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Python List Index: Find First, Last or All Occurrences

  • February 28, 2022 February 28, 2022

Python List Index Find First, Last or All Occurrences Cover Image

In this tutorial, you’ll learn how to use the Python list index method to find the index (or indices) of an item in a list . The method replicates the behavior of the indexOf() method in many other languages, such as JavaScript. Being able to work with Python lists is an important skill for a Pythonista of any skill level. We’ll cover how to find a single item, multiple items, and items meetings a single condition.

By the end of this tutorial, you’ll have learned:

  • How the Python list.index() method works
  • How to find a single item’s index in a list
  • How to find the indices of all items in a list
  • How to find the indices of items matching a condition
  • How to use alternative methods like list comprehensions to find the index of an item in a list

Table of Contents

Python List Index Method Explained

The Python list.index() method returns the index of the item specified in the list . The method will return only the first instance of that item. It will raise a ValueError is that item is not present in the list.

Let’s take a look at the syntax of the index() method:

Let’s break these parameters down a little further:

  • element= represents the element to be search for in the list
  • start= is an optional parameter that indicates which index position to start searching from
  • end= is an optional parameter that indicates which index position to search up to

The method returns the index of the given element if it exists. Keep in mind, it will only return the first index. Additionally, if an item doesn’t exist, a ValueError will be raised.

In the next section, you’ll learn how to use the .index() method.

Find the Index Position of an Item in a Python List

Let’s take a look at how the Python list.index() method works. In this example, we’ll search for the index position of an item we know is in the list.

Let’s imagine we have a list of the websites we open up in the morning and we want to know at which points we opened 'datagy' .

We can see that the word 'datagy' was in the first index position. We can see that the word 'twitter' appears more than once in the list. In the next section, you’ll learn how to find every index position of an item.

Finding All Indices of an Item in a Python List

In the section above, you learned that the list.index() method only returns the first index of an item in a list. In many cases, however, you’ll want to know the index positions of all items in a list that match a condition.

Unfortunately, Python doesn’t provide an easy method to do this. However, we can make use of incredibly versatile enumerate() function and a for-loop to do this . The enumerate function iterates of an item and returns both the index position and the value.

Let’s see how we can find all the index positions of an item in a list using a for loop and the enumerate() function:

Let’s break down what we did here:

  • We defined a function, find_indices() , that takes two arguments: the list to search and the item to find
  • The function instantiates an empty list to store any index position it finds
  • The function then loops over the index and item in the result of the enumerate() function
  • For each item, the function evaludates if the item is equal to the search term. If it is, the index is appended to the list
  • Finally, this list is returned

We can also shorten this list for a more compact version by using a Python list comprehension . Let’s see what this looks like:

One of the perks of both these functions is that when an item doesn’t exist in a list, the function will simply return an empty list, rather than raising an error.

Find the Last Index Position of an Item in a Python List

In this section, you’ll learn how to find the last index position of an item in a list. There are different ways to approach this. Depending on the size of your list, you may want to choose one approach over the other.

For smaller lists, let’s use this simpler approach:

In this approach, the function subtracts the following values:

  • len(search_list) returns the length of the list
  • 1 , since indices start at 0
  • The .index() of the reversed list

There are two main problems with this approach:

  • If an item doesn’t exist, an ValueError will be raised
  • The function makes a copy of the list. This can be fine for smaller lists, but for larger lists this approach may be computationally expensive.

Let’s take a look at another approach that loops over the list in reverse order. This saves the trouble of duplicating the list:

In the example above we loop over the list in reverse, by starting at the last index. We then evaluate if that item is equal to the search term. If it is we return the index position and the loop ends. Otherwise, we decrement the value by 1 using the augmented assignment operator .

Index of an Element Not Present in a Python List

By default, the Python list.index() method will raise a ValueError if an item is not present in a list. Let’s see what this looks like. We’ll search for the term 'pinterest' in our list:

When Python raises this error, the entire program stops. We can work around this by nesting it in a try-except block.

Let’s see how we can handle this error:

Working with List Index Method Parameters

The Python list.index() method also provides two additional parameters, start= and stop= . These parameters, respectively, indicate the positions at which to start and stop searching.

Let’s say that we wanted to start searching at the second index and stop at the sixth, we could write:

By instructing the method to start at index 2 , the method skips over the first instance of the string 'twitter' .

Finding All Indices of Items Matching a Condition

In this final section, we’ll explore how to find the index positions of all items that match a condition. Let’s say, for example, that we wanted to find all the index positions of items that contain the letter 'y' . We could use emulate the approach above where we find the index position of all items. However, we’ll add in an extra condition to our check:

The main difference in this function to the one shared above is that we evaluate on a more “fuzzy” condition.

In this tutorial, you learned how to use the index list method in Python. You learned how the method works and how to use it to find the index position of a search term. You also learned how to find the index positions of items that exist more than once, as well as finding the last index position of an item.

Finally, you learned how to handle errors when an item doesn’t exist as well as how to find the indices of items that match a condition.

Additional Resources

To learn more about related topics, check out the tutorials below:

  • Python Lists: A Complete Overview
  • Python Zip Lists – Zip Two or More Lists in Python
  • Python IndexError: List Index Out of Range Error Explained
  • Python List Index: Official Documentation

Nik Piepenbreier

Nik is the author of datagy.io and has over a decade of experience working with data analytics, data science, and Python. He specializes in teaching developers how to use Python for data science using hands-on tutorials. View Author posts

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Indexing in Python – A Complete Beginners Guide

Indexing In Python

Data structures in Python include lists, tuples, etc. These data structures can have multiple elements in them, each having some different properties but the problem is how to refer to a particular element from the hundreds of elements they contain. Here indexing comes into action. Indexing is a simple but fundamental concept that is important to learn before further processing with Python data structures.

This tutorial will explain everything you need to know about indexing in Python. But first, let’s take a quick look at iterables.

Prerequisite – What are Iterables?

