我有一个看起来像这样的代码:
def somefunction(somelist):
for item in somelist:
if len(item) > 10:
do something
elif len(item) > 6:
do something
elif len(item) > 3:
do something
else:
do something
因为我多次调用 len(item) ，这样做效率低吗？编写如下代码更好，还是它们在性能上完全相同？
def somefunction(somelist):
for item in somelist:
x = len(item)
if x > 10:
do something
elif x > 6:
do something
elif x > 3:
do something
else:
do something
请您参考如下方法:
第二种方法肯定更好，因为调用 len() 的次数减少了:
In [16]: import dis
In [18]: lis=["a"*10000,"b"*10000,"c"*10000]*1000
In [19]: def first():
for item in lis:
if len(item)<100:
pass
elif 100<len(item)<200:
pass
elif 300<len(item)<400:
pass
....:
In [20]: def second():
for item in lis:
x=len(item)
if x<100:
pass
elif 100<x<200:
pass
elif 300<x<400:
pass
....:
您始终可以使用 timeit 模块为您的代码计时:
In [21]: %timeit first()
100 loops, best of 3: 2.03 ms per loop
In [22]: %timeit second()
1000 loops, best of 3: 1.66 ms per loop
使用dis.dis()看Python字节码反汇编成助记词
In [24]: dis.dis(first)
2 0 SETUP_LOOP 109 (to 112)
3 LOAD_GLOBAL 0 (lis)
6 GET_ITER
>> 7 FOR_ITER 101 (to 111)
10 STORE_FAST 0 (item)
3 13 LOAD_GLOBAL 1 (len)
16 LOAD_FAST 0 (item)
19 CALL_FUNCTION 1
22 LOAD_CONST 1 (100)
25 COMPARE_OP 0 (<)
28 POP_JUMP_IF_FALSE 34
4 31 JUMP_ABSOLUTE 7
5 >> 34 LOAD_CONST 1 (100)
37 LOAD_GLOBAL 1 (len)
40 LOAD_FAST 0 (item)
43 CALL_FUNCTION 1
46 DUP_TOP
47 ROT_THREE
48 COMPARE_OP 0 (<)
51 JUMP_IF_FALSE_OR_POP 63
54 LOAD_CONST 2 (200)
57 COMPARE_OP 0 (<)
60 JUMP_FORWARD 2 (to 65)
>> 63 ROT_TWO
64 POP_TOP
>> 65 POP_JUMP_IF_FALSE 71
6 68 JUMP_ABSOLUTE 7
7 >> 71 LOAD_CONST 3 (300)
74 LOAD_GLOBAL 1 (len)
77 LOAD_FAST 0 (item)
80 CALL_FUNCTION 1
83 DUP_TOP
84 ROT_THREE
85 COMPARE_OP 0 (<)
88 JUMP_IF_FALSE_OR_POP 100
91 LOAD_CONST 4 (400)
94 COMPARE_OP 0 (<)
97 JUMP_FORWARD 2 (to 102)
>> 100 ROT_TWO
101 POP_TOP
>> 102 POP_JUMP_IF_FALSE 7
8 105 JUMP_ABSOLUTE 7
108 JUMP_ABSOLUTE 7
>> 111 POP_BLOCK
>> 112 LOAD_CONST 0 (None)
115 RETURN_VALUE
In [25]: dis.dis(second)
2 0 SETUP_LOOP 103 (to 106)
3 LOAD_GLOBAL 0 (lis)
6 GET_ITER
>> 7 FOR_ITER 95 (to 105)
10 STORE_FAST 0 (item)
3 13 LOAD_GLOBAL 1 (len)
16 LOAD_FAST 0 (item)
19 CALL_FUNCTION 1
22 STORE_FAST 1 (x)
4 25 LOAD_FAST 1 (x)
28 LOAD_CONST 1 (100)
31 COMPARE_OP 0 (<)
34 POP_JUMP_IF_FALSE 40
5 37 JUMP_ABSOLUTE 7
6 >> 40 LOAD_CONST 1 (100)
43 LOAD_FAST 1 (x)
46 DUP_TOP
47 ROT_THREE
48 COMPARE_OP 0 (<)
51 JUMP_IF_FALSE_OR_POP 63
54 LOAD_CONST 2 (200)
57 COMPARE_OP 0 (<)
60 JUMP_FORWARD 2 (to 65)
>> 63 ROT_TWO
64 POP_TOP
>> 65 POP_JUMP_IF_FALSE 71
7 68 JUMP_ABSOLUTE 7
8 >> 71 LOAD_CONST 3 (300)
74 LOAD_FAST 1 (x)
77 DUP_TOP
78 ROT_THREE
79 COMPARE_OP 0 (<)
82 JUMP_IF_FALSE_OR_POP 94
85 LOAD_CONST 4 (400)
88 COMPARE_OP 0 (<)
91 JUMP_FORWARD 2 (to 96)
>> 94 ROT_TWO
95 POP_TOP
>> 96 POP_JUMP_IF_FALSE 7
9 99 JUMP_ABSOLUTE 7
102 JUMP_ABSOLUTE 7
>> 105 POP_BLOCK
>> 106 LOAD_CONST 0 (None)
109 RETURN_VALUE
