{"id":3093,"date":"2021-11-27T16:59:50","date_gmt":"2021-11-27T16:59:50","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=3093"},"modified":"2025-03-28T03:42:17","modified_gmt":"2025-03-28T03:42:17","slug":"python-raise-from","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-raise-from\/","title":{"rendered":"Python raise from"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn how to use the Python raise from statement to raise an exception with extra information.<\/p>\n\n\n\n

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

The raise from<\/code> statement has the following syntax:<\/p>\n\n\n

raise<\/span> <ExceptionType> from<\/span> <cause><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Technically, it’s equivalent to the following:<\/p>\n\n\n
ex = ExceptionType\nex.__cause__ = cause\nraise<\/span> ex<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Python raise from statement example #<\/a><\/h2>\n\n\n\n
The following divide()<\/code> function divides a number by another and returns the result of the division:<\/p>\n\n\n
def<\/span> divide<\/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 be zero'<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The divide()<\/code> function has an exception handler that catches the ZeroDivisionError<\/code> exception. Inside the handler, we raise a new ValueError<\/code> exception. <\/p>\n\n\n\n
If you pass zero to the second argument of the divide()<\/code> function: <\/p>\n\n\n
def<\/span> divide<\/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 be zero'<\/span>) from<\/span> ex\n\n\ndivide(10<\/span>, 0<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
you’ll get the following stack trace:<\/p>\n\n\n
Traceback (most recent call last):\n  File \"c:\/python\/app.py\"<\/span>, line 3<\/span>, in<\/span> divide\n    return<\/span> 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:\/python\/app.py\"<\/span>, line 8<\/span>, in<\/span> <module>\n    divide(10<\/span>, 0<\/span>)\n  File \"c:\/python\/app.py\"<\/span>, line 5<\/span>, in<\/span> divide\n    raise<\/span> ValueError('b must not be zero'<\/span>)\nValueError: b must not<\/span> be zero<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The import message is:<\/p>\n\n\n
During handling of the above exception, another exception occurred:<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
It means that while you were handling the ZeroDivisionError<\/code> exception, the ValueError<\/code> exception occurred.<\/p>\n\n\n\n
To instruct Python that you want to modify and forward the ZeroDivisionError<\/code> exception, you can use the raise from<\/code> statement like this:<\/p>\n\n\n
def<\/span> divide<\/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 be zero'<\/span>) from<\/span> ex\n\ndivide(10<\/span>, 0<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
When you run the code, you’ll get the following stack trace:<\/p>\n\n\n
Traceback (most recent call last):\n  File \"c:\/python\/app.py\"<\/span>, line 3<\/span>, in<\/span> divide\n    return<\/span> a \/ b\nZeroDivisionError: division by zero\n\nThe above exception was the direct cause of the following exception:\n\nTraceback (most recent call last):\n  File \"c:\/python\/app.py\"<\/span>, line 8<\/span>, in<\/span> <module>\n    divide(10<\/span>, 0<\/span>)\n  File \"c:\/python\/app.py\"<\/span>, line 5<\/span>, in<\/span> divide\n    raise<\/span> ValueError('b must not be zero'<\/span>) from<\/span> ex\nValueError: b must not<\/span> be zero<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Now, you receive the ValueError<\/code> exception with a cause added to the __cause__ attribute of the exception object. <\/p>\n\n\n\n
The following modifies the above code to show the __cause__<\/code> attribute of the ValueError<\/code> exception:<\/p>\n\n\n
def<\/span> divide<\/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 be zero'<\/span>) from<\/span> ex\n\n\ntry<\/span>:\n    divide(10<\/span>, 0<\/span>)\nexcept<\/span> ValueError as<\/span> ex:\n    print('cause:'<\/span>, ex.__cause__)\n    print('exception:'<\/span>, ex)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
cause: division by zero\nexception: b must not<\/span> be zero<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Python raise exception from None #<\/a><\/h2>\n\n\n\n
If the cause of the exception is not important, you can omit the cause by using the raise exception from None<\/code> statement:<\/p>\n\n\n
raise<\/span> <ExceptionType> from<\/span> None<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
 For example, you can hide the cause of the ValueError<\/code> exception in the divide()<\/code> function as follows:<\/p>\n\n\n
def<\/span> divide<\/span>(a, b)<\/span>:<\/span>\n    try<\/span>:\n        return<\/span> a \/ b\n    except<\/span> ZeroDivisionError:\n        raise<\/span> ValueError('b must not be zero'<\/span>) from<\/span> None<\/span>\n\n\ntry<\/span>:\n    divide(10<\/span>, 0<\/span>)\nexcept<\/span> ValueError as<\/span> ex:\n    print('cause:'<\/span>, ex.__cause__)\n    print('exception:'<\/span>, ex)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
cause: None<\/span>\nexception: b must not<\/span> be zero<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Now, the __cause__<\/code> is None<\/code>. Also, the divide()<\/code> function raises the ValueError<\/code> exception without any additional information.<\/p>\n\n\n\n
If you remove the try<\/code> statement in the code that calls the divide()<\/code> function:<\/p>\n\n\n
def<\/span> divide<\/span>(a, b)<\/span>:<\/span>\n    try<\/span>:\n        return<\/span> a \/ b\n    except<\/span> ZeroDivisionError:\n        raise<\/span> ValueError('b must not be zero'<\/span>) from<\/span> None<\/span>\n\n\ndivide(10<\/span>, 0<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
you’ll get the following stack trace:<\/p>\n\n\n
Traceback (most recent call last):\n  File \"c:\/python\/app.py\"<\/span>, line 8<\/span>, in<\/span> <module>\n    divide(10<\/span>, 0<\/span>)\n  File \"c:\/python\/app.py\"<\/span>, line 5<\/span>, in<\/span> divide\n    raise<\/span> ValueError('b must not be zero'<\/span>) from<\/span> None<\/span>\nValueError: b must not<\/span> be zero<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
    \n
  • Use the Python raise from<\/code> statement to modify and forward an existing exception.<\/li>\n\n\n\n
  • Use the raise exception from None<\/code>  statment to hide the cause of the exception.<\/li>\n<\/ul>\n
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Try it<\/a><\/p>\n\n\n\n
By default, the __cause__<\/code> attribute on exception<\/a> objects is always initialized to None<\/a><\/code>. <\/p>\n\n\n\n