.