Python Lists: A Comprehensive Guide to Using Lists in Python
One of the most fundamental data structures in Python is the list. Lists are ordered collections of items, which can be of different data types, such as integers, strings, or even other lists. In this blog post, we will explore the basics of Python lists, including how to access, change, add, and remove list items.
Lists are created using square brackets [].
List Items
List items are:
- Ordered: The items have a defined order, and the new items will be placed at the end of the list.
- Changeable: We can change, add, and remove items in a list after it has been created.
- Allow duplicate values: Lists are indexed, meaning it can have items that have the same value.
List Length
To determine how many items a list contains, we can use Python's built-in function len(). The len function takes a list as an argument and returns an integer, indicating the number of items in that given list.
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
print(len(fruits)) #outputs: 5
Data Types
List items can be of any data type, including strings, numbers, booleans, and even other collection types:
list_1 = [10, 8, 6, 7, 25]
list_2 = ["string 1", "string 2", "string 3"]
list_3 = [True, False, True]
list_4 = [
[10, 8, 6, 7, 25],
["string 1", "string 2", "string 3"]
]
Accessing List Items
Accessing individual items in a list is a fundamental operation that you'll need to perform frequently. In Python, you can access a list item by referring to its index, which is an integer value that represents the item's position in the list. The first item in the list has an index of 0, the second item has an index of 1, and so on.
Here's an example:
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
print(fruits[0]) # Output: 'apple'
print(fruits[2]) # Output: 'cherry'
You can also access list items using negative indexing, which starts from the end of the list. The last item in the list has an index of -1, the second-to-last item has an index of -2, and so on.
Here's an example:
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
print(fruits[-1]) # Output: 'kiwi'
print(fruits[-3]) # Output: 'cherry'
Changing List Items
Lists are mutable, which means that you can change their contents after they have been created. To modify an item in a list, you can simply assign a new value to its index.
Here's an example:
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
fruits[1] = 'pear'
print(fruits) # Output: ['apple', 'pear', 'cherry', 'orange', 'kiwi']
Adding List Items
You can add new items to a list using several methods. One way is to use the append() method, which adds an item to the end of the list.
Here's an example:
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
fruits.append('pear')
print(fruits) # Output: ['apple', 'banana', 'cherry', 'orange', 'kiwi', 'pear']
Another way to add items to a list is to use the insert() method, which allows you to specify the index where the new item should be inserted.
Here's an example:
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
fruits.insert(1, 'pear')
print(fruits) # Output: ['apple', 'pear', 'banana', 'cherry', 'orange', 'kiwi']
Removing List Items
You can remove items from a list using the remove() method, which removes the first occurrence of the specified item.
Here's an example:
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
fruits.remove('banana')
print(fruits) # Output: ['apple', 'cherry', 'orange', 'kiwi']
Another way to remove items from a list is to use the del statement, which can remove an item at a specific index or remove the entire list.
Here's an example:
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
del fruits[1]
print(fruits) # Output: ['apple', 'cherry', 'orange', 'kiwi']
del fruits # remove the entire list
Loop Lists
To loop through the items of a list we can use a for loop.
Example: print all the items of the list, one by one
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
for fruit in fruits:
print(fruit) #outputs all list items one by one
A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).
Loop through index numbers
We can also loop through the list items by referring to their index number. By using Python's range() and len() functions we can iterate through the list items by using their index numbers.
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
for i in range(len(fruits)):
print(fruits[i]) #outputs all list items one by one
Using a while loop
We can also loop through the list items using a while loop, where we use the len() function to determine the length of the list, then start at index 0 and loop your way through the list items by referring to their indexes.
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
i = 0
while i < len(fruits):
print(fruits[i]) #outputs all list items one by one
i += 1
Remember to increase the index by 1, in this case the variable i, otherwise you will be stuck in an infinite loop.
Sort Lists
Python has a built-in function sort(), which sorts the list by default alphanumerically and ascending,
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
fruits.sort()
print(fruits) #outputs: ['apple', 'banana', 'cherry', 'kiwi', 'orange']
If you want to sort the items in reverse order(descending), pass the reverse = True argument to the sort method:
fruits = ['apple', 'banana', 'cherry', 'orange', 'kiwi']
fruits.sort(reverse = True)
print(fruits) #outputs: ['orange', 'kiwi', 'cherry', 'banana', 'apple']
Custom Sort Function
You can create a custom function of your own and pass it to the sort method as a keyword argument key = function, to sort the list items based on a condition you set.
def myFunction(n):
return abs(n - 50)
my_list = [100, 50, 65, 82, 23]
my_list.sort(key = myFunction)
print(my_list)
Case Insensitive Sort
By default the sort() built-in function is case-sensitive, meaning that all capital letters are sorted before lower case letters:
Example:
fruits = ['apple', 'banana', 'Cherry', 'orange', 'Kiwi']
fruits.sort()
print(fruits) #outputs: ['Cherry', 'Kiwi', 'apple', 'banana', 'orange']
So to sort the list case-insensitive, we can pass the str.lower built-in function as a key function in the sort() method:
fruits = ['apple', 'banana', 'Cherry', 'orange', 'Kiwi']
fruits.sort(key = str.lower)
print(fruits) #outputs: ['apple', 'banana', 'Cherry', 'Kiwi', 'orange']
Reverse Order
If you want to reverse the order of a list, you can use the reverse() method which reverses the current sorting order of the elements regardless of the alphabet.
fruits = ['apple', 'banana', 'Cherry', 'orange', 'Kiwi']
fruits.reverse()
print(fruits) #outputs: ['Kiwi', 'orange', 'Cherry', 'banana', 'apple']
Join Lists
There are several ways to join two or more lists Python:
Using the
+operator:fruits = ['apple', 'banana', 'cherry']
more_fruits = ['orange', 'kiwi']
all_fruits = fruits + more_fruits
print(all_fruits) #outputs: ['apple', 'banana', 'cherry', 'orange', 'kiwi']Using the
extend()method (this method modifies the original list):fruits = ['apple', 'banana', 'cherry']
more_fruits = ['orange', 'kiwi']
fruits.extend(more_fruits)
print(fruits) #outputs: ['apple', 'banana', 'cherry', 'orange', 'kiwi'](Not preferred) Another way is to iterate through the items of the list and append them to the wanted list, one by one:
fruits = ['apple', 'banana', 'cherry']
more_fruits = ['orange', 'kiwi']
for item in more_fruits:
fruits.append(item)
print(fruits) #outputs: ['apple', 'banana', 'cherry', 'orange', 'kiwi']
List Methods
Python has built-in functions that we can use on lists, some of which we already used on the above examples:
| Method | Description |
|---|---|
append() | Adds an element at the end of the list |
clear() | Removes all the elements from the list |
copy() | Returns a copy of the list |
count() | Returns the number of elements with the specified value |
extend() | Add the elements of a list (or any iterable), to the end of the current list |
index() | Returns the index of the first element with the specified value |
insert() | Adds an element at the specified position |
pop() | Removes the element at the specified position |
remove() | Removes the item with the specified value |
reverse() | Reverses the order of the list |
sort() | Sorts the list |
Conclusion
Python lists are versatile and powerful data structures that you will use frequently in your coding journey. Understanding how to access, change, add, and remove list items is essential for manipulating lists effectively. By mastering these fundamental skills, you'll be well on your way to becoming a proficient Python programmer.