Question

What is the difference between:

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

and

some_list2 = [] some_list2 += ['something'] 

Answer 1

For your case the only difference is performance: append is twice as fast.

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 

In general case append will add one item to the list, while += will copy all elements of right-hand-side list into the left-hand-side list.

Update: perf analysis

Comparing bytecodes we can assume that append version wastes cycles in LOAD_ATTR + CALL_FUNCTION, and += version — in BUILD_LIST. Apparently BUILD_LIST outweighs 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 

We can improve performance even more by removing LOAD_ATTR overhead:

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