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Carmine Scarpitta
Carmine Scarpitta

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2 reasons to use extend() instead of append() in Python

Whether you are a Python noob or an expert, you have surely heard of list. list is one of Python's built-in data types.

Creating an empty list in Python is very simple:

my_list = []
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Once you have created an empty list, you can add items to it.

Python offers two methods for adding items to a list: append() and extend().

Understanding how these methods work is important to choose which one to use.

append()

append() adds an item to the end of the list. The item can be any Python object such as a number or a string.

The following example shows how to add the string 'hello' to the end of the my_list list.

my_list.append('hello')
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After the append() operation, my_list will contain a single element:

['hello']
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extend()

extend() adds all the items of an iterable to the end of a list. An iterable is a Python object capable of returning its members one at a time. Examples of iterables are lists, tuples and strings.

The following example adds several items to the end of the my_list list:

items = ['hello', 'world']
my_list.extend()
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After the extend() operation my_list will contain two elements:

['hello', 'world']
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Difference between append() and extend()

The main difference between append() and extend() is the number of items that can be added to the list. append() adds only a single item to the list. Instead, extend() can add all the items of an iterable to the list.

To add many items to a list using append(), we can define a loop, as shown in the following example:

items = ['hello', 'world']
for item in items:
    my_list.append(item)
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The above for loop iterates on the list of items and adds all the items to my_list one by one. Basically for each iteration it adds an item of items to my_list.

Are append() and extend() equivalent?

Not exactly. There are two differences:

  1. extend() makes the code more readable;
  2. extend() is more efficient than append() (i.e. it requires less time to add the items to the list).

Why extend() is more efficient?

When we want to add items to the list, the Python interpreter needs to expand the list. In order to expand the list, it allocates new memory and copies the new items to the newly allocated memory. This is common to both append() and extend() operations.

What happens when we add several items to a list using append() in a loop?

items = ['hello', 'world']
for item in items:
    my_list.append(item)
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In the above example, the Python interpreter iterates on each element of the items list. In each iteration, it allocates new memory in my_list. The size of the allocated memory is such as to contain an element of items.

First iteration: it allocates memory to store the 'hello' string and copies the 'hello' string to the newly allocated memory.

Second iteration: it allocates memory to store the 'world' string and copies the 'world' string to the newly allocated memory.

In conclusion, append() performs two allocate operations and two copy operations.

Now let's see what happens when we use extend() to add all the items to my_list:

items = ['hello', 'world']
my_list.extend(items)
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In this case, the Python interpreter allocates new memory to store both 'hello' and 'world' strings in my_list.

So, extend() performs only one allocate operation.

Conclusion

Adding many items to a list using append() in a loop can require many allocate operations. In order to mitigate this inefficiency, Python developers implemented a over-allocation, which reduces the number of allocate operations required to append elements to a list. Over-allocation is out of scope for this post.

Allocation operations can be very costly.

extend() performs only one allocate operation, which results in an improved efficiency. Consequently, less time is required to add the items to the list.

In conclusion, you should always use extend() to add a set of items to a list.

That's all for this post. If you liked this post, follow me on dev.to and Twitter (@cscarpitta94) to get notified when I publish new posts. 😊

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