5 Best Ways to Write a List of Integers to a File in Python

💡 Problem Formulation: Python developers often need to save a list of integers to a file. Say, you have a list [1, 2, 3, 4, 5] and you want to write this list to a text file such that each number appears on a new line. This article will walk you through several methods to accomplish this task, showing the advantages and drawbacks of each.

Method 1: Using a Loop with File.write()

This method involves opening a file in write mode and using a for loop to iterate over the list of integers. Each integer is converted to a string and written to the file followed by a newline character.

Here’s an example:

ints_list = [1, 2, 3, 4, 5]
with open('output.txt', 'w') as file:
    for number in ints_list:
        file.write(f"{number}\n")

Output:

This code opens “output.txt” for writing, iterates through each element in ints_list, converts it to a string, and writes it to the file on a new line. The f-string formatting allows for easy and readable inline variable insertion.

Method 2: Using writelines() with List Comprehension

This approach utilizes the writelines() method which expects an iterable of strings. We can convert the list of integers to a list of strings with newline characters using list comprehension.

Here’s an example:

ints_list = [1, 2, 3, 4, 5]
with open('output.txt', 'w') as file:
    file.writelines([f"{number}\n" for number in ints_list])

Output:

Instead of writing each integer one by one, this example uses list comprehension to create a list of strings where each string is an integer followed by a newline. The writelines() method then writes this list to the file in one go, making it a concise and efficient method.

Method 3: Using Map and writelines()

Another way to convert and write the list of integers to file is by using the map() function along with writelines(). The map() function applies a function to all items in an input list.

Here’s an example:

ints_list = [1, 2, 3, 4, 5]
with open('output.txt', 'w') as file:
    file.writelines(map(lambda x: f"{x}\n", ints_list))

Output:

In this code, the map() function is used to apply a lambda function that formats each integer with a newline to each item of the list, creating an iterator. Then writelines() method writes the formatted strings to the file.

Method 4: Using JSON

For a more structured data storage, Python’s json module can be used to write the list of integers to a file in JSON format. JSON is easily readable and widely used for data exchange.

Here’s an example:

import json

ints_list = [1, 2, 3, 4, 5]
with open('output.json', 'w') as file:
    json.dump(ints_list, file)

Output:

[1,2,3,4,5]

This code snippet serializes ints_list to a JSON formatted stream using json.dump(), which writes it directly to the file specified. This method is great for interoperability with other systems that use or understand JSON.

Bonus One-Liner Method 5: Using Join and File.write()

The most concise method merges all integers into a single string with newlines between them and writes it in one operation using file.write().

Here’s an example:

ints_list = [1, 2, 3, 4, 5]
with open('output.txt', 'w') as file:
    file.write('\n'.join(map(str, ints_list)) + '\n')

Output:

This snippet demonstrates the elegant use of join() to concatenate the string representations of the integers, separated by newline characters. This one-liner is an efficient way to write a list to a file when code brevity is desired.

Summary/Discussion

Method 1: Loop with File.write(). Strengths: Simple and clear. Weaknesses: Not the most efficient with larger lists due to multiple write operations.
Method 2: Writelines with List Comprehension. Strengths: More efficient with a single write operation. Weaknesses: Slightly less readable for beginners.
Method 3: Map and writelines(). Strengths: Functional programming approach, can be efficient. Weaknesses: Lambda may be less intuitive for those unfamiliar with functional concepts.
Method 4: Using JSON. Strengths: Highly interoperable and structured. Weaknesses: Overhead of importing json module, not a plain list in the file.
Method 5: Join and File.write(). Strengths: Very concise and efficient. Weaknesses: Might be less easy to modify or debug due to its condensed nature.