Python raise from

Summary: in this tutorial, you will learn how to use the Python raise from statement to raise an exception with extra information.

Introduction to the Python raise from statement #

The raise from statement has the following syntax:

raise <ExceptionType> from <cause>

Technically, it’s equivalent to the following:

ex = ExceptionType
ex.__cause__ = cause
raise ex

By default, the __cause__ attribute on exception objects is always initialized to None.

Python raise from statement example #

The following divide() function divides a number by another and returns the result of the division:

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError as ex:
        raise ValueError('b must not be zero')

The divide() function has an exception handler that catches the ZeroDivisionError exception. Inside the handler, we raise a new ValueError exception.

If you pass zero to the second argument of the divide() function:

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError as ex:
        raise ValueError('b must not be zero') from ex


divide(10, 0)

you’ll get the following stack trace:

Traceback (most recent call last):
  File "c:/python/app.py", line 3, in divide
    return a / b
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "c:/python/app.py", line 8, in <module>
    divide(10, 0)
  File "c:/python/app.py", line 5, in divide
    raise ValueError('b must not be zero')
ValueError: b must not be zero

The import message is:

During handling of the above exception, another exception occurred:

It means that while you were handling the ZeroDivisionError exception, the ValueError exception occurred.

To instruct Python that you want to modify and forward the ZeroDivisionError exception, you can use the raise from statement like this:

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError as ex:
        raise ValueError('b must not be zero') from ex

divide(10, 0)

When you run the code, you’ll get the following stack trace:

Traceback (most recent call last):
  File "c:/python/app.py", line 3, in divide
    return a / b
ZeroDivisionError: division by zero

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "c:/python/app.py", line 8, in <module>
    divide(10, 0)
  File "c:/python/app.py", line 5, in divide
    raise ValueError('b must not be zero') from ex
ValueError: b must not be zero

Now, you receive the ValueError exception with a cause added to the __cause__ attribute of the exception object.

The following modifies the above code to show the __cause__ attribute of the ValueError exception:

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError as ex:
        raise ValueError('b must not be zero') from ex


try:
    divide(10, 0)
except ValueError as ex:
    print('cause:', ex.__cause__)
    print('exception:', ex)

Output:

cause: division by zero
exception: b must not be zero

Python raise exception from None #

If the cause of the exception is not important, you can omit the cause by using the raise exception from None statement:

raise <ExceptionType> from None

For example, you can hide the cause of the ValueError exception in the divide() function as follows:

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        raise ValueError('b must not be zero') from None


try:
    divide(10, 0)
except ValueError as ex:
    print('cause:', ex.__cause__)
    print('exception:', ex)

Try it

Output:

cause: None
exception: b must not be zero

Now, the __cause__ is None. Also, the divide() function raises the ValueError exception without any additional information.

If you remove the try statement in the code that calls the divide() function:

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        raise ValueError('b must not be zero') from None


divide(10, 0)

you’ll get the following stack trace:

Traceback (most recent call last):
  File "c:/python/app.py", line 8, in <module>
    divide(10, 0)
  File "c:/python/app.py", line 5, in divide
    raise ValueError('b must not be zero') from None
ValueError: b must not be zero

Summary #

Use the Python raise from statement to modify and forward an existing exception.
Use the raise exception from None statment to hide the cause of the exception.

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