Python - Working with Text Files

Working with Text Files in Python

Introduction

Text files are one of the most fundamental components in computing. From storing configuration data to logging events or saving user input, text files are everywhere. In Python, working with text files is simple and powerful thanks to the built-in file handling capabilities.

Python provides functions that allow developers to open, read, write, and manipulate text files in various ways. This document covers everything you need to know about working with text files in Python—ranging from basic reading and writing to advanced file manipulation techniques.

What is a Text File?

A text file is a file that contains human-readable characters and can be opened using a text editor like Notepad, Sublime Text, or Visual Studio Code. Text files typically have extensions like .txt, .csv, .log, or .html. These files store plain text, and unlike binary files, they do not contain formatting, images, or compiled code.

Opening a Text File

The open() Function

The open() function is used to open files in Python. It returns a file object that can be used to read from or write to the file.

file = open("example.txt", "r")

The two main parameters are:

• Filename: The name (and path) of the file
• Mode: How the file is to be opened (read, write, etc.)

Common File Modes for Text Files

• 'r' – Read (default)
• 'w' – Write (overwrites existing)
• 'a' – Append (adds to the end)
• 'r+' – Read and Write
• 'w+' – Write and Read (overwrites)
• 'a+' – Append and Read

Reading from a Text File

read() Method

The read() method reads the entire content of a file as a single string.

with open("example.txt", "r") as file:
    content = file.read()
    print(content)

readline() Method

Reads a single line from the file. Useful when processing large files line by line.

with open("example.txt", "r") as file:
    line1 = file.readline()
    print(line1)

readlines() Method

Returns a list containing all lines in the file.

with open("example.txt", "r") as file:
    lines = file.readlines()
    for line in lines:
        print(line.strip())

Writing to a Text File

write() Method

The write() method writes a string to the file. If the file does not exist, it will be created. In write mode, existing content is erased.

with open("example.txt", "w") as file:
    file.write("Hello, world!")

writelines() Method

The writelines() method writes a list of strings to the file.

lines = ["First line\n", "Second line\n", "Third line\n"]
with open("example.txt", "w") as file:
    file.writelines(lines)

Appending to a Text File

Appending data means adding new data at the end of the file without altering the existing content.

with open("example.txt", "a") as file:
    file.write("This is an appended line.\n")

Reading and Writing Simultaneously

You can use modes like r+, w+, and a+ to read and write simultaneously.

with open("example.txt", "r+") as file:
    file.write("Start of the file\n")
    file.seek(0)
    print(file.read())

Working with File Paths

Absolute vs Relative Paths

- Absolute path: full path to the file (e.g., /home/user/docs/file.txt)
- Relative path: relative to the script’s location (e.g., docs/file.txt)

Using os and pathlib

import os

current_dir = os.getcwd()
file_path = os.path.join(current_dir, "example.txt")

from pathlib import Path

file_path = Path("example.txt")
print(file_path.resolve())

Using with Statement

Using the with statement is the safest and most efficient way to work with files in Python. It ensures the file is properly closed after its suite finishes.

with open("example.txt", "r") as file:
    data = file.read()

Why Use with ?

• Automatic file closure
• Cleaner code
• Handles exceptions gracefully

Working with Large Text Files

When dealing with large text files, reading the entire file at once can be memory-intensive. It is better to process the file line by line using a loop.

with open("largefile.txt", "r") as file:
    for line in file:
        print(line.strip())

Modifying Contents of a Text File

To modify a file’s contents, you generally:

1. Read the file content
2. Make modifications in memory
3. Write the updated content back

with open("example.txt", "r") as file:
    lines = file.readlines()

lines[0] = "Modified first line\n"

with open("example.txt", "w") as file:
    file.writelines(lines)

Handling File Exceptions

try:
    with open("nonexistent.txt", "r") as file:
        data = file.read()
except FileNotFoundError:
    print("The file was not found.")
except IOError:
    print("An I/O error occurred.")

Working with CSV Text Files

CSV (Comma Separated Values) files are plain text files often used for storing tabular data.

import csv

# Reading CSV
with open("data.csv", "r") as file:
    reader = csv.reader(file)
    for row in reader:
        print(row)

# Writing CSV
with open("output.csv", "w", newline='') as file:
    writer = csv.writer(file)
    writer.writerow(["Name", "Age"])
    writer.writerow(["Alice", "30"])

Working with JSON Text Files

JSON files are also text files that store data in key-value pairs.

import json

# Writing JSON
data = {"name": "John", "age": 25}
with open("data.json", "w") as file:
    json.dump(data, file)

# Reading JSON
with open("data.json", "r") as file:
    content = json.load(file)
    print(content)

Counting Words and Lines

with open("example.txt", "r") as file:
    lines = file.readlines()
    print(f"Lines: {len(lines)}")

    words = sum(len(line.split()) for line in lines)
    print(f"Words: {words}")

Search and Replace in a Text File

with open("example.txt", "r") as file:
    content = file.read()

content = content.replace("old", "new")

with open("example.txt", "w") as file:
    file.write(content)

Removing Blank Lines

with open("example.txt", "r") as file:
    lines = file.readlines()

non_blank = [line for line in lines if line.strip() != ""]

with open("example.txt", "w") as file:
    file.writelines(non_blank)

Using File Encoding

Text files can be encoded in formats like UTF-8, ASCII, etc. It is often a good practice to specify encoding explicitly.

with open("example.txt", "r", encoding="utf-8") as file:
    print(file.read())

Summary of Best Practices

• Use with statement to manage file resources
• Always specify file modes appropriately
• Be careful with 'w' mode—it overwrites files
• Use encoding explicitly when dealing with non-ASCII text
• Handle exceptions to avoid runtime errors
• Use readlines() with caution on large files

Working with text files in Python is a fundamental skill that opens the door to a wide range of applications, from data logging to configuration handling and file-based databases. Python's simple and intuitive syntax makes file manipulation both easy and powerful. Whether you're just reading a file line by line or performing complex updates and transformations, Python provides all the tools necessary to interact with text files safely and effectively.

Understanding how to open, read, write, append, and modify text files, as well as how to handle exceptions and work with encoding, will prepare you to manage text-based data in real-world projects. Practicing these operations and exploring additional modules like csv  and json will further strengthen your proficiency in Python file handling.