Python list push front

8 ways to add an element to the beginning of a list and string in Python

Let’s look at different methods for adding elements to the beginning of a list and string: concatenation, insert, append, extend, rjust, and f-strings.

How to add an item to the top of a Python list: 4 methods

Lists in Python are mutable and ordered data structures designed to store a group of values. If you need to insert an element in the first position of an existing list during the program execution, you can use one of the methods described below.

Method 1: insert

This method is implemented using the built-in insert() method of lists. This method works with any type of data and allows you to insert the desired element at any position in the existing list, including the first one. The insert() method takes two parameters – the index (position) at which the element is to be inserted, and the element itself. Counting positions in Python starts from zero – Accordingly, to insert an element at the beginning of the list , you need to specify 0 , and not, as the first parameter 1 . Let’s take an example. Suppose we have a list [1, 2, 3] where we want to insert the number 5 at the beginning. Use the insert() method:

arr = [1, 2, 3] arr.insert(0, 5) print(arr) 

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Prepend a List in Python: 4 Best Methods (with code)

Every python programmer should know all the basics of lists, and one of the most important tasks to learn is to add items to them at specified positions. The order of items in a list is significant and sometimes, you might want to add new items to the beginning. In this article, we will learn how to prepend a list in python with code. We will examine several different approaches to it.

But first, let’s revisit some important concepts of python lists.

What is list and prepending a list mean?

A list in Python is a collection of ordered items that can be changed, meaning it is mutable. Items in a list can be of any data type, including numbers, strings, or other lists.

The item is identified by its index which represents its position in the list. Python lists are zero-indexed. It means that the first element in a list has an index of 0, the second element has an index of 1, and so on. There is also negative indexing from the end of the sequence, which starts from -1.

The following image will help you better understand it:

The process of adding an item to the front of a list is referred to as prepending a list in python. A new item is inserted at the top of the list, with all other items shifting to the right to make room for the new item.

How to prepend to a list in python?

There are various methods to prepend a list in python. We will discuss 4 such ways in detail.

1) insert() method

The insert() method in Python is a built-in function that allows you to insert an item into a list at a given position. You can prepend a list by inserting an item in the first position of index 0.

The insert() method has the following syntax:

where the list is the list you want to prepend, the index is the position you want to insert the item, and the item is the item you want to insert. To prepend an item, we need to set the index to 0. Here is an example:

numbers = [1, 2, 3, 4, 5] numbers.insert(0, 0) print(numbers)

However, keep in mind that inserting an element into a list can be an expensive operation, especially if the list is very large since it requires shifting all the elements after the insertion point to make room for the new element.

2) The ‘+’ operator

The ‘+’ operator is another way to prepend a list. The ‘+’ operator joins two lists together. To prepend a list, make a new list with the item to be prepended and concatenate it with the original list.

The following is the syntax:

where the item is the item you want to prepend and the list is the original list. For example:

numbers = [1, 2, 3, 4, 5] new_list = [0] + numbers print(new_list)

3) The ‘*’ operator

In Python, the ‘*’ operator can also be used to prepend a list by making a new list with the item you want to prepend and multiplying it by the number of times you want it to be repeated.

numbers = [1, 2, 3, 4, 5] new_list = [0] * (len(numbers) + 1) new_list[1:] = numbers print(new_list)

4) Using slicing

Slicing in python is used for extracting a portion of a list. To prepend a list, make a new list with the item you want to prepend and then combine it with the original list using slicing.

The following is the syntax:

where the item is the item you want to prepend and the list is the original list. For example:

numbers = [1, 2, 3, 4, 5] new_list = [0] + numbers[:] print(new_list)

Each of these methods has advantages and disadvantages, so it is critical to select the one that is best suited to your specific use case. Note that all these methods operate almost at the same speed so code readability is an important factor when deciding which method to use.

Conclusion

Lists are a frequently used data structure in python. Items can be added to the front or at the end of the list. Adding items to a list, in the beginning, is called pre-pending. Here we learned different ways for how to prepened a list in python.

