17
Lesson 17
Common file handling exceptions
Objective
By the end of this lesson, students will understand how to handle common file handling exceptions in Python, such as FileNotFoundError, PermissionError, and others. They will learn to implement error handling using try-except blocks to create robust file operations.
1. Introduction to Exceptions:
In Python, exceptions are events that disrupt the normal flow of a program. When working with file operations, various exceptions may arise, making it crucial to handle them properly to avoid program crashes and ensure smooth execution.
2. Understanding common file handling Exceptions:
By the end of this lesson, students will understand how to handle common file handling exceptions in Python, such as FileNotFoundError, PermissionError, and others. They will learn to implement error handling using try-except blocks to create robust file operations.
1. Introduction to Exceptions:
In Python, exceptions are events that disrupt the normal flow of a program. When working with file operations, various exceptions may arise, making it crucial to handle them properly to avoid program crashes and ensure smooth execution.
2. Understanding common file handling Exceptions:
- FileNotFoundError: Raised when attempting to access a file that does not exist. This is one of the most common exceptions encountered when working with files.
- PermissionError: Raised when trying to open a file in a way that the user does not have permission to perform (e.g., trying to write to a read-only file).
- IsADirectoryError: Raised when an operation intended for a file is attempted on a directory.
- IOError: A more general error raised for input/output operations that fail, though it is less common in Python 3 as it has been integrated into the more specific exceptions.
3. Handling Exceptions with Try-Except Blocks:
Using try-except blocks allows you to gracefully handle exceptions without crashing the program. This enables you to manage errors and provide user-friendly messages.
try:
with open("non_existent_file.txt", "r") as file:
content = file.read()
except FileNotFoundError:
print("Error: The file was not found.")4. Handling multiple Exceptions:
You can handle multiple exceptions by specifying them in a tuple. This is useful when you want to manage different types of errors that can occur during file operations.
try:
with open("example.txt", "r") as file:
content = file.read()
except (FileNotFoundError, PermissionError) as e:
print(f"Error: {e}")5. Catching general Exceptions:
While it's usually best to catch specific exceptions, you can also catch all exceptions using Exception. However, this should be done with caution, as it may mask unexpected errors.
try:
with open("another_file.txt", "r") as file:
content = file.read()
except Exception as e:
print(f"An unexpected error occurred: {e}")6. Using 'finally' clause:
The 'finally' clause allows you to execute code regardless of whether an exception occurred or not. This is often used for cleanup actions, such as closing files.
try:
file = open("sample.txt", "r")
content = file.read()
except FileNotFoundError:
print("Error: The file was not found.")
finally:
file.close() # Ensure the file is closed if it was opened7. Raising Exceptions:
You can raise exceptions explicitly using the 'raise' statement. This is useful for triggering errors when certain conditions are met in your code.
def read_file(file_name):
if not file_name.endswith('.txt'):
raise ValueError("Only .txt files are allowed.")
with open(file_name, "r") as file:
return file.read()
try:
read_file("document.pdf")
except ValueError as e:
print(e) # Output: Only .txt files are allowed.8. Best practices for Exception handling:
- Be Specific: Catch specific exceptions to avoid masking other bugs.
- Provide Feedback: Give informative messages to the user when an error occurs.
- Use Finally for cleanup**: Ensure resources are released (like file handles) by using 'finally'.
9. Practical use cases:
- File Operations: Handling exceptions when reading from or writing to files to prevent crashes.
- User input validation: Raising exceptions for invalid inputs in file-related functions.
- Resource management: Using 'finally' to ensure files are closed properly.
10. Exercises:
Exercise 1: FileNotFoundError handling
1. Write a function that attempts to read a file and handles FileNotFoundError, printing a custom error message.
Exercise 2: PermissionError handling
1. Create a script that tries to write to a read-only file and handles PermissionError, displaying a message indicating the issue.
Exercise 3: Multiple Exceptions
1. Write a program that attempts to open a file and handles both FileNotFoundError and PermissionError in one block.
Exercise 4: General Exception handling
1. Create a function that opens a file and prints its content but catches any exceptions, providing a message for unexpected errors.
Exercise 5: Raising custom Exceptions
1. Write a function that checks if a file has the correct extension and raises a custom 'ValueError' if it does not.
Conclusion
In this lesson, students learned about common file handling exceptions in Python and how to manage them using try-except blocks. They explored best practices for exception handling and gained skills necessary for writing robust and user-friendly file operations.