Before we get started with indexing, let’s understand what iterables are and what is their main function. The knowledge of iterables is much needed to go behind indexing.

Iterables in Python

Iterables are a special type of objects in Python that you can iterate over. Meaning you can traverse through all the different elements or entities contained within the object. It can be easily achieved using the for loops .

Under the hood, what all these iterable items carry are two unique methods called __iter__() or __getitem__() that implement the Sequence Semantics .

Besides lists in Python, strings and tuples are also iterable.

Now that we know what Iterables are in Python. How is this related to indexing?

What is Indexing in Python?

Indexing in Python is a way to refer to the individual items within an iterable by their position. In other words, you can directly access your elements of choice within an iterable and do various operations depending on your needs.

Before we get into examples of Indexing in Python, there’s an important thing to Note:

In Python, objects are “zero-indexed” meaning the position count starts at zero. Many other programming languages follow the same pattern. So, if there are 5 elements present within a list. Then the first element (i.e. the leftmost element) holds the “zeroth” position, followed by the elements in the first, second, third, and fourth positions.

Python Index Method

The index of a specific item within a list can be revealed when the index() method is called on the list with the item name passed as an argument.

Where item is an element that index value we want to get.

Python Index Operator

The Python Index Operator is represented by opening and closing square brackets: []. The syntax, however, requires you to put a number inside the brackets.

Where n is just an integer number representing the position of the element we want to access.

We can see how our print function accesses different elements within our string object to get the specific characters we want.

Negative Indexing in Python

We’ve recently learned how to use indexing in Lists and Strings to get the specific items of our interest. Although in all our previous cases we’ve used a positive integer inside our index operator (the square brackets), it’s not necessarily needed to be that way.

Often, if we are interested in the last few elements of a list or maybe we just want to index the list from the opposite end, we can use negative integers. This process of indexing from the opposite end is called Negative Indexing.

Note: In negative Indexing, the last element is represented by -1 and not -0.

In this tutorial, we’ve learned that indexing is just a way of referencing the elements of an iterable. We have used the Python index() method to get the index of a particular element. We’ve also looked at the Python index operator and what negative indexing is. Hope you enjoyed the tutorial and learned how to implement indexing in your next project.

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The index() method returns the index of the specified element in the list .

Syntax of List index()

The syntax of the list index() method is:

list index() parameters

The list index() method can take a maximum of three arguments:

  • element - the element to be searched
  • start (optional) - start searching from this index
  • end (optional) - search the element up to this index

Return Value from List index()

  • The index() method returns the index of the given element in the list.
  • If the element is not found, a ValueError exception is raised.

Note: The index() method only returns the first occurrence of the matching element.

Example 1: Find the index of the element

Example 2: index of the element not present in the list, example 3: working of index() with start and end parameters.

Also Read: Python Program to Access Index of a List Using for Loop

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Python intro, next episode, lists and indexing.

Overview Teaching: 60 min Exercises: 0 min Questions How can we group similar values or data into one variable? Objectives Create lists to group similar values or items together Learn to modify lists by adding or deleting items Be able to access a specific item or items in a list using list indexing and slicing

The variable types we have seen so far, such as floats , ints (short for integers), and strings let us store one piece of data in a single variable. However, it is often useful to store multiple pieces of data in a single variable. For example, imagine we have calculated total colony area in pixels for four different images of plates. Instead of making four different variables, we can use a list to keep them all together. This is handy because we only have to keep track of one variable name! In the next lesson you’ll learn how to perform the same operations on each item in a list.

Creating lists

Lists are denoted using square brackets, with the items separated by commas. The code below will create and print a populated list called areas .

We can also make lists of strings, or even lists of lists!

It can also be useful to make an empty list:

This will seem more useful after we have talked about adding items to lists!

Modifying lists: adding, removing, and altering values

Add/delete objects (list.append(value), list.remove(value)

Unlike floats, ints, and strings, lists can be modified after we make them. For example, we can add objects using the append command:

Returns the output

Now, add an item:

This returns the output

We can even make an empty list and add objects to it!

We can also modify a list by removing items

0-based list indexing

We can also access specific items of the list based on their position in the list. We can use the list name followed by the number of the position we want in square brackets.

What do you think samples[1] will result in?

This code returns the output

Notice that giving the index “1” returned the second item in the list. This is because Python (like many programming languages) begins counting at 0 .

List Indexing How would you get the first item in the list samples ? How would you get the last item? Solution print ( samples [ 0 ]) #Returns the first item, at index position 0 print ( samples [ 2 ]) #Returns the first item, at index position 2 (the list contains 3 items) samples[0] returns the output Plate16 and samples[2] returns the output Plate17 .
Using -1 and negative indexes to count backwards The index -1 is a special index that always refers to the last item of the list. Similarly, we can use other negative numbers to count backwards from the end of a list. This can be very useful if we know we want to get items from the end of a list, but we don’t know the list’s length! print ( samples [ - 1 ]) Returns the output Plate17
List Indexing With Negative Numbers How would you use negative numbers to get the next-to-last item in the list samples ? How would you get the third-to-last item? Solution print ( samples [ - 2 ]) #Returns the next-to-last item print ( samples [ - 3 ]) #Returns the third-to-last item (which is the same as the first item in this example) samples[-2] returns the output Plate293 and samples[-3] returns the output Plate16 .

We can also use list indexing to replace specific items in the list. For example,

Accessing a range of objects with slicing (up to, but not inclusive)

We can also use similar syntax to return a slice or subsection of the list. We separate the first and last indices for the items we want with a colon. Note that the second index is up to, but not inclusive. For example,

This returns a list of areas[0] and areas[1] . It does not include areas[2] .

If we want to start at the beginning of the list, we can leave out the first coordinate. Similarly, if we want to start our slice at a specific index and go to the end of the list, we can leave out the second coordinate.

Finding the length of a list

We may not always know (or remember) the length of a list. If we need to get this number, we can use the len() function to find it.