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About The Author

Abrar Ahmed

An ambivert individual with a thirst for knowledge and passion to achieve. Striving to connect Artificial Intelligence in all aspects of life. I am also an avid coder and partake in coding challenges all the time on Leetcode and CodeChef.

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Prepend to a Python a List (Append at beginning)

In this tutorial, you’ll learn how to use Python to prepend a list. While Python has a method to append values to the end of a list, there is no prepend method. By the end of this tutorial, you’ll have learned how to use the .insert() method and how to concatenate two lists to prepend.

You’ll also learn how to use the deque object to insert values at the front of a list. In many cases, this is the preferred method, as it’s significantly more memory efficient than other methods.

The Problem with Prepending a Python List

Python lists are a mutable container data type, meaning that they can be altered. Because of this, it can be tempting to add an item to a list. However, depending on the size of your list, this process can be immensely memory intensive.

The reason for this is that there is no “room” at the beginning of the list. When you add an item to the front of a list, Python needs to shift all other items forward. Later in this tutorial, you’ll learn about the deque data structure, which represents a double-ended queue. In these deque objects, items can be inserted freely at the beginning or end.

Using Insert to Prepend to a Python List

An intuitive way to add an item to the front of a Python list is to use the insert method. The .insert() method takes two parameters:

Let’s take a look at an example:

# Using .insert() to prepend to a Python list words = ['welcome', 'to', 'datagy'] words.insert(0, 'hello') print(words) # Returns: ['hello', 'welcome', 'to', 'datagy']

The operation takes place in-place, meaning that a new object doesn’t need to be created. In the following example, you’ll learn how to use the + operator to prepend an item to a list.

Using the + Operator to Prepend to a Python List

The Python + operator is an incredibly versatile operator. When combined with two lists, the two lists are added together, in the order in which they appear. This means that when we want to prepend an item (or multiple items), we can create a list containing this item.

We then only need to apply the + operator between the two lists to combine them. Let’s take a look at another example:

# Using + to prepend an item to a list words = ['welcome', 'to', 'datagy'] prefix = ['hello'] words = prefix + words print(words) # Returns: ['hello', 'welcome', 'to', 'datagy']

Similarly, we can use the augmented assignment operator to prepend an item to a list. The difference here is that we need to reassign to the prefix, not the other way around. This is demonstrated below:

# Using the augmented assignment operator to prepend an item to a list words = ['welcome', 'to', 'datagy'] prefix = ['hello'] prefix += words print(prefix) # Returns: ['hello', 'welcome', 'to', 'datagy']

In the next section, you’ll learn how to use list slicing to prepend to a Python list.

Using List Slicing to Prepend to a Python List

This method can feel a bit awkward, but it can also be a useful way to assign an item to the front of a list. We assign a list with a single value to the slice of [:0] of another list. This forces the item to be added before the 0th item.

Let’s take a look at an example:

# Using List Slicing to prepend to a Python List words = ['welcome', 'to', 'datagy'] words[:0] = ['hello'] print(words) # Returns: ['hello', 'welcome', 'to', 'datagy']

In the final section, you’ll learn the most memory-efficient way to add items to the front of a Python list.

Using Deque to Prepend to a Python List

Part of the incredibly versatile collections library is the deque class. This class represents a double-ended queue, which represent stacks or queues from other languages. The benefit of this is that the class allows you to prepend items to the left, without the memory implications of shifting all values.

Let’s take a look at an example:

# Using deque to prepend to a list in Python from collections import deque words = deque(['welcome', 'to', 'datagy']) words.appendleft('hello') print(words) # Returns: ['hello', 'welcome', 'to', 'datagy']

The benefit of this approach is noticeable when working with large lists. With smaller lists, you may not notice a benefit, but it’s a good idea to keep performance in mind.

Conclusion

In this tutorial, you learned how to prepend to a Python list. You first learned why prepending to a list can be a troublesome idea. Then, you learned how to use three different list methods, including .insert() , the + operator, and list indexing to prepend to a list. Finally, you learned how to use the collections deque object to prepend to a list in a memory-efficient manner.

Additional Resources

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

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