5 Best Ways to Write a Python List to a Text File

💡 Problem Formulation: Python developers often encounter the need to persist sequences of data for later use or processing. A common requirement is to convert a Python list into a text file, storing each item of the list on a separate line. For example, given a list ['apple', 'banana', 'cherry'], the desired output would be a text file containing each fruit on a new line.

Method 1: Using a Simple for Loop With File Write

This traditional method involves iterating through the list and writing each element to the file followed by a newline character. It uses basic file handling operations provided by Python.

Here’s an example:

fruits = ['apple', 'banana', 'cherry']

with open('fruits.txt', 'w') as file:
    for fruit in fruits:
        file.write(f"{fruit}\n")

Output: A text file named fruits.txt with each fruit on a new line.

This snippet opens a file in write mode and iterates through the list of fruits. Each fruit is written to the file followed by a newline, resulting in each fruit name on a separate line in the text file.

Method 2: Using the writelines() Method

The writelines() function can be used to write a list of strings to a file where each string is assumed to include a newline at the end. This method reduces the need to explicitly loop through the list elements.

Here’s an example:

fruits = ['apple\n', 'banana\n', 'cherry\n']

with open('fruits.txt', 'w') as file:
    file.writelines(fruits)

Output: A text file fruits.txt with each element of the list on a separate line.

The code uses writelines() to write each string in the list to the file. Each string in the list already contains a newline character, saving the need to append one during the write operation.

Method 3: Using List Comprehension

List comprehension can be combined with file writing to create a concise way to write a list to a file. This method leverages Python’s ability to perform actions within a single line of code.

Here’s an example:

fruits = ['apple', 'banana', 'cherry']

with open('fruits.txt', 'w') as file:
    [file.write(f"{fruit}\n") for fruit in fruits]

Output: The fruits.txt file, containing each item of the list on a separate line.

This line of code demonstrates the use of list comprehension to iterate through the list and write each item, followed by a newline, to the file.

Method 4: Using a Join Operation With a Single Write

For efficiency, the entire list can be converted into a single string – with each item separated by a newline – and then written to the file in one go. This minimizes the number of write operations performed.

Here’s an example:

fruits = ['apple', 'banana', 'cherry']

with open('fruits.txt', 'w') as file:
    file.write("\n".join(fruits))

Output: A text file fruits.txt with apple, banana, and cherry each on their own line.

The example creates a single string from the list by joining each element with a newline character between them, and then writes this string to the file, thus reducing the file operations to just one.

Bonus One-Liner Method 5: Using a Generator Expression

A generator expression can be used for writing items to a file. This method is memory-efficient as it does not require the creation of an intermediary list.

Here’s an example:

fruits = ['apple', 'banana', 'cherry']

with open('fruits.txt', 'w') as file:
    file.writelines(f"{fruit}\n" for fruit in fruits)

Output: A text file fruits.txt where ‘apple’, ‘banana’, and ‘cherry’ appear each on a separate line.

The generator expression within writelines() efficiently processes each item, appending a newline and passing it directly to the file object for writing.

Summary/Discussion

• Method 1: Using a Simple for Loop With File Write. Easy to understand. Can be slow for very large lists.
• Method 2: Using the writelines() Method. More concise than Method 1. Requires that newline characters be part of the list elements beforehand.
• Method 3: Using List Comprehension. Compact code. Might be less readable for beginners.
• Method 4: Using a Join Operation With a Single Write. Efficient for IO operations. Not ideal for extremely large lists, as it creates a large string in memory first.
• Method 5: Using a Generator Expression. Memory efficient and concise. May be less familiar to new Python programmers.