Note that this returns the expected length of the list. Even though areas[3] is the last item of the list (0-based counting!), the length is returned as 4.

We can also use similar indexing principles to access specific characters or substrings of a string variable.

Indexing characters in a string Assume you have the following variable: element = 'oxygen' What are the values of the following? 1. element[:4] 2. element[4:] 3. element[:] Solution Create code with print statements to try this out. element[:4] starts at the beginning of the list and goes up to (but does not include) position 4, returning 'oxyg' element[4:] starts at position 4 ( 'e' ) and goes through the end of the string, returning 'en' element[:] starts at the beginning and goes through the end of the string, returning 'oxygen'
String and list indices Assume you have the following variable: element = 'oxygen' Explain what element[1:-1] does. Solution If you create and run code with a print statement, you’ll get the following output: element[1:-1] will return all but the first and last characters: 'xyge' . Remember that position 1 is the second character of the string (0-based counting!) and that the slice goes up to , but doesn’t include the character at the index position after the colon. Since element[-1] refers to the last character, the slice will stop just before 'n' , and only include up to the ‘e’ at the next-to-last position.
Adding a value to a list Assume you have the following list: areas = [8536.47, 11359.3, 17743.4] Create a new empty list areas2 = [] Now add items to areas2 so that it is identical to areas except that 7798.02 has been inserted between 8536.47 and 11359.3. You should refer to the list areas and avoid typing in numbers whenever possible. The final value of areas2 should be areas2 = [8536.47, 7798.02, 11359.3, 17743.4] Solution #Create the list areas areas = [ 8536.47 , 11359.3 , 17743.4 ] #Create the empty list areas2 areas2 = [] #Now, add items to areas2 areas2 . append ( areas [ 0 ]) areas2 . append ( 7798.02 ) areas2 . append ( areas [ 1 ]) areas2 . append ( areas [ 2 ]) #print areas and areas2 print ( 'the value of areas is' , areas ) print ( 'the value of areas2 is' , areas2 ) Note that this did not change the original list areas! You could also do it like this, using the .insert() command #Create the list areas areas = [8536.47, 11359.3, 17743.4] #Create the list areas2 by copying areas areas2 = areas #Now, use the insert command to insert the new number at index 1 areas2.insert(1, 7798.02) #print areas and areas2 print('the value of areas is', areas) print('the value of areas2 is', areas2) However, this will also change the original list areas ! This is because areas2=areas does not create a brand-new copy of the list. Instead, it points to the same location in memory as the original variable name areas .
Lists of lists The code below creates a list of lists: salsas = [[ 'tomatoes' , 'onion' , 'cilantro' , 'jalepeno' ], [ 'mango' , 'cucumber' , 'jalepeno' , 'red onion' , 'lime juice' ], [ 'avocado' , 'lime juice' , 'onion' , 'salt' , 'jalepeno' ]] print ( salsas [ 1 ]) #prints the second list (index 1 in the first list) print ( salsas [ 1 ][ 0 ]) #prints the first item (index 0) of the second list and returns the following output: [‘mango’, ‘cucumber’, ‘jalepeno’, ‘red onion’, ‘lime juice’] mango

Write code that will print each instance of the word 'jalepeno' in the list of lists salsas .

Solution salsas = [[ 'tomatoes' , 'onion' , 'cilantro' , 'jalepeno' ], [ 'mango' , 'cucumber' , 'jalepeno' , 'red onion' , 'lime juice' ], [ 'avocado' , 'lime juice' , 'onion' , 'salt' , 'jalepeno' ]] print ( salsas [ 0 ][ 3 ]) print ( salsas [ 1 ][ 2 ]) print ( salsas [ 2 ][ 4 ]) How does this code work? As we have seen before, salsas[0] , salsas[1] , and salsas[2] refer to specific items in the list salsas . But in this case, each item in salsas is itself a list! Therefore, salsas[0] refers to the list ['tomatoes', 'onion', 'cilantro', 'jalepeno'] . So salsas[0][3] refers to the item at index 3 of that list. The number in the first set of square brackets selects the individual list that we want to look at; the number in the second set of square brackets selects a specific item within that list.
Key Points Lists can be used to group numbers (ints or floats), strings, lists, or other kinds of values together in one variable Lists are ordered- objects stay in the order you add them Lists are indexed using 0-based indexing You can use square bracket notation to return a single item in a list or subset of items
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Python Indexerror: list assignment index out of range Solution

In python, lists are mutable as the elements of a list can be modified. But if you try to modify a value whose index is greater than or equal to the length of the list then you will encounter an Indexerror: list assignment index out of range.  

Python Indexerror: list assignment index out of range Example

If ‘fruits’ is a list, fruits=[‘Apple’,’ Banana’,’ Guava’]and you try to modify fruits[5] then you will get an index error since the length of fruits list=3 which is less than index asked to modify for which is 5.

So, as you can see in the above example, we get an error when we try to modify an index that is not present in the list of fruits.

Method 1: Using insert() function

The insert(index, element) function takes two arguments, index and element, and adds a new element at the specified index.

Let’s see how you can add Mango to the list of fruits on index 1.

It is necessary to specify the index in the insert(index, element) function, otherwise, you will an error that the insert(index, element) function needed two arguments.

Method 2: Using append()

The append(element) function takes one argument element and adds a new element at the end of the list.

Let’s see how you can add Mango to the end of the list using the append(element) function.

Python IndexError FAQ

Q: what is an indexerror in python.

A: An IndexError is a common error that occurs when you try to access an element in a list, tuple, or other sequence using an index that is out of range. It means that the index you provided is either negative or greater than or equal to the length of the sequence.

Q: How can I fix an IndexError in Python?

