Handling File Errors

When working with files, many things can go wrong:

• The file does not exist.
• You do not have permission to read it.
• The disk is full when writing.

If you do not handle these situations, your program crashes. Let us learn how to make file operations safe.

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Catching FileNotFoundError

try:
    with open("config.txt", "r") as file:
        data = file.read()
        print(data)
except FileNotFoundError:
    print("Error: config.txt not found! Using default settings.")

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Checking If a File Exists Before Opening

Python's os.path module lets you check if a file exists before trying to open it:

import os

file_path = "data.csv"

if os.path.exists(file_path):
    with open(file_path, "r") as file:
        print(file.read())
else:
    print(f"'{file_path}' does not exist.")

Which approach is better?

• Use try-except when you expect the file to usually exist (EAFP — Easier to Ask Forgiveness than Permission).
• Use os.path.exists() when you want to check upfront and take a different action.

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Handling Permission Errors

try:
    with open("/etc/shadow", "r") as file:
        print(file.read())
except PermissionError:
    print("Error: You do not have permission to read this file.")
except FileNotFoundError:
    print("Error: File not found.")

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Quick Summary

• FileNotFoundError: Raised when opening a non-existent file path in read mode.
• PermissionError: Raised when attempting to access restricted or locked OS files.
• os.path.exists(path): Pre-checks if a target file exists before attempting to open it.
• Defensive Practice: Always wrap file operations in structured try-except blocks.

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What's Next?

Now let's explore structured tabular datasets and spreadsheets using Python's built-in CSV Basics in the next lesson!