Errors are problems in a program that prevent it from running correctly. They can be syntax errors (invalid code structure) or runtime errors (errors that occur while the program is executing).
Exceptions are events that disrupt the normal flow of a program. They are raised when an error occurs and can be handled to prevent program crashes.
Types of Errors
- Syntax errors: These occur due to incorrect syntax (e.g., missing colon, mismatched parentheses).
- Runtime errors: These occur during program execution (e.g., division by zero, accessing an index out of range).
- Logical errors: These are errors in the program’s logic that produce incorrect results.
Exception Handling
Python uses the try-except block to handle exceptions:
Python
try:
# code that might raise an exception
except ExceptionType:
# code to handle the exception
Example:
Python
def divide(x, y):
try:
result = x / y
return result
except ZeroDivisionError:
print("Error: Division by zero")
return None
try, except, else, and finally
try: Encloses code that might raise an exception.except: Handles specific exceptions.else: Executes if no exception occurs.finally: Always executes, regardless of whether an exception occurs.
Python
try:
f = open("myfile.txt")
# Perform file operations
except FileNotFoundError:
print("File not found")
else:
print("File found")
finally:
f.close()
Raising Exceptions
You can raise custom exceptions using the raise keyword:
Python
raise ValueError("Invalid value")
Common Built-in Exceptions
IndexError: When a sequence index is out of range.ValueError: When a built-in operation or function receives an argument with an invalid type or value.TypeError: When an operation or function is applied to an object of inappropriate type.ZeroDivisionError: When the second argument of a division or modulo operation is zero.FileNotFoundError: When a file or directory is not found.
By effectively handling exceptions, you can create more robust and reliable Python programs.
Error and Exception Handling
What is the difference between an error and an exception?
An error is a problem in the code that prevents it from running, while an exception is an event that occurs during program execution that disrupts the normal flow.
Why is exception handling important?
It prevents program crashes, provides informative error messages, and allows for graceful recovery.
How does a try-except block work?
The try block contains code that might raise an exception. If an exception occurs, the except block handles it.
How do I raise a custom exception?
Use the raise keyword with an exception class.
When should I raise a custom exception?
When you want to signal a specific error condition.
Should I handle all exceptions?
1. No, handle only the exceptions you can reasonably recover from.
2. Provide informative error messages to help with debugging.
3. Use specific exception types for better error handling.