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
the statement has the following syntax:
raise <ExceptionType> from <cause>
Code language: Python (python)
Technically, it’s equivalent to the following:
ex = ExceptionType
ex.__