python - 在 Python 中， ".append()"和 "+= []"有什么区别？

Question

有什么区别:

some_list1 = []
some_list1.append("something")

和

some_list2 = []
some_list2 += ["something"]

Answer 1

对于您的情况，唯一的区别是性能:追加速度是原来的两倍。

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

一般情况下，append 将添加一项到列表中，而 += 将复制右侧列表的 所有 元素进入左侧列表。

更新:性能分析

比较字节码，我们可以假设 append 版本在 LOAD_ATTR + CALL_FUNCTION 和 += version -- 在 BUILD_LIST 中浪费了周期。显然 BUILD_LIST 胜过 LOAD_ATTR + CALL_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

我们可以通过移除 LOAD_ATTR 开销来进一步提高性能:

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