A: To fix an IndexError, you can take the following steps:

  • Check the index value: Make sure the index you’re using is within the valid range for the sequence. Remember that indexing starts from 0, so the first element is at index 0, the second at index 1, and so on.
  • Verify the sequence length: Ensure that the sequence you’re working with has enough elements. If the sequence is empty, trying to access any index will result in an IndexError.
  • Review loop conditions: If the IndexError occurs within a loop, check the loop conditions to ensure they are correctly set. Make sure the loop is not running more times than expected or trying to access an element beyond the sequence’s length.
  • Use try-except: Wrap the code block that might raise an IndexError within a try-except block. This allows you to catch the exception and handle it gracefully, preventing your program from crashing.

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Lists are used to store multiple items in a single variable.

Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are Tuple , Set , and Dictionary , all with different qualities and usage.

Lists are created using square brackets:

Create a List:

List items are ordered, changeable, and allow duplicate values.

List items are indexed, the first item has index [0] , the second item has index [1] etc.

When we say that lists are ordered, it means that the items have a defined order, and that order will not change.

If you add new items to a list, the new items will be placed at the end of the list.

Note: There are some list methods that will change the order, but in general: the order of the items will not change.

The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.

Allow Duplicates

Since lists are indexed, lists can have items with the same value:

Lists allow duplicate values:

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List Length

To determine how many items a list has, use the len() function:

Print the number of items in the list:

List Items - Data Types

List items can be of any data type:

String, int and boolean data types:

A list can contain different data types:

A list with strings, integers and boolean values:

From Python's perspective, lists are defined as objects with the data type 'list':

What is the data type of a list?

The list() Constructor

It is also possible to use the list() constructor when creating a new list.

Using the list() constructor to make a List:

Python Collections (Arrays)

There are four collection data types in the Python programming language:

  • List is a collection which is ordered and changeable. Allows duplicate members.
  • Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
  • Set is a collection which is unordered, unchangeable*, and unindexed. No duplicate members.
  • Dictionary is a collection which is ordered** and changeable. No duplicate members.

*Set items are unchangeable, but you can remove and/or add items whenever you like.

**As of Python version 3.7, dictionaries are ordered . In Python 3.6 and earlier, dictionaries are unordered .

When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.

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Python indexerror: list assignment index out of range Solution

An IndexError is nothing to worry about. It’s an error that is raised when you try to access an index that is outside of the size of a list. How do you solve this issue? Where can it be raised?

In this article, we’re going to answer those questions. We will discuss what IndexErrors are and how you can solve the “list assignment index out of range” error. We’ll walk through an example to help you see exactly what causes this error.

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Without further ado, let’s begin!

The Problem: indexerror: list assignment index out of range

When you receive an error message, the first thing you should do is read it. An error message can tell you a lot about the nature of an error.

Our error message is: indexerror: list assignment index out of range.

IndexError tells us that there is a problem with how we are accessing an index . An index is a value inside an iterable object, such as a list or a string.

The message “list assignment index out of range” tells us that we are trying to assign an item to an index that does not exist.

In order to use indexing on a list, you need to initialize the list. If you try to assign an item into a list at an index position that does not exist, this error will be raised.

An Example Scenario

The list assignment error is commonly raised in for and while loops .

We’re going to write a program that adds all the cakes containing the word “Strawberry” into a new array. Let’s start by declaring two variables:

The first variable stores our list of cakes. The second variable is an empty list that will store all of the strawberry cakes. Next, we’re going to write a loop that checks if each value in “cakes” contains the word “Strawberry”.

If a value contains “Strawberry”, it should be added to our new array. Otherwise, nothing will happen. Once our for loop has executed, the “strawberry” array should be printed to the console. Let’s run our code and see what happens:

As we expected, an error has been raised. Now we get to solve it!

The Solution

Our error message tells us the line of code at which our program fails:

The problem with this code is that we are trying to assign a value inside our “strawberry” list to a position that does not exist.

When we create our strawberry array, it has no values. This means that it has no index numbers. The following values do not exist:

We are trying to assign values to these positions in our for loop. Because these positions contain no values, an error is returned.

We can solve this problem in two ways.

Solution with append()

First, we can add an item to the “strawberry” array using append() :

The append() method adds an item to an array and creates an index position for that item. Let’s run our code: [‘Strawberry Tart’, ‘Strawberry Cheesecake’].

Our code works!

Solution with Initializing an Array

Alternatively, we can initialize our array with some values when we declare it. This will create the index positions at which we can store values inside our “strawberry” array.

To initialize an array, you can use this code:

This will create an array with 10 empty values. Our code now looks like this:

Let’s try to run our code:

Our code successfully returns an array with all the strawberry cakes.

This method is best to use when you know exactly how many values you’re going to store in an array.

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Our above code is somewhat inefficient because we have initialized “strawberry” with 10 empty values. There are only a total of three cakes in our “cakes” array that could possibly contain “Strawberry”. In most cases, using the append() method is both more elegant and more efficient.

IndexErrors are raised when you try to use an item at an index value that does not exist. The “indexerror: list assignment index out of range” is raised when you try to assign an item to an index position that does not exist.

To solve this error, you can use append() to add an item to a list. You can also initialize a list before you start inserting values to avoid this error.

Now you’re ready to start solving the list assignment error like a professional Python developer!

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How to Fix “IndexError: List Assignment Index Out of Range” in Python

How to Fix “IndexError: List Assignment Index Out of Range” in Python

Table of Contents

The IndexError: List Assignment Index Out of Range error occurs when you assign a value to an index that is beyond the valid range of indices in the list. As Python uses zero-based indexing, when you try to access an element at an index less than 0 or greater than or equal to the list’s length, you trigger this error.

It’s not as complicated as it sounds. Think of it this way: you have a row of ten mailboxes, numbered from 0 to 9. These mailboxes represent the list in Python. Now, if you try to put a letter into mailbox number 10, which doesn't exist, you'll face a problem. Similarly, if you try to put a letter into any negative number mailbox, you'll face the same issue because those mailboxes don't exist either.

The IndexError: List Assignment Index Out of Range error in Python is like trying to put a letter into a mailbox that doesn't exist in our row of mailboxes. Just as you can't access a non-existent mailbox, you can't assign a value to an index in a list that doesn't exist.

