Python Exceptions: An Introduction

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December 01, 2024
Python Exceptions: An Introduction
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Python exceptions provide a mechanism for handling errors that occur during the execution of a program. Unlike syntax errors, which are detected by the parser, Python raises exceptions when an error occurs in syntactically correct code. Knowing how to raise, catch, and handle exceptions effectively helps to ensure your program behaves as expected, even when encountering errors.

In Python, you handle exceptions using a try … except block. This structure allows you to execute code normally while responding to any exceptions that may arise. You can also use else to run code if no exceptions occur, and the finally clause to execute code regardless of whether an exception was raised.

By the end of this tutorial, you’ll understand that:

  • Exceptions in Python occur when syntactically correct code results in an error.
  • You can handle exceptions using the try, except, else, and finally keywords.
  • The try … except block lets you execute code and handle exceptions that arise.
  • Python 3 introduced more built-in exceptions compared to Python 2, making error handling more granular.
  • It’s bad practice to catch all exceptions at once using except Exception or the bare except clause.
  • Combining try, except, and pass allows your program to continue silently without handling the exception.
  • Using try … except is not inherently bad, but you should use it judiciously to handle only known issues appropriately.

In this tutorial, you’ll get to know Python exceptions and all relevant keywords for exception handling by walking through a practical example of handling a platform-related exception. Finally, you’ll also learn how to create your own custom Python exceptions.

Take the Quiz: Test your knowledge with our interactive “Python Exceptions: An Introduction” quiz. You’ll receive a score upon completion to help you track your learning progress:

An Introduction to Python Exceptions

Interactive Quiz

Python Exceptions: An Introduction

In this quiz, you'll test your understanding of Python exceptions. You'll cover the difference between syntax errors and exceptions and learn how to raise exceptions, make assertions, and use the try and except block.

Understanding Exceptions and Syntax Errors

Syntax errors occur when the parser detects an incorrect statement. Observe the following example:

Python Traceback 
>>> print(0 / 0))
  File "<stdin>", line 1
    print(0 / 0))
                ^
SyntaxError: unmatched ')'

The arrow indicates where the parser ran into the syntax error. Additionally, the error message gives you a hint about what went wrong. In this example, there was one bracket too many. Remove it and run your code again:

Python 
>>> print(0 / 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

This time, you ran into an exception error. This type of error occurs whenever syntactically correct Python code results in an error. The last line of the message indicates what type of exception error you ran into.

Instead of just writing exception error, Python details what type of exception error it encountered. In this case, it was a ZeroDivisionError. Python comes with various built-in exceptions as well as the possibility to create user-defined exceptions.

Raising an Exception in Python

There are scenarios where you might want to stop your program by raising an exception if a condition occurs. You can do this with the raise keyword:

Illustration of raise statement usage

You can even complement the statement with a custom message. Assume that you’re writing a tiny toy program that expects only numbers up to 5. You can raise an error when an unwanted condition occurs:

Python low.py 
number = 10
if number > 5:
    raise Exception(f"The number should not exceed 5. ({number=})")
print(number)

In this example, you raised an Exception object and passed it an informative custom message. You built the message using an f-string and a self-documenting expression.

When you run low.py, you’ll get the following output:

Python Traceback 
Traceback (most recent call last):
  File "./low.py", line 3, in <module>
    raise Exception(f"The number should not exceed 5. ({number=})")
Exception: The number should not exceed 5. (number=10)

The program comes to a halt and displays the exception to your terminal or REPL, offering you helpful clues about what went wrong. Note that the final call to print() never executed, because Python raised the exception before it got to that line of code.

With the raise keyword, you can raise any exception object in Python and stop your program when an unwanted condition occurs.

Debugging During Development With assert

Read the full article at https://realpython.com/python-exceptions/ »

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December 01, 2024 at 07:30PM
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