L = [1,2,3,4]
L.append(13) # (1)
L = L + [13] # (2)
What is difference in above statements?
Asked
Active
Viewed 93 times
0
Alexei Levenkov
- 98,904
- 14
- 127
- 179
Memorial
- 57
- 6
-
There isn't really a difference, though in the latter it seems you are unnecessarily creating a new list `[13]` – MarkHarley Dec 13 '15 at 18:42
-
2And `+=` is different again – Padraic Cunningham Dec 13 '15 at 18:45
-
There is a huge difference, and `+=` is *not* different from the last line. – temporary_user_name Dec 13 '15 at 18:46
-
Are you asking the difference in the performance? – fodma1 Dec 13 '15 at 18:46
-
@Aerovistae, really? – Padraic Cunningham Dec 13 '15 at 18:47
-
3@Aerovistae: `+=` is different, try it ;) – Andrea Corbellini Dec 13 '15 at 18:47
-
I just did. It's not different. `L += [13]` is shorthand for `L = L + [13]`. – temporary_user_name Dec 13 '15 at 18:49
-
Also see http://stackoverflow.com/questions/725782/in-python-what-is-the-difference-between-append-and and http://stackoverflow.com/questions/3638486/what-is-the-difference-between-list-append1-and-list-list-1-python – Mazdak Dec 13 '15 at 18:50
-
1@Aerovistae, It's different in that it extends the existing list and doesn't create a new one, unlike `L = L + [13]`. – Mark Tolonen Dec 13 '15 at 18:50
-
@MarkTolonen INTERESTING!!! Didn't know *that.* – temporary_user_name Dec 13 '15 at 18:52
-
To check the difference use `print(id(L))` on `L` after each command. – bastelflp Dec 13 '15 at 18:59
2 Answers
5
L.append(13) appends 13 to an existing list.
L = L + [13] creates a new list.
stranac
- 26,638
- 5
- 25
- 30
-
2This is (at the time of this comment) the only answer that addresses the difference between mutating a list and assigning a new list value to the original name. – chepner Dec 13 '15 at 18:45
-
2
L.append(13) adds a single item, the int 13, to the end of the list.
L = L + [13] adds every item in a secondary list to the end of the first list. So you could have written L = L + [12, 4, 13] and it would add all three.
Moreover, append adds 13 to the end of an existing list....in the computer's memory, L is still the same list, just with a new item added into it. But whenever you use the = operator*, you're creating something new. So L = L + [13] is creating a new list in the computer's memory, assigning it the name L and filling it with the contents of the old L concatenated with the list [13].
*If you do var1 = var2, however, it's not creating something new but rather assigning the name var1 to point to the same place in memory as var2.
temporary_user_name
- 35,956
- 47
- 141
- 220
-
1"But whenever you use the = operator, you're creating something new." This glosses over the difference between rebinding and mutation, and makes it seem like `M = L` will create a new list in the computer's memory. – DSM Dec 13 '15 at 18:52