Let’s take a look at example code that raises this error and some strategies to prevent it from occurring in the first place.

Example of “IndexError: List Assignment Index Out of Range”

Remember, assigning a value at an index that is negative or out of bounds of the valid range of indices of the list raises the error.

How to resolve “IndexError: List Assignment Index Out of Range”

You can use methods such as append() or insert() to insert a new element into the list.

How to use the append() method

Use the append() method to add elements to extend the list properly and avoid out-of-range assignments.

How to use the insert() method

Use the insert() method to insert elements at a specific position instead of direct assignment to avoid out-of-range assignments.

Now one big advantage of using insert() is even if you specify an index position which is way out of range it won’t give any error and it will just append the element at the end of the list.

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Python Find in List – How to Find the Index of an Item or Element in a List

In this article you will learn how to find the index of an element contained in a list in the Python programming language.

There are a few ways to achieve this, and in this article you will learn three of the different techniques used to find the index of a list element in Python.

The three techniques used are:

  • finding the index using the index() list method,
  • using a for-loop ,
  • and finally, using list comprehension and the enumerate() function.

Specifically, here is what we will cover in depth:

  • How indexing works
  • Use the index() method to find the index of an item 1. Use optional parameters with the index() method
  • Use a for-loop to get indices of all occurrences of an item in a list
  • Use list comprehension and the enumerate() function to get indices of all occurrences of an item in a list

What are Lists in Python?

Lists are a built-in data type in Python, and one of the most powerful data structures.

They act as containers and store multiple, typically related, items under the same variable name.

Items are placed and enclosed inside square brackets, [] . Each item inside the square brackets is separated by a comma, , .

From the example above, you can see that lists can contain items that are of any data type, meaning list elements can be heterogeneous.

Unlike arrays that only store items that are of the same type, lists allow for more flexibility.

Lists are also mutable , which means they are changeable and dynamic. List items can be updated, new items can be added to the list, and any item can be removed at any time throughout the life of the program.

An Overview of Indexing in Python

As mentioned, lists are a collection of items. Specifically, they are an ordered collection of items and they preserve that set and defined order for the most part.

Each element inside a list will have a unique position that identifies it.

That position is called the element's index .

Indices in Python, and in all programming languages, start at 0 and not 1 .

Let's take a look at the list that was used in the previous section:

The list is zero-indexed and counting starts at 0 .

The first list element, "John Doe" , has an index of 0 . The second list element, 34 , has an index of 1 . The third list element, "London" , has an index of 2 . The forth list element, 1.76 , has an index of 3 .

Indices come in useful for accessing specific list items whose position (index) you know.

So, you can grab any list element you want by using its index.

To access an item, first include the name of the list and then in square brackets include the integer that corresponds to the index for the item you want to access.

Here is how you would access each item using its index:

But what about finding the index of a list item in Python?

In the sections that follow you will see some of the ways you can find the index of list elements.

Find the Index of an Item using the List index() Method in Python

So far you've seen how to access a value by referencing its index number.

What happens though when you don't know the index number and you're working with a large list?

You can give a value and find its index and in that way check the position it has within the list.

For that, Python's built-in index() method is used as a search tool.

The syntax of the index() method looks like this:

Let's break it down:

  • my_list is the name of the list you are searching through.
  • .index() is the search method which takes three parameters. One parameter is required and the other two are optional.
  • item is the required parameter. It's the element whose index you are searching for.
  • start is the first optional parameter. It's the index where you will start your search from.
  • end the second optional parameter. It's the index where you will end your search.

Let's see an example using only the required parameter:

In the example above, the index() method only takes one argument which is the element whose index you are looking for.

Keep in mind that the argument you pass is case-sensitive . This means that if you had passed "python", and not "Python", you would have received an error as "python" with a lowercase "p" is not part of the list.

The return value is an integer, which is the index number of the list item that was passed as an argument to the method.

Let's look at another example:

If you try and search for an item but there is no match in the list you're searching through, Python will throw an error as the return value - specifically it will return a ValueError .

This means that the item you're searching for doesn't exist in the list.

A way to prevent this from happening, is to wrap the call to the index() method in a try/except block.

If the value does not exist, there will be a message to the console saying it is not stored in the list and therefore doesn't exist.

Another way would be to check to see if the item is inside the list in the first place, before looking for its index number. The output will be a Boolean value - it will be either True or False.

How to Use the Optional Parameters with the index() Method

Let's take a look at the following example:

In the list programming_languages there are three instances of the "Python" string that is being searched.

As a way to test, you could work backwards as in this case the list is small.

You could count and figure out their index numbers and then reference them like you've seen in previous sections:

There is one at position 1 , another one at position 3 and the last one is at position 5 .

Why aren't they showing in the output when the index() method is used?

When the index() method is used, the return value is only the first occurence of the item in the list. The rest of the occurrences are not returned.

The index() method returns only the index of the position where the item appears the first time.

You could try passing the optional start and end parameters to the index() method.

You already know that the first occurence starts at index 1 , so that could be the value of the start parameter.

For the end parameter you could first find the length of the list.

To find the length, use the len() function:

The value for end parameter would then be the length of the list minus 1. The index of the last item in a list is always one less than the length of the list.

So, putting all that together, here is how you could try to get all three instances of the item:

The output still returns only the first instance!

Although the start and end parameters provide a range of positions for your search, the return value when using the index() method is still only the first occurence of the item in the list.

How to Get the Indices of All Occurrences of an Item in A List

Use a for-loop to get the indices of all occurrences of an item in a list.

Let's take the same example that we've used so far.

That list has three occurrences of the string "Python".

First, create a new, empty list.

This will be the list where all indices of "Python" will be stored.

Next, use a for-loop . This is a way to iterate (or loop) through the list, and get each item in the original list. Specifically, we loop over each item's index number.

You first use the for keyword.

Then create a variable, in this case programming_language , which will act as a placeholder for the position of each item in the original list, during the iterating process.

