[python] Python에서“.append ()”와“+ = []”의 차이점은 무엇입니까?

귀하의 경우 유일한 차이점은 성능입니다. 추가는 두 배 빠릅니다.

Python 3.0 (r30:67507, Dec  3 2008, 20:14:27) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import timeit
>>> timeit.Timer('s.append("something")', 's = []').timeit()
0.20177424499999999
>>> timeit.Timer('s += ["something"]', 's = []').timeit()
0.41192320500000079

Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC v.1310 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import timeit
>>> timeit.Timer('s.append("something")', 's = []').timeit()
0.23079359499999999
>>> timeit.Timer('s += ["something"]', 's = []').timeit()
0.44208112500000141

일반적으로 append목록에 하나의 항목을 추가 하고 오른쪽 목록의 모든 요소를 왼쪽 목록에 +=복사 합니다.

업데이트 : 성능 분석

바이트 코드를 비교하면 append버전이 LOAD_ATTR+ CALL_FUNCTION에서 사이클을 낭비 하고 + = version-in BUILD_LIST. 분명히 + BUILD_LIST보다 큽니다 .LOAD_ATTRCALL_FUNCTION

>>> import dis
>>> dis.dis(compile("s = []; s.append('spam')", '', 'exec'))
  1           0 BUILD_LIST               0
              3 STORE_NAME               0 (s)
              6 LOAD_NAME                0 (s)
              9 LOAD_ATTR                1 (append)
             12 LOAD_CONST               0 ('spam')
             15 CALL_FUNCTION            1
             18 POP_TOP
             19 LOAD_CONST               1 (None)
             22 RETURN_VALUE
>>> dis.dis(compile("s = []; s += ['spam']", '', 'exec'))
  1           0 BUILD_LIST               0
              3 STORE_NAME               0 (s)
              6 LOAD_NAME                0 (s)
              9 LOAD_CONST               0 ('spam')
             12 BUILD_LIST               1
             15 INPLACE_ADD
             16 STORE_NAME               0 (s)
             19 LOAD_CONST               1 (None)
             22 RETURN_VALUE

LOAD_ATTR오버 헤드 를 제거하여 성능을 더욱 향상시킬 수 있습니다 .

>>> timeit.Timer('a("something")', 's = []; a = s.append').timeit()
0.15924410999923566

답변

>>> a = []
>>> id(a)
11814312
>>> a.append("hello")
>>> id(a)
11814312

>>> b = []
>>> id(b)
11828720
>>> c = b + ["hello"]
>>> id(c)
11833752
>>> b += ["hello"]
>>> id(b)
11828720

>>> a=[]
>>> a.append([1,2])
>>> a
[[1, 2]]
>>> a=[]
>>> a+=[1,2]
>>> a
[1, 2]

l= []

def a1(x):
    l.append(x) # works

def a2(x):
    l= l+[x] # assign to l, makes l local
             # so attempt to read l for addition gives UnboundLocalError

def a3(x):
    l+= [x]  # fails for the same reason

>>> l1= []
>>> l2= l1

>>> l1.append('x')
>>> l1 is l2
True

>>> l1= l1+['x']
>>> l1 is l2
False

>>> l2= l1
>>> l1+= ['x']
>>> l1 is l2
True

 some_list2 += ["something"]

 some_list2.extend(["something"])

myList = [ ]
listA = [1,2,3]
listB = ["a","b","c"]

>> myList.append(listA)
>> myList.append(listB)
>> myList
[[1,2,3],['a',b','c']]

>> myList += listA + listB
>> myList
[1,2,3,"a","b","c"]