What's the difference between plus and append in python for list manipulation? [duplicate]

There are two major differences. The first is that + is closer in meaning to extend than to append:

>>> a = [1, 2, 3]
>>> a + 4
Traceback (most recent call last):
  File "<pyshell#13>", line 1, in <module>
    a + 4
TypeError: can only concatenate list (not "int") to list
>>> a + [4]
[1, 2, 3, 4]
>>> a.append([4])
>>> a
[1, 2, 3, [4]]
>>> a.extend([4])
>>> a
[1, 2, 3, [4], 4]

The other, more prominent, difference is that the methods work in-place: extend is actually like += - in fact, it has exactly the same behavior as += except that it can accept any iterable, while += can only take another list.

Using list.append modifies the list in place - its result is None. Using + creates a new list.

>>> L1 = [1,2,3]
>>> L2 = [97,98,99]
>>>
>>> # Mutate L1 by appending more values:
>>> L1.append(4)
>>> L1
[1, 2, 3, 4]
>>>
>>> # Create a new list by adding L1 and L2 together
>>> L1 + L2
[1, 2, 3, 4, 97, 98, 99]
>>> # L1 and L2 are unchanged
>>> L1
[1, 2, 3, 4]
>>> L2
[97, 98, 99]
>>>
>>> # Mutate L2 by adding new values to it:
>>> L2 += [999]
>>> L2
[97, 98, 99, 999]