In Python, what is the difference between ".append()" and "+= []"?

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 

Answer 2

In the example you gave, there is no difference, in terms of output, between append and +=. But there is a difference between append and + (which the question originally asked about).

>>> 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 

As you can see, append and += have the same result; they add the item to the list, without producing a new list. Using + adds the two lists and produces a new list.