Next, you need to specify the set amount of iterations the for-loop should perform.

In this case, the loop will iterate through the full length of the list, from start to finish. The syntax range(len(programming_languages)) is a way to access all items in the list programming_languages .

The range() function takes a sequence of numbers that specify the number it should start counting from and the number it should end the counting with.

The len() function calculates the length of the list, so in this case counting would start at 0 and end at - but not include - 6 , which is the end of the list.

Lastly, you need to set a logical condition.

Essentially, you want to say: "If during the iteration, the value at the given position is equal to 'Python', add that position to the new list I created earlier".

You use the append() method for adding an element to a list.

Use List Comprehension and the enumerate() Function to Get the Indices of All Occurrences of an Item in A List

Another way to find the indices of all the occurrences of a particular item is to use list comprehension.

List comprehension is a way to create a new list based on an existing list.

Here is how you would get all indices of each occurrence of the string "Python", using list comprehension:

With the enumerate() function you can store the indices of the items that meet the condition you set.

It first provides a pair ( index, item ) for each element in the list ( programming_languages ) that is passed as the argument to the function.

index is for the index number of the list item and item is for the list item itself.

Then, it acts as a counter which starts counting from 0 and increments each time the condition you set is met, selecting and moving the indices of the items that meet your criteria.

Paired with the list comprehension, a new list is created with all indices of the string "Python".

And there you have it! You now know some of the ways to find the index of an item, and ways to find the indices of multiple occurrences of an item, in a list in Python.

I hope you found this article useful.

To learn more about the Python programming language, check out freeCodeCamp's Scientific Computing with Python Certification .

You'll start from the basics and learn in an interacitve and beginner-friendly way. You'll also build five projects at the end to put into practice and help reinforce what you've learned.

Thanks for reading and happy coding!

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IndexError: list assignment index out of range in Python

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Last updated: Jan 29, 2023 Reading time · 9 min

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# Table of Contents

  • IndexError: list assignment index out of range
  • (CSV) IndexError: list index out of range
  • sys.argv[1] IndexError: list index out of range
  • IndexError: pop index out of range
Make sure to click on the correct subheading depending on your error message.

# IndexError: list assignment index out of range in Python

The Python "IndexError: list assignment index out of range" occurs when we try to assign a value at an index that doesn't exist in the list.

To solve the error, use the append() method to add an item to the end of the list, e.g. my_list.append('b') .

indexerror list assignment index out of range

Here is an example of how the error occurs.

assignment to index out of range

The list has a length of 3 . Since indexes in Python are zero-based, the first index in the list is 0 , and the last is 2 .

Trying to assign a value to any positive index outside the range of 0-2 would cause the IndexError .

# Adding an item to the end of the list with append()

If you need to add an item to the end of a list, use the list.append() method instead.

adding an item to end of list with append

The list.append() method adds an item to the end of the list.

The method returns None as it mutates the original list.

# Changing the value of the element at the last index in the list

If you meant to change the value of the last index in the list, use -1 .

change value of element at last index in list

When the index starts with a minus, we start counting backward from the end of the list.

# Declaring a list that contains N elements and updating a certain index

Alternatively, you can declare a list that contains N elements with None values.

The item you specify in the list will be contained N times in the new list the operation returns.

Make sure to wrap the value you want to repeat in a list.

If the list contains a value at the specific index, then you are able to change it.

# Using a try/except statement to handle the error

If you need to handle the error if the specified list index doesn't exist, use a try/except statement.

The list in the example has 3 elements, so its last element has an index of 2 .

We wrapped the assignment in a try/except block, so the IndexError is handled by the except block.

You can also use a pass statement in the except block if you need to ignore the error.

The pass statement does nothing and is used when a statement is required syntactically but the program requires no action.

# Getting the length of a list

If you need to get the length of the list, use the len() function.

The len() function returns the length (the number of items) of an object.

The argument the function takes may be a sequence (a string, tuple, list, range or bytes) or a collection (a dictionary, set, or frozen set).

If you need to check if an index exists before assigning a value, use an if statement.

This means that you can check if the list's length is greater than the index you are trying to assign to.

# Trying to assign a value to an empty list at a specific index

Note that if you try to assign to an empty list at a specific index, you'd always get an IndexError .

You should print the list you are trying to access and its length to make sure the variable stores what you expect.

# Use the extend() method to add multiple items to the end of a list

If you need to add multiple items to the end of a list, use the extend() method.

The list.extend method takes an iterable (such as a list) and extends the list by appending all of the items from the iterable.

The list.extend method returns None as it mutates the original list.

# (CSV) IndexError: list index out of range in Python

The Python CSV "IndexError: list index out of range" occurs when we try to access a list at an index out of range, e.g. an empty row in a CSV file.

To solve the error, check if the row isn't empty before accessing it at an index, or check if the index exists in the list.

csv indexerror list index out of range

Assume we have the following CSV file.

And we are trying to read it as follows.

# Check if the list contains elements before accessing it

One way to solve the error is to check if the list contains any elements before accessing it at an index.

The if statement checks if the list is truthy on each iteration.

All values that are not truthy are considered falsy. The falsy values in Python are:

  • constants defined to be falsy: None and False .
  • 0 (zero) of any numeric type
  • empty sequences and collections: "" (empty string), () (empty tuple), [] (empty list), {} (empty dictionary), set() (empty set), range(0) (empty range).

# Check if the index you are trying to access exists in the list

Alternatively, you can check whether the specific index you are trying to access exists in the list.

This means that you can check if the list's length is greater than the index you are trying to access.

# Use a try/except statement to handle the error

Alternatively, you can use a try/except block to handle the error.

We try to access the list of the current iteration at index 1 , and if an IndexError is raised, we can handle it in the except block or continue to the next iteration.

# sys.argv [1] IndexError: list index out of range in Python

The sys.argv "IndexError: list index out of range in Python" occurs when we run a Python script without specifying values for the required command line arguments.

