Lists are one of the six fundamental data types in the Python programming language. To work effectively with Python, you need to know the functions and methods that work with lists. And that’s what we’ll explain in this article.
In Python, lists can be used to store multiple elements in a single variable. Moreover, a single Python iterate list can harbor elements of multiple data types. Lists (like arrays in other programming languages) can also be nested – i.e. lists can contain other lists.
Python provides multiple ways to iterate over lists; each one has its benefits and drawbacks. In this article, we shall look at how Python lists are iterated and present an example for each method. If this all seems new, we recommend trying our Learn Programming with Python track to get a head start in Python programming. This track will help you understand the fundamentals of programming, including lists and iteration.
Without further delay, let's dive right in!
7 Ways You Can Iterate Through a List in Python
1. A Simple for Loop
Using a Python
for loop is one of the simplest methods for iterating over a list or any other sequence (e.g. tuples, sets, or dictionaries).
Python for loops are a powerful tool, so it is important for programmers to understand their versatility. We can use them to run the statements contained within the loop once for each item in a list. For example:
fruits = ["Apple", "Mango", "Banana", "Peach"]for fruit in fruits: print(fruit)
Running the function results in the following output:
for loop has printed each of the list items. In other words, the loop has called the
print() function four times, each time printing the current item in the list – i.e. the name of a fruit.
2. List Comprehension
List comprehension is similar to the for loop; however, it allows us to create a list and iterate through it in a single line. Due to its utter simplicity, this method is considered one of the most robust ways of iterating over Python lists. Check out this article on lists and list comprehension in Python for more details. For now, let’s look at an example:
fruits = ["Apple", "Mango", "Banana", "Peach"][print(fruit + " juice") for fruit in fruits]
You’ll notice that we’re using what looks like another for loop:
for fruit in fruits. The key here is that the command and the
for..in structure are enclosed with the
print() command in square brackets; that’s what makes it a list comprehension.
Here’s the output:
Apple juiceMango juiceBanana juicePeach juice
As you can see, we created the fruits list just as we did in the previous example. However, this time we used list comprehension to do two things: add the word ‘juice’ to the end of the list item and print it.
3. A for Loop with range()
Another method for looping through a Python list is the
range() function along with a
range() generates a sequence of integers from the provided starting and stopping indexes. (An index refers to the position of elements in a list. The first item has an index of 0, the second list item is 1, and so on.) The syntax of the range function is as follows:
range(start, stop, step)
The start and step arguments are optional; only the stop argument is required. The step determines if you skip list items; this is set as 1 by default, meaning no items are skipped. If you only specify one parameter (i.e. the stop index), the function constructs a range object containing all elements from 0 to stop-1.
Here’s an example that will print the fruit name and its index in the list:
fruits = ["Apple", "Mango", "Banana", "Peach"]# Constructs range object containing elements from 0 to 3for i in range(len(fruits)): print("The list at index", i, "contains a", fruits[i])
This results in the following output:
The list at index 0 contains a AppleThe list at index 1 contains a Mango The list at index 2 contains a BananaThe list at index 3 contains a Peach
A slightly different approach would be to print only some of the fruits based on their index. We’d do this by specifying the starting and ending index for the for loop using the
fruits = ["Apple", "Mango", "Banana", "Peach"]# Constructs range object containing only 1 and 2for i in range(1, 3): print(fruits[i])
Here’s the output:
As we asked, it’s returned only those fruits at index 1 and 2; remember, 3 is the stopping point, and 0 is the first index in Python.
4. A for Loop with enumerate()
Sometimes you want to know the index of the element you are accessing in the list. The
enumerate() function will help you here; it adds a counter and returns it as something called an ‘enumerate object’. This object contains elements that can be unpacked using a simple Python for loop. Thus, an enumerate object reduces the overhead of keeping a count of the number of elements in a simple iteration.
Here’s an example:
fruits = ["Apple", "Mango", "Banana", "Peach"]for index, element in enumerate(fruits): print(index, ":", element)
Running the above code returns this list of the elements and their indexes:
0 : Apple1 : Mango2 : Banana3 : Peach
5. A for Loop with lambda
lambda function is an anonymous function in which a mathematical expression is evaluated and then returned. As a result,
lambda can be used as a function object. Let’s see how to use
lambda as we loop through a list.
We’ll make a
for loop to iterate over a list of numbers, find each number's square, and save or append it to the list. Finally, we’ll print a list of squares. Here’s the code:
lst1 = [1, 2, 3, 4, 5]lst2 =  # Lambda function to square numbertemp = lambda i:i**2for i in lst1: # Add to lst2 lst2.append(temp(i)) print(lst2)
lambda to iterate through the list and find the square of each value. To iterate through
for loop is used. Each integer is passed in a single iteration; the
append() function saves it to
We can make this code even more efficient using the
lst1 = [1, 2, 3, 4, 5] lst1 = list(map(lambda v: v ** 2, lst1)) print(lst1)
After applying the provided function to each item in a specified iterable,
map() produces a map object (which is an iterator) of the results.
Both these codes give the exact same output:
[1, 4, 9, 16, 25]
6. A while Loop
We can also iterate over a Python list using a
while loop. This is one of the first loops beginning programmers meet. It's also one of the easiest to grasp. If you consider the name of the loop, you'll soon see that the term "while" has to do with an interval or time period. The term "loop" refers to a piece of code that is executed repeatedly. So, a
while loop executes until a certain condition is met.
In the code below, that condition is the length of the list; the
i counter is set to zero, then it adds 1 every time the loop prints one item in the list. When
i becomes greater than the number of items in the list, the
while loop terminates. Check out the code:
fruits = ["Apple", "Mango", "Banana", "Peach"]i = 0while i < len(fruits): print(fruits[i]) i = i + 1
Can you guess what the output will be?
It is important to note the
i = i + 1 in the code above can also be shortened as
i += 1.
Our code ensures that the condition
i < len(fruits) will be satisfied after a certain number of iterations. Ending
while loops properly is critical; you can learn more about how to end loops in Python here.
7. The NumPy Library
The methods we’ve discussed so far used a small lists. However, efficiency is essential when you’re working with larger amounts of data. Suppose you have large single-dimensional lists with a single data type. In this case, an external library like NumPy is the best way to loop through big lists.
NumPy reduces the overhead by making iteration more efficient. This is done by converting the lists into NumPy arrays. As with lists, the for loop can also be used to iterate over these arrays.
It is important to note that the method we present here can only be used for arrays of single data types.
import numpy as npnums = np.array([1, 2, 3, 4, 5])for num in nums: print(num)
Running the code above gives the following output:
Although we’ve used
for num in nums: for its simplicity in this example, it’s usually better to use
for num in np.nditer(nums): when you’re working with large lists. The
np.nditer function returns an iterator that can traverse the NumPy array, which is computationally more efficient than using a simple for loop.
Time To Practice Lists and Loops in Python!
Python loops are useful because they allow you to repeat a piece of code. You'll frequently find yourself in circumstances where you'll need to perform the same operations over and over again; loops help you do so efficiently.
You now know many ways to use Python to loop through a list. If you want to practice what you’ve learned (and solidify your understanding of Python) check out our Python Practice Set. The exercises are straightforward and intuitive. Plus, there aren’t many tricky questions, and you will always be able to count on help and hints. So visit this course now and embark on your journey to become a Pythonista.