{"id":672,"date":"2018-05-21T00:24:10","date_gmt":"2018-05-21T00:24:10","guid":{"rendered":"http:\/\/www.afternerd.com\/blog\/?p=672"},"modified":"2019-01-13T03:22:08","modified_gmt":"2019-01-13T03:22:08","slug":"append-vs-extend","status":"publish","type":"post","link":"https:\/\/www.afternerd.com\/blog\/append-vs-extend\/","title":{"rendered":"Python Lists: Append vs Extend (With Examples)"},"content":{"rendered":"

Two of the most common List<\/a> methods in Python are the append<\/span> and extend<\/span> methods.<\/p>\n

However, these two methods are the cause of a lot of confusion and misunderstanding among Python beginners.<\/p>\n

In this article, I will explain what each method does and show you exactly the difference between them.<\/p>\n

First: Append<\/h2>\n

\"\"<\/p>\n

The append<\/span> method is used to add an object to a list.<\/p>\n

This object can be of any data type<\/strong>, a string, an integer, a boolean, or even another list.<\/p>\n

Say you want to\u00a0append<\/strong>\u00a0an item to a list L>\/span> that initially has 4 elements<\/span><\/p>\n

>>> L = [1, 2, 3, 4]\r\n>>> L.append(5)\r\n>>> L\r\n[1, 2, 3, 4, 5]<\/code><\/pre>\n
As you can see, the append method adds the new item 5 to the list.<\/p>\n
Needless to say, the length of the list has increased by one (and only one) because the append method adds only one object<\/strong> to the list.<\/p>\n
This is an important distinction because you will see later that this is not necessarily the case with extend<\/span>.<\/p>\n
Alright, just out of curiosity let’s try to append a list<\/strong> to our list.<\/p>\n
>>> L = [1, 2, 3, 4]\r\n>>> L.append([5, 6, 7])\r\n>>> L\r\n[1, 2, 3, 4, [5, 6, 7]]<\/code><\/pre>\n
So what we did here is we appended one object<\/strong> (which happens to be of type list) to our list L<\/span><\/p>\n
Again, after the modification the list length grew by only one.<\/p>\n
Now let’s take a look at a similar, yet different, method.<\/p>\n
Second: Extend<\/h2>\n
\"\"<\/p>\n
extend<\/span> is another very common list method.<\/p>\n
Unlike append that can take an object of any type<\/strong> as an argument, extend can only take an iterable object<\/a> as an argument.<\/p>\n
An iterable object is an object that you can iterate through like strings, lists, tuples, dicts, or any object with the __iter__()<\/span> method.<\/p>\n
What extend<\/span> does is very straightforward, it iterates through the iterable<\/strong> object one item at a time and appends each item to the list.<\/p>\n
For example, let’s try to extend a list by another list.<\/p>\n
>>> L = [1, 2, 3, 4]\r\n>>> L.extend([5, 6, 7])\r\n>>> L\r\n[1, 2, 3, 4, 5, 6, 7]<\/code><\/pre>\n
As you can see in the example above, extend<\/span> takes a list (which is an iterable) as an argument and appends each item of the list to L.<\/p>\n
Three integer objects were appended to the list and the list size grew by three.<\/p>\n
This behavior is obviously different from that of the append<\/span> method.<\/p>\n
Let’s look at another example with a different iterable object, strings.<\/p>\n
>>> L = [1, 2, 3, 4]\r\n>>> L.extend(\"hi\")\r\n>>> L\r\n[1, 2, 3, 4, 'h', 'i']<\/code><\/pre>\n
Same thing!<\/p>\n
extend<\/span> iterates through the characters of “hi” and appends each character to L.<\/p>\n
Side Note: How is Extend different from the (+) Operator?<\/h2>\n
One question you might ask is how the extend method is different from using the + operator.<\/p>\n
>>> L = [1, 2, 3, 4]\r\n>>> L + [5, 6, 7]\r\n[1, 2, 3, 4, 5, 6, 7]<\/code><\/pre>\n
This above code seems to have the same effect of extending a list by another list, right?<\/p>\n
Well there are two major differences:<\/p>\n
1. you can’t use the (+) operator to extend a list by any iterable other than a list<\/h3>\n
For example, you can’t use the (+) operator to extend a list with a string, or a tuple, or a dict.<\/p>\n
2. extend modifies the list in place whereas the (+) operator creates a new list<\/h3>\n
>>> L = [1, 2, 3, 4]\r\n>>> L + [5, 6]\r\n[1, 2, 3, 4, 5, 6]\r\n>>> L\r\n[1, 2, 3, 4]<\/code><\/pre>\n
In the previous example, notice that L has not changed at all.<\/p>\n
This is because the plus operator creates a new list object that holds the concatenated list.<\/p>\n
To use the resulting list, you might need to store it in another variable first.<\/p>\n
>>> c = L + [5, 6]\r\n>>> c\r\n[1, 2, 3, 4, 5, 6]<\/code><\/pre>\n
Read:<\/strong> Python Lists vs Tuples<\/a><\/p><\/blockquote>\n
Conclusion<\/h2>\n
1- Both extend<\/strong> and append<\/strong> are list methods that are used to add items to a list.<\/p>\n
2- append<\/strong> adds one object of any type to a list.<\/p>\n
3- extend<\/strong> operates on iterable objects<\/em> and appends every item in the iterable to the list.<\/p>\n
4- using the (+) operator is not equivalent to using the extend<\/strong> method.<\/p>\n
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