File Handling

Reading and Writing Data

Introduction

File handling is an essential skill in programming, allowing you to work with external data sources such as text files, CSV files, and JSON files. In this post, we’ll cover how to read from and write to files in Python, handle common errors, and manage file paths effectively.

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1. Basics of File Handling

Python uses the built-in open() function to work with files.

Opening a File

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file = open("example.txt", mode="r")  # Open in read mode

File Modes:

• r: Read (default)
• w: Write (overwrites existing content)
• a: Append (adds content without overwriting)
• r+: Read and write

Closing a File

Always close files to free resources:

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file.close()

Alternatively, use a with statement to handle files automatically:

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

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2. Reading Files

Reading Entire Content

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

Reading Line by Line

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with open("example.txt", "r") as file:
    for line in file:
        print(line.strip())  # Remove trailing newlines

Reading Specific Number of Characters

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with open("example.txt", "r") as file:
    snippet = file.read(10)  # Read first 10 characters
    print(snippet)

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3. Writing Files

Writing New Content

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

Appending Content

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with open("example.txt", "a") as file:
    file.write("Adding more content.\n")

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4. Working with CSV Files

Python’s csv module makes it easy to read and write CSV files.

Reading CSV Files

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import csv

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

Writing CSV Files

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with open("output.csv", "w", newline="") as file:
    writer = csv.writer(file)
    writer.writerow(["Name", "Age", "City"])
    writer.writerow(["Alice", 30, "New York"])

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5. Working with JSON Files

Python’s json module is useful for handling JSON data.

Reading JSON

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import json

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

Writing JSON

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with open("output.json", "w") as file:
    json.dump({"name": "Alice", "age": 30}, file, indent=4)

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6. Handling Errors

When working with files, errors like missing files or permissions issues can occur. Use try-except to handle them gracefully.

Example:

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try:
    with open("nonexistent.txt", "r") as file:
        content = file.read()
except FileNotFoundError:
    print("File not found!")
except IOError:
    print("An error occurred while accessing the file.")

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7. Managing File Paths

Python’s os and pathlib modules help manage file paths across platforms.

Using os

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import os

file_path = os.path.join("folder", "file.txt")
print(file_path)  # Output depends on your OS

Using pathlib

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from pathlib import Path

file_path = Path("folder") / "file.txt"
print(file_path)

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8. Practical Example

Problem: Processing a Log File

Read a log file, filter specific entries, and save them to a new file.

Code Example:

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

filtered = [line for line in lines if "ERROR" in line]

with open("error_logs.txt", "w") as file:
    file.writelines(filtered)

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Conclusion

File handling in Python opens doors to working with a wide variety of data sources. Whether it’s reading plain text, parsing CSV files, or handling JSON data, mastering file operations is an indispensable skill.