To solve the error, provide values for the required arguments, e.g. python main.py first second .

sys argv indexerror list index out of range

I ran the script with python main.py .

The sys.argv list contains the command line arguments that were passed to the Python script.

# Provide all of the required command line arguments

To solve the error, make sure to provide all of the required command line arguments when running the script, e.g. python main.py first second .

Notice that the first item in the list is always the name of the script.

It is operating system dependent if this is the full pathname or not.

# Check if the sys.argv list contains the index

If you don't have to always specify all of the command line arguments that your script tries to access, use an if statement to check if the sys.argv list contains the index that you are trying to access.

I ran the script as python main.py without providing any command line arguments, so the condition wasn't met and the else block ran.

We tried accessing the list item at index 1 which raised an IndexError exception.

You can handle the error or use the pass keyword in the except block.

# IndexError: pop index out of range in Python

The Python "IndexError: pop index out of range" occurs when we pass an index that doesn't exist in the list to the pop() method.

To solve the error, pass an index that exists to the method or call the pop() method without arguments to remove the last item from the list.

indexerror pop index out of range

The list has a length of 3 . Since indexes in Python are zero-based, the first item in the list has an index of 0 , and the last an index of 2 .

If you need to remove the last item in the list, call the method without passing it an index.

The list.pop method removes the item at the given position in the list and returns it.

You can also use negative indices to count backward, e.g. my_list.pop(-1) removes the last item of the list, and my_list.pop(-2) removes the second-to-last item.

Alternatively, you can check if an item at the specified index exists before passing it to pop() .

This means that you can check if the list's length is greater than the index you are passing to pop() .

An alternative approach to handle the error is to use a try/except block.

If calling the pop() method with the provided index raises an IndexError , the except block is run, where we can handle the error or use the pass keyword to ignore it.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

  • IndexError: index 0 is out of bounds for axis 0 with size 0
  • IndexError: invalid index to scalar variable in Python
  • IndexError: pop from empty list in Python [Solved]
  • Replacement index 1 out of range for positional args tuple
  • IndexError: too many indices for array in Python [Solved]
  • IndexError: tuple index out of range in Python [Solved]

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5 Best Ways to Replace an Element in a Python List By Index

Method 1: direct assignment.

The most straightforward method to replace an element in a list is by directly assigning a new value to the desired index. This operation is intuitive and very efficient because it directly accesses the list by its index and changes the value.

Here’s an example:

Output: ['apple', 'blueberry', 'cherry']

In the provided example, we use index 1 to directly replace ‘banana’ with ‘blueberry’ in the fruits list. This approach is best used when you know the exact index of the element that needs to be replaced.

Method 2: Using the pop() and insert() Methods

Replacing an element can also be done by first removing the item at the target index using pop() and then inserting the new item at the same index using insert() . This method is useful when you want to perform actions both with the replaced and new element.

The fruits.pop(1) removes ‘banana’ from the list and returns it, which we store in old_fruit . We then immediately use fruits.insert(1, 'blueberry') to place ‘blueberry’ at the vacated position. This two-step process can be handy but is less efficient than direct assignment.

Method 3: Using List Comprehension

List comprehension can be used to generate a new list where one or more elements are replaced based on their index. It is a concise and Pythonic way to manipulate lists. However, this method creates a new list instead of modifying the existing one.

The list comprehension iterates over fruits with the help of enumerate() to retain both the element and its index. For each element, if the index i is not 1 , the element remains unchanged; otherwise, ‘blueberry’ is inserted.

Method 4: Using the slice Assignment

Slice assignment lets you replace elements within a range in a list. This is an optimal choice when needing to replace a sequence of items with others of the same length. It mutates the original list.

The slice notation fruits[1:2] specifies that we are targeting the list starting at index 1 up to (but not including) index 2 . We then assign a list with a single element, ‘blueberry’, to this slice, replacing ‘banana’.

Bonus One-Liner Method 5: Using the enumerate() Function Within a Loop

For a one-liner solution without creating a new list, you can use a loop with enumerate() to replace an item by checking its index. This method is compact but less readable than others.

Here, we use a list comprehension that loops through each item and its index in fruits . Whenever the index i equals 1 , it calls fruits.__setitem__(i, 'blueberry') , which replaces the element at index i with ‘blueberry’.

Summary/Discussion

  • Method 1: Direct Assignment. Simple and most efficient way for replacing a single element. Cannot handle complex conditions easily.
  • Method 2: pop() and insert() Methods. Allows access to the replaced element. Involves two operations, thus is less efficient than direct assignment.
  • Method 3: List Comprehension. Pythonic and useful for replacing multiple items based on conditions. However, it results in a new list, which may not be desired.
  • Method 4: Slice Assignment. Ideal for replacing a range of elements. Directly mutates the original list, preserving its identity.
  • Method 5: One-Liner with enumerate() . Compact syntax for in-place replacement. Its dense structure may impact code readability.

Emily Rosemary Collins is a tech enthusiast with a strong background in computer science, always staying up-to-date with the latest trends and innovations. Apart from her love for technology, Emily enjoys exploring the great outdoors, participating in local community events, and dedicating her free time to painting and photography. Her interests and passion for personal growth make her an engaging conversationalist and a reliable source of knowledge in the ever-evolving world of technology.

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COMMENTS

  1. Populating a list/array by index in Python?

    Populating a list/array by index in Python? - Stack Overflow Populating a list/array by index in Python? Ask Question Asked 14 years, 9 months ago Modified 4 years, 11 months ago Viewed 95k times 35 Is this possible: myList = [] myList [12] = 'a' myList [22] = 'b' myList [32] = 'c' myList [42] = 'd' When I try, I get:

  2. Python List index()

    List index () method searches for a given element from the start of the list and returns the position of the first occurrence. Example: Python Animals= ["cat", "dog", "tiger"] print(Animals.index ("dog")) Output 1 Definition of Python List index () Python list index () method is used to find position of element in list Python.

