In this article, you’ll learn how to sort a list of tuples by the second value in Python.

To make it more fun, we have the following running scenario:

BridgeTech is a bridge restoration company. They have asked you to sort and return the Top 10 elements from the Periodic Table based on the ‘Atomic Radius’ in descending order.

The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron.

Wikpedia

Click here to download the Periodic Table. Save this file as periodic_table.csv and move it to the current working directory.

Question: How would you write the Python code to accomplish this task?

We can accomplish this task by one of the following options:

• Method 1: Use sort() and lambda
• Method 2: Use sort() and itemgetter()
• Method 3: Use sorted() and lambda
• Method 4: Use a Bubble Sort

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Preparation

Before any data manipulation can occur, one (1) new library will require installation.

• The Pandas library enables access to/from a DataFrame.

To install this library, navigate to an IDE terminal. At the command prompt ($), execute the code below. For the terminal used in this example, the command prompt is a dollar sign ($). Your terminal prompt may be different.

$ pip install pandas

Hit the <Enter> key on the keyboard to start the installation process.

If the installation was successful, a message displays in the terminal indicating the same.

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Feel free to view the PyCharm installation guide for the required library.

• How to install Pandas on PyCharm

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Add the following code to the top of each code snippet. This snippet will allow the code in this article to run error-free.

import numpy as np

from operator import itemgetter

 Note: The operator library is built-in to Python and does not require installation.

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Method 1: Use Sort and a Lambda

To sort a list of tuples based on the second element, use sort() and lambda in the one-liner expression tups.sort(key=lambda x: x[1], reverse=True).

Here’s an example:

df = pd.read_csv('periodic_table.csv', usecols=['Name', 'AtomicRadius'])

tups = [tuple(x) for x in df.values.tolist()]

tups.sort(key=lambda x: x[1], reverse=True)

print(tups[0:10])

The CSV file is read in preparation, and two (2) columns save to a DataFrame. The DataFrame then converts to a list of tuples (tups) using List Comprehension.

We are ready to sort!

A lambda is passed as a parameter to sort() indicating the sort element (x[1]), and the sort order is set to descending (reverse=True). The results save to tups.

To complete the process, slicing is performed, and the Top 10 elements are sent to the terminal.

Output

[('Francium', 348.0), ('Cesium', 343.0), ('Rubidium', 303.0), ('Radium', 283.0), ('Potassium', 275.0), ('Barium', 268.0), ('Actinium', 260.0), ('Strontium', 249.0), ('Curium', 245.0), ('Californium', 245.0)]

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Method 2: Use Sort & Itemgetter

To sort a list of tuples by the second element, use the sort() and itemgetter() functions in the expression tuples.sort(key=itemgetter(1), reverse=True).

Here’s an example:

df = pd.read_csv('periodic_table.csv', usecols=['Name', 'AtomicRadius'])

tups = [tuple(x) for x in df.values.tolist()]

tups.sort(key=itemgetter(1), reverse=True)

print(tups[0:10])

The CSV file is read in preparation, and two (2) columns save to a DataFrame. The DataFrame then converts to a List of Tuples (tups) using List Comprehension.

We are ready to sort!

The sort() function passes a key (itemgetter(n)) where n is the sort element (itemgetter(1)), and the sort order is set to descending (reverse=True).

The results save to tups.

To complete the process, slicing is performed, and the Top 10 elements are sent to the terminal.

 Note: The itemgetter() function is slightly faster than a lambda. Use itemgetter if speed and memory are a factor.

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Method 3: Use Sorted & Lambda

To sort a list of tuples by the second element, combine the functions sorted() and lambda in the expression sorted(tups, key=lambda x:(x[1]), reverse=True) and assign the resulting sorted list to the original variable tups.

Here’s an example:

df = pd.read_csv('periodic_table.csv', usecols=['Name', 'AtomicRadius'])

tups = [tuple(x) for x in df.values.tolist()]

tups = sorted(tups, key=lambda x:(x[1]), reverse=True)

print(tups[0:10])

The CSV file is read in preparation, and two (2) columns save to a DataFrame. The DataFrame then converts to a List of Tuples (tups) using List Comprehension.

We are ready to sort!

A lambda is passed as a parameter to sorted(), indicating the sort element (x[1]), and the sort order is set to descending (reverse=True). The results save to tups.

To complete the process, slicing is performed, and the Top 10 elements are sent to the terminal.

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Method 4: Use Bubble Sort

To sort a List of Tuples by the second element, you can also modify a sorting algorithm from scratch such as Bubble Sort to access the second (or n-th) tuple value as a basis for sorting.

Here’s an example:

df = pd.read_csv('periodic_table.csv', usecols=['Name', 'AtomicRadius'])

tups = [tuple(x) for x in df.values.tolist()] def sort_tuples_desc(tups, idx): length = len(tups) for i in range(0, length): for j in range(0, length-i-1): if (tups[j][idx] < tups[j + 1][idx]): tmp = tups[j] tups[j] = tups[j+1] tups[j+1] = tmp return tups

print(sort_tuples_desc(tups, 1)[0:10])

The CSV file is read in preparation, and two (2) columns save to a DataFrame. The DataFrame then converts to a List of Tuples (tups) using List Comprehension.

We are ready to sort!

