Investigating Adding and Appending to Lists

0. + and append

0.1. same behavior?

Example 0. + and append do the same thing

In [1]: list1 = [1, 2, 3, 4]

In [2]: list1 = list1 + [5]

In [3]: print list1
[1, 2, 3, 4, 5]

In [4]: list2 = [1, 2, 3, 4]

In [5]: list2.append(5)

In [6]: print list2
[1, 2, 3, 4, 5]

[2] & [4]:

From these two lines of codes, they do the same thing: add one more element to the list

0.2. Then, what’s the difference?

Difference 1:

• append mutates the lists.
• list1 = list1 + [element] creates a new list

Example 1. Difference 1

def proc(mylist):
    mylist.append(6)

def proc2(mylist):
    mylist  = mylist + [6]

In [9]: print list1
[1, 2, 3, 4, 5]

In [10]: proc(list1)

In [11]: print list1
[1, 2, 3, 4, 5, 6]

In [12]: print list2
[1, 2, 3, 4, 5]

In [13]: proc2(list2)

In [14]: print list2
[1, 2, 3, 4, 5]

[2] & [11]:

append mutates the lists. So, even though there is no return statement in proc function, we could still see the mutation on list1.

[5] & [14]:

list1 = list1 + [6] creates a new list. So, if there is no return statement, the list [1, 2, 3, 4, 5, 6] only exists inside the function as a newly created list. That’s why calling the variable outside of the function, it still prints [1, 2, 3, 4, 5].

Difference 2:

• append can only add one object to the list. It cannot concatenate two lists.
• + can concatenate two lists.

Example 2. Difference 2: append

In [15]: list2.append([7,8,9])

In [16]: list2
Out[16]: [1, 2, 3, 4, 5, 6, [7, 8, 9]]

[15] & [16]:

So here, append add a list ([7,8,9]) as one element to the list. It does not produce [1,2,3,4,5,6,7,8,9], which is the concatenation of two strings.

Example 3. Difference 2: +

In [17]: list1 = list1 + [7,8,9]

In [18]: list1
Out[18]: [1, 2, 3, 4, 5, 7, 8, 9]

[17] & [18]:

+ actually two lists: [1,2,3,4,5,6] and [7,8,9] to form a new list [1,2,3,4,5,6,7,8,9]

1. +=

Example 4. += works the same as append I

In [19]: list3 = [1,2,3,4]

In [20]: list3 += [5]

In [21]: list3
Out[21]: [1, 2, 3, 4, 5]

[19] & [21]:

Here, += works the same as append: add 5 to the end of the list

Example 5. += works the same as append II

def proc3(mylis):
    mylist += [6]

In [23]: print list3
[1, 2, 3, 4, 5]

In [24]: proc3(list3)

In [25]: print list3
[1, 2, 3, 4, 5, 6]

[25]:

so += works the same as append: it mutates the original list.

Example 6. += concatenates the lists

In [26]: list3 += [7,8,9]

In [27]: list3
Out[27]: [1, 2, 3, 4, 5, 6, 7, 8, 9]

2. Summary

In terms of mutation of the list:

• append and += mutates the list.
• + does not mutate the original list. Instead, it creates a new list.

In terms of modifying the list:

• append only add element to the list. It cannot concatenate the list.
• + and += can concatenate the list.