{"id":3049,"date":"2021-11-27T02:57:54","date_gmt":"2021-11-27T02:57:54","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=3049"},"modified":"2025-03-28T03:40:22","modified_gmt":"2025-03-28T03:40:22","slug":"python-exceptions","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-exceptions\/","title":{"rendered":"Python Exceptions"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the Python exceptions and how to handle them gracefully in programs.<\/p>\n\n\n\n

Introduction to Python exceptions #<\/a><\/h2>\n\n\n\n

In Python, exceptions are objects of the exception classes<\/a>. All exception classes are the subclasses of the BaseException<\/code> class. <\/p>\n\n\n\n

However, almost all built-in exception classes inherit from the Exception<\/code> class, which is the subclass of the BaseException<\/code> class:<\/p>\n\n\n

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\"python<\/figure>\n<\/div>\n\n\n

This page shows a complete class hierarchy for built-in exceptions in Python<\/a>.<\/p>\n\n\n\n

The following example defines a list of three elements and attempts to access the fourth one:<\/p>\n\n\n

colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>]\n\nprint(colors[3<\/span>])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The invalid index caused the IndexError<\/code> exception as expected:<\/p>\n\n\n
IndexError: list index out of range<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>]\n\ntry<\/span>:\n    print(colors[3<\/span>])\nexcept<\/span> IndexError as<\/span> e:\n    print(e)\n\n\nprint('Continue to run'<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
<class 'IndexError'> - list index out of range\nContinue to run<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, we use the try...except<\/code> statement to handle the IndexError<\/code> exception. As you can see from the output, the program continues to run after the try...except<\/code> statement.<\/p>\n\n\n\n
The IndexError<\/code> class inherits from the LookupError<\/code> class which inherits from the Exception<\/code> class:<\/p>\n\n\n
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And you can catch either LookupError<\/code> or Exception<\/code> class when an IndexError<\/code> exception occurs. For example:<\/p>\n\n\n
colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>]\n\ntry<\/span>:\n    print(colors[3<\/span>])\nexcept<\/span> LookupError as<\/span> e:\n    print(e.__class__, '-'<\/span>, e)\n\nprint('Continue to run'<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
<class 'IndexError'> - list index out of range\nContinue to run<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the exception is still IndexError<\/code> even though we catch the LookupError<\/code> exception. Therefore, when you handle an exception, the exception handler will catch the exception type you specify and any of its subclasses.<\/p>\n\n\n\n
The program runs the same if you use the Exception<\/code> class instead of the LookupError<\/code> class:<\/p>\n\n\n
colors = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>]\n\ntry<\/span>:\n    print(colors[3<\/span>])\nexcept<\/span> Exception as<\/span> e:\n    print(e.__class__, '-'<\/span>, e)\n\nprint('Continue to run'<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
<class 'IndexError'> - list index out of range\nContinue to run<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
In practice, you should catch the exceptions as specific as possible so that you know how to deal with each exception in a specific way.<\/p>\n\n\n\n
Python exception handling example #<\/a><\/h2>\n\n\n\n
The following example defines a division<\/code> function that returns the result of a is divided by b:<\/p>\n\n\n
def<\/span> division<\/span>(a, b)<\/span>:<\/span>\n    return<\/span> a \/ b\n\n\nc = division(10<\/span>, 0<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
ZeroDivisionError: division by zero<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, if b is zero, the ZeroDivisionError<\/code> exception will occur. To handle the ZeroDivisionError<\/code> exception, you use the try...except<\/code> statement as follows:<\/p>\n\n\n
def<\/span> division<\/span>(a, b)<\/span>:<\/span>\n    try<\/span>:\n        return<\/span> {\n            'success'<\/span>: True<\/span>,\n            'message'<\/span>: 'OK'<\/span>,\n            'result'<\/span>: a \/ b\n        }\n    except<\/span> ZeroDivisionError as<\/span> e:\n        return<\/span> {\n            'success'<\/span>: False<\/span>,\n            'message'<\/span>: 'b cannot be zero'<\/span>,\n            'result'<\/span>: None<\/span>\n        }\n\n\nresult = division(10<\/span>, 0<\/span>)\nprint(result)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
{'success'<\/span>: False<\/span>, 'message'<\/span>: 'b cannot be zero'<\/span>, 'result'<\/span>: None}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the function returns a dictionary that has three elements:<\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
When an exception occurs, Python stops the program unless you handle it. To handle an exception, you use the try...except<\/a><\/code> statement. For example:<\/p>\n\n\n