A sort function sort_tuples_desc is created and passed two (2) parameters: a List of Tuples (tups), and the sort element (idx). Then, the infamous Bubble Sort is performed on the elements.

This function returns a List of Tuples sorted in descending order.

To complete the process, slicing is performed, and the Top 10 elements are sent to the terminal.

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Summary

These four (4) methods of sorting a List of Tuples based on the second element should give you enough information to select the best one for your coding requirements.

Good Luck & Happy Coding!

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Problem Statement: How to write a list to a file with Python?

Mostly Python programmers use persistent storage systems like databases or files to store serialized data structures like arrays, lists, and dictionaries. It is because databases and files are reusable, i.e. after analyzing the given data, we can store it in the file, and later that data can be read to use in an application. There are many different ways to write a list to the file in Python. Let’s look at some of them:

Method 1- Using Read And Write

Python facilitates us with standard methods used to read the data from the file and to write the data to a file. While dealing with single lines, we can use the read() and write() methods, respectively. Suppose we have the following list of strings and we have to store each string in a file using Python:

colors = ["red", "black", "white", "yellow", "blue"]

To write the list in to file, we have to follow the steps given below:

• Firstly, open the file in write mode by passing the file path and access mode “w” to the open() function.
• Next, we have to use the “for” loop to iterate the list. In each iteration, we will get a list item that we need to write in the file using the write() method.
•  After iterating through the whole list, we need to ensure that we have closed the file. For that, we use the close() method.

Let’s visualize the above demonstration with the help of the following snippet:

# List of colours

colors = ["red", "black", "white", "yellow", "blue"]

# Opening the file in write mode

file = open('colors.txt', 'w')

# Writing the list to the file

for color in colors: file.write(color + '\n')

# Closing the file

file.close()

Output:

red black white yellow blue

Note: The ‘\n‘ character is used for a new line at the end of each item in the list.

Let’s have a look at a situation that demonstrates how we can read the list from the file:

Example:

# Empty list that will read from the file

colors = []

# Opening the file in read mode

with open(r'colors.txt', 'r') as file: for color in file: x = color[:-1] colors.append(x)

print(colors)

Output:

["red", "black", "white", "yellow", "blue"]

Recommended Read: How to Read a File Line-By-Line and Store Into a List?

Method 2- Using Writelines() Method

While dealing with multiple lines, we have to use the readlines() and writelines() file methods in Python.  Hence we can write the entire list into a file using the writelines() method.

Example:

# List of colours

colors = ["red", "black", "white", "yellow", "blue"]

# Opening the file in write mode

with open('colors.txt', 'w') as file: # Writing the entire list to the file file.writelines("\n" % color for color in colors)

Output:

red black white yellow blue

⦿ The following example shows how to use readlines() to read the entire list from a file in Python:

Example:

# Empty list that will read from the file

colors = []

# Opening the file in read mode

with open(r'colors.txt', 'r') as file: colors = [color.rstrip() for color in file.readlines()]

Output:

["red", "black", "white", "yellow", "blue"]

Method 3- Using The Pickle Module

Pickle is a module in Python that is used to serialize or de-serialize an object structure. We can use this module to serialize a list for later use in the same file.  The dump() method from the module is used to write the list into a file and it takes the reference of the file and list as its parameters. The method stores the list efficiently as a binary data stream. As it uses a binary stream, the file can even be opened in binary writing mode (wb).  Using the module, we can convert any object like a list or dictionary into a character stream. The character stream has the information to reconstruct the object in the future.

Approach: To write a list into the file, we have to first import the pickle module at the start of the program. Then we will use the access mode to open the file. The open() function checks if the file exists or not and if it exists, it gets truncated. The function creates a new one if the file doesn’t already exist. Further, the dump() method converts the object and writes it into the file.

Example:

# Importing the pickle module

import pickle

# Writing the list to the binary file

def writel(a): # Storing the list in binary file (wb) mode with open('file', 'wb') as fp: pickle.dump(colors, fp) print('Completed the process of writing the list into a binary file')

# Reading the list to memory

def readl(): # Reading the list in binary file (rb) mode with open('sample', 'rb') as fp: n = pickle.load(fp) return n

# List of colors

colors = ["red", "black", "white", "yellow", "blue"]

# Calling the writel method

writel(colors)

color = readl()

# Printing the list

print(color)

Output:

Completed the process of writing the list into a binary file ["red", "black", "white", "yellow", "blue"]

Method 4- Using The Json Module

We can use the JSON module to convert the list into a JSON format and then write it into a file using the JSON dump() method. Generally, when we execute a GET request, we will receive a response in JSON format. We can then store the JSON response in a file for any future use.

# Importing the JSON module

import JSON

def writel(a): with open("colors.json", "w") as fp: json.dump(a, fp) print('Completed the process of writing json data into json file')

# Reading the list to memory

def readl(): with open('colors.json', 'rb') as fp: n = json.load(fp) return n

# List of colors

colors = ["red", "black", "white", "yellow", "blue"]

writel(colors)

color = readl()

# Printing the list

print(color)

Output:

Completed the process of writing json data into json file ["red", "black", "white", "yellow", "blue"]

Conclusion

That’s all about how to write a list to a file with Python. I hope you found it helpful. Please stay tuned and subscribe for more interesting articles. Happy learning!

Recommended: Correct Way to Write line To File in Python

Authors: Rashi Agarwal and Shubham Sayon