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

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 win32Type "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()0.41192320500000079Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC v.1310 32 bit (Intel)] on win32Type "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

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.

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

See that append adds a single element to the list, which may be anything. +=[] joins the lists.