5 Effective Ways to Append a List of Strings to a File in Python

💡 Problem Formulation: You’ve got a list of strings, say ['Python', 'is', 'fun!'], and you want to append these strings to an existing file, each on a new line. The desired output is that each string from the list is added to the end of the file, preserving any existing content in the file. This article walks you through different methods to achieve this appending in Python.

Method 1: Using file object’s write() method

The file object’s write() method in Python allows you to write a string to a file. If you want to append multiple strings from a list, you can iterate through the list and write each string followed by a newline character to the file.

Here’s an example:

list_of_strings = ['Python', 'is', 'fun!']
with open('example.txt', 'a') as file:
    for string in list_of_strings:
        file.write(f"{string}\n")

Output in ‘example.txt’:

Python
is
fun!

This code snippet opens ‘example.txt’ in append mode (‘a’), iterates over each string in our list, and writes each with a newline added. It’s simple and effective for lists that aren’t too large.

Method 2: Using file object’s writelines() method

The writelines() method takes a list of strings and writes them to the file. To append each string on a new line, you’ll need to ensure each string ends with a newline character before calling writelines().

Here’s an example:

list_of_strings = ['Python', 'rocks', 'indeed!']
with open('example.txt', 'a') as file:
    file.writelines(f"{string}\n" for string in list_of_strings)

Output in ‘example.txt’:

Python
rocks
indeed!

Here we open ‘example.txt’ in append mode, use a generator expression to add newline characters to all strings, then call writelines() to append them in one go. It’s efficient for larger lists, reducing the I/O overhead by making a single method call.

Method 3: Using the print() function

The print() function in Python can be used to write text to a file. By using the file argument, print will direct the output to a file instead of the console. Each call automatically appends a newline at the end.

Here’s an example:

list_of_strings = ['Save', 'the', 'code!']
with open('example.txt', 'a') as file:
    for string in list_of_strings:
        print(string, file=file)

Output in ‘example.txt’:

Save
the
code!

This snippet uses a file context to open ‘example.txt’ and loops through the list of strings, using print() with the file argument to append each string to the file on a new line. It’s intuitive for beginners and maintains readability.

Method 4: Using the Pathlib module

The Pathlib module provides an object-oriented interface to handle filesystem paths. It includes a write_text() method for writing text to files, and when combined with reading, it can be used for appending.

Here’s an example:

from pathlib import Path

list_of_strings = ['Embrace', 'the', 'AI!']
file_path = Path('example.txt')
file_content = file_path.read_text() + "\n" + "\n".join(list_of_strings)
file_path.write_text(file_content)

Output in ‘example.txt’:

Embrace
the
AI!

In this example, we use Path to represent the file path. We read the current content, append the newline character and the new strings, and then write everything back. While it’s a clean approach, it may not be efficient for large files due to memory usage.

Bonus One-Liner Method 5: Using the ‘+’ operator with writelines()

For a quick one-liner solution, you can combine the newline character with each string in the list and pass the concatenated result to writelines().

Here’s an example:

open('example.txt', 'a').writelines(f"{s}\n" for s in ['Coding', 'is', 'bliss!'])

Output in ‘example.txt’:

Coding
is
bliss!

This method opens the file and writes lines in a single line of code, using list comprehension to handle the newline concatenation. It’s concise, but lacks the context manager’s safety for closing the file.

Summary/Discussion

Method 1:

Method 2:

Method 3:

Method 4:

Method 5: