How to Append One List to Another in Python

Key Takeaways

1. append() adds a list as a single nested element.
2. extend() and + operator add elements individually.
3. Use a loop with append() for custom processing.

In Python, lists are versatile data structures that allow you to store collections of items. Often, you may need to combine two lists. Python provides several methods to achieve this, each with its own use cases and behaviors. This article explores these methods in detail.

Using append()

The append() method adds its argument as a single element to the end of a list. When you use append() to add a list to another list, the entire list is added as a single element, resulting in a nested list.

Example:

list1 = ['apple', 'banana', 'cherry']
list2 = ['orange', 'grape']

list1.append(list2)

print(list1)
# Output: ['apple', 'banana', 'cherry', ['orange', 'grape']]

In this example, list2 is appended to list1 as a single element. The resulting list1 contains the original elements followed by list2 as a nested list.

Using extend()

If you want to add elements of one list to another individually, rather than as a single nested list, you can use the extend() method. This method iterates over its argument and adds each element to the list.

Example:

list1 = ['apple', 'banana', 'cherry']
list2 = ['orange', 'grape']

list1.extend(list2)

print(list1)
# Output: ['apple', 'banana', 'cherry', 'orange', 'grape']

Here, each element of list2 is added to list1 individually. The extend() method modifies the original list in place.

Using the + Operator

Another way to combine lists is by using the + operator, which concatenates lists and returns a new list.

Example:

list1 = ['apple', 'banana', 'cherry']
list2 = ['orange', 'grape']

combined_list = list1 + list2

print(combined_list)
# Output: ['apple', 'banana', 'cherry', 'orange', 'grape']

This method does not modify the original lists but creates a new list containing elements from both.

Using a Loop with append()

You can also use a loop to append each element of one list to another individually.

Example:

list1 = ['apple', 'banana', 'cherry']
list2 = ['orange', 'grape']

for item in list2:
    list1.append(item)

print(list1)
# Output: ['apple', 'banana', 'cherry', 'orange', 'grape']

This approach gives you more control over how elements are added, allowing for additional processing if needed.

Choosing the Right Method

• Use append() when you want to add the entire second list as a single element, resulting in a nested list.
• Use extend() or the + operator when you want to combine lists element-wise.
• Use a loop with append() when you need to add elements individually with additional processing.

Understanding these methods and their behaviors will help you manipulate lists effectively in Python.

FAQs

What's the difference between append() and extend()?

append() adds a list as one item, while extend() adds each item individually.

Does the + operator modify the original list?

No, it returns a new list without altering the originals.

When should I use a loop to add lists?

When you need to process or filter elements before appending.

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