{"id":3076,"date":"2021-11-27T09:57:50","date_gmt":"2021-11-27T09:57:50","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=3076"},"modified":"2021-11-27T10:07:59","modified_gmt":"2021-11-27T10:07:59","slug":"python-raise-exception","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-raise-exception\/","title":{"rendered":"Python Raise Exception"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to raise exceptions by using the Python raise<\/code> statement.<\/p>\n\n\n\n

Introduction to the Python raise statement #<\/a><\/h2>\n\n\n\n

To raise an exception<\/a>, you use the raise<\/code> statement:<\/p>\n\n\n

raise<\/span> ExceptionType()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The ExceptionType()<\/code> must be subclass of the BaseException<\/code> class. Typically, it is a subclass of the Exception<\/code> class. Note that the ExceptionType<\/code> doesn’t need to be directly inherited from the Exception<\/code> class. It can indirectly inherit from a class that is a subclass of the Exception class.<\/p>\n\n\n\n
The BaseException<\/code> class has the __init__<\/code> method that accepts an *args<\/code> argument. It means that you can pass any number of arguments to the exception object when raising an exception.<\/p>\n\n\n\n
The following example uses the raise<\/code> statement to raise a ValueError<\/code> exception. It passes three arguments to the ValueError<\/code> __init__<\/code> method:<\/p>\n\n\n
try<\/span>:\n    raise<\/span> ValueError('The value error exception'<\/span>, 'x'<\/span>, 'y'<\/span>)\nexcept<\/span> ValueError as<\/span> ex:\n    print(ex.args)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
('The value error exception', 'x', 'y')<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Reraise the current exception #<\/a><\/h2>\n\n\n\n
Sometimes, you want to log an exception and raise the same exception again. In this case, you can use the raise<\/code> statement without specifying the exception object. <\/p>\n\n\n\n
For example, the following defines a division()<\/code> function that returns the division of two numbers:<\/p>\n\n\n
def<\/span> division<\/span>(a, b)<\/span>:<\/span>\n    try<\/span>:\n        return<\/span> a \/ b\n    except<\/span> ZeroDivisionError as<\/span> ex:\n        print('Logging exception:'<\/span>, str(ex))\n        raise<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If you pass zero to the second argument of the division()<\/code> function, the ZeroDivisionError<\/code> exception will occur. However, instead of handling the exception, you can log the exception and raise it again.<\/p>\n\n\n\n
Note that you don’t need to specify the exception object in the raise<\/code> statement. In this case, Python knows that the raise<\/code> statement will raise the current exception that has been caught by the except<\/code> clause.<\/p>\n\n\n\n
The following code causes a ZeroDivisionError<\/code> exception:<\/p>\n\n\n
division(1<\/span>, 0<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
And you’ll see both the logging message and the exception in the output:<\/p>\n\n\n
Logging exception: division by zero\nTraceback (most recent call last):\n  File \"c:\/pythontutorial\/app.py\", line 9, in <module>\n    division(1, 0)\n  File \"C:\/pythontutorial\/app.py\", line 3, in division\n    return a \/ b\nZeroDivisionError: division by zero<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Raise another exception during handling an exception #<\/a><\/h2>\n\n\n\n
When handling an exception, you may want to raise another exception. For example:<\/p>\n\n\n
def<\/span> division<\/span>(a, b)<\/span>:<\/span>\n    try<\/span>:\n        return<\/span> a \/ b\n    except<\/span> ZeroDivisionError as<\/span> ex:\n        raise<\/span> ValueError('b must not zero'<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In the division()<\/code> function, we raise a ValueError<\/code>exception if the ZeroDivisionError<\/code> occurs.<\/p>\n\n\n\n
If you run the following code, you’ll get the detail of the stack trace:<\/p>\n\n\n
division(1<\/span>, 0<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Traceback (most recent call last):\n  File \"C:\/pythontutorial\/app.py\", line 3, in division\n    return a \/ b\nZeroDivisionError: division by zero\n\nDuring handling of the above exception, another exception occurred:\n\nTraceback (most recent call last):\n  File \"C:\/pythontutorial\/app.py\", line 8, in <module>\n    division(1, 0)\n  File \"C:\/pythontutorial\/app.py\", line 5, in division\n    raise ValueError('b must not zero')\nValueError: b must not zero<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
First, the ZeroDivisionError<\/code> exception occurs:<\/p>\n\n\n
Traceback (most recent call last):\n  File \"C:\/pythontutorial\/app.py\", line 3, in division\n    return a \/ b\nZeroDivisionError: division by zero<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, during handling the ZeroDivisionError<\/code> exception, the ValueError<\/code> exception occurs:<\/p>\n\n\n
Traceback (most recent call last):\n  File \"C:\/pythontutorial\/app.py\", line 8, in <module>\n    division(1, 0)\n  File \"C:\/pythontutorial\/app.py\", line 5, in division\n    raise ValueError('b must not zero')\nValueError: b must not zero<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
  • Use the Python raise<\/code> statement to raise an exception.<\/li>
  • When handling exception, you can raise the same or another exception.<\/li><\/ul>\n
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