  3. Python List Index Function

    What is Indexing in Python? In Python, indexing refers to the process of accessing a specific element in a sequence, such as a string or list, using its position or index number. Indexing in Python starts at 0, which means that the first element in a sequence has an index of 0, the second element has an index of 1, and so on.

  4. Python List index() Method

    Definition and Usage The index () method returns the position at the first occurrence of the specified value. Syntax list .index ( elmnt ) Parameter Values More Examples Example What is the position of the value 32: fruits = [4, 55, 64, 32, 16, 32] x = fruits.index (32) Try it Yourself »

  5. Python List Index: Find First, Last or All Occurrences • datagy

    February 28, 2022 In this tutorial, you'll learn how to use the Python list index method to find the index (or indices) of an item in a list. The method replicates the behavior of the indexOf () method in many other languages, such as JavaScript. Being able to work with Python lists is an important skill for a Pythonista of any skill level.

  6. Indexing in Python

    In Python, objects are "zero-indexed" meaning the position count starts at zero. Many other programming languages follow the same pattern. So, if there are 5 elements present within a list. Then the first element (i.e. the leftmost element) holds the "zeroth" position, followed by the elements in the first, second, third, and fourth ...

  7. Python List index()

    Example 1: Find the index of the element # vowels list vowels = ['a', 'e', 'i', 'o', 'i', 'u'] # index of 'e' in vowels index = vowels.index ( 'e') print('The index of e:', index) # element 'i' is searched # index of the first 'i' is returned index = vowels.index ( 'i') print('The index of i:', index) Run Code Output

  8. Python Intro: Lists and Indexing

    Lists can be used to group numbers (ints or floats), strings, lists, or other kinds of values together in one variable. Lists are ordered- objects stay in the order you add them. Lists are indexed using 0-based indexing. You can use square bracket notation to return a single item in a list or subset of items.

  9. Python Indexerror: list assignment index out of range Solution

    Method 1: Using insert () function The insert (index, element) function takes two arguments, index and element, and adds a new element at the specified index. Let's see how you can add Mango to the list of fruits on index 1. Python3 fruits = ['Apple', 'Banana', 'Guava'] print("Original list:", fruits) fruits.insert (1, "Mango")

  10. The Basics of Indexing and Slicing Python Lists

    Accessing the items in a list (and in other iterables like tuples and strings) is a fundamental skill for Python coders, and many Python tools follow similar conventions for indexing and slicing (e.g. numpy Arrays and pandas DataFrames ). So it's worth being familiar with the ins and outs. Definitions and Stage-Setting

  11. Python Lists

    Python Variables Variable Names Assign Multiple Values Output Variables Global Variables Variable Exercises. ... the first item has index [0], the second item has index [1] etc. ... There are four collection data types in the Python programming language: List is a collection which is ordered and changeable. Allows duplicate members.

  12. Python's Assignment Operator: Write Robust Assignments

    To create a new variable or to update the value of an existing one in Python, you'll use an assignment statement. This statement has the following three components: A left operand, which must be a variable. The assignment operator ( =) A right operand, which can be a concrete value, an object, or an expression.

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    1 Note that lists are indexed starting at 0, so even with that change you would have gotten an IndexError when i=2. - shriakhilc Jan 13, 2022 at 21:11 1 In addition to what @GreenCloakGuy noted, l only has two items in it and range (3) is too large. - JonSG Jan 13, 2022 at 21:11 1

  14. Python indexerror: list assignment index out of range Solution

    The message "list assignment index out of range" tells us that we are trying to assign an item to an index that does not exist. In order to use indexing on a list, you need to initialize the list. If you try to assign an item into a list at an index position that does not exist, this error will be raised. An Example Scenario

  15. How to Fix "IndexError: List Assignment Index Out of Range" in Python

    How to use the insert () method. Use the insert () method to insert elements at a specific position instead of direct assignment to avoid out-of-range assignments. my_list = [ 10, 20, 30 ] my_list.insert ( 3, 987) #Inserting element at index 3 print (my_list) Now one big advantage of using insert () is even if you specify an index position ...

  16. Python Find in List

    How indexing works Use the index () method to find the index of an item 1. Use optional parameters with the index () method Get the indices of all occurrences of an item in a list Use a for-loop to get indices of all occurrences of an item in a list

  17. IndexError: list assignment index out of range in Python

    The Python "IndexError: list assignment index out of range" occurs when we try to assign a value at an index that doesn't exist in the list. To solve the error, use the append() method to add an item to the end of the list, e.g. my_list.append('b') .

  18. Python error: IndexError: list assignment index out of range

    this makes the size of the list just big enough to hold 2 elements, the two you added, which has an index of 0 and 1 (python lists are 0-based). In your code, further down, you then specify the contents of element j which starts at 2, and your code blows up immediately because you're trying to say "for a list of 2 elements, please store the ...

  19. 5 Best Ways to Replace an Element in a Python List By Index

    Method 1: Direct Assignment. The most straightforward method to replace an element in a list is by directly assigning a new value to the desired index. This operation is intuitive and very efficient because it directly accesses the list by its index and changes the value. In the provided example, we use index to directly replace 'banana ...

  20. python

    python - Assignment of variable to list using indexing - Stack Overflow Assignment of variable to list using indexing Ask Question Asked 1 year, 7 months ago 1 year, 7 months ago Viewed 420 times 0 The following is a simple function: def func (first): third = first [0] first [0] [0] = 5 print (third) first = [ [3,4]] func (first)

  21. python

    3 Answers. Sorted by: 2. You can set the index directly: In [11]: df.index = ['row0', 'row1', 'row2', 'row3', 'row4', 'row5'] In [12]: df Out [12]: Col1 Col2 Col3 row0 -1.094278 -0.689078 -0.465548 row1 1.555546 -0.388261 1.211150 row2 -0.143557 1.769561 -0.679080 row3 -0.064910 1.959216 0.227133 row4 -0.383729 0.113739 -0.954082 row5 0.434357 ...