append将元素添加到列表中。extend将第一个列表与另一个列表/可迭代列表连接起来。

>>> xs = ['A', 'B']
>>> xs
['A', 'B']

>>> xs.append("D")
>>> xs
['A', 'B', 'D']

>>> xs.append(["E", "F"])
>>> xs
['A', 'B', 'D', ['E', 'F']]

>>> xs.insert(2, "C")
>>> xs
['A', 'B', 'C', 'D', ['E', 'F']]

>>> xs.extend(["G", "H"])
>>> xs
['A', 'B', 'C', 'D', ['E', 'F'], 'G', 'H']

extend（L）通过附加给定列表L中的所有项目来扩展列表。

>>> a
[1, 2, 3]
a.extend([4])  #is eqivalent of a[len(a):] = [4]
>>> a
[1, 2, 3, 4]
a = [1, 2, 3]
>>> a
[1, 2, 3]
>>> a[len(a):] = [4]
>>> a
[1, 2, 3, 4]

append在列表末尾附加指定的对象：

>>> x = [1, 2, 3]
>>> x.append([4, 5])
>>> print(x)
[1, 2, 3, [4, 5]]

extend通过从指定的iterable中附加元素来扩展列表：

>>> x = [1, 2, 3]
>>> x.extend([4, 5])
>>> print(x)
[1, 2, 3, 4, 5]

这相当于使用+运算符进行追加和扩展：

>>> x = [1,2,3]
>>> x
[1, 2, 3]
>>> x = x + [4,5,6] # Extend
>>> x
[1, 2, 3, 4, 5, 6]
>>> x = x + [[7,8]] # Append
>>> x
[1, 2, 3, 4, 5, 6, [7, 8]]

以下两个片段在语义上是等价的：

for item in iterator:
    a_list.append(item)

and

a_list.extend(iterator)

后者可能更快，因为循环是在C中实现的。

append（object）通过将对象添加到列表来更新列表。

x = [20]
# List passed to the append(object) method is treated as a single object.
x.append([21, 22, 23])
# Hence the resultant list length will be 2
print(x)
--> [20, [21, 22, 23]]

extend（list）本质上连接两个列表。

x = [20]
# The parameter passed to extend(list) method is treated as a list.
# Eventually it is two lists being concatenated.
x.extend([21, 22, 23])
# Here the resultant list's length is 4
print(x)
--> [20, 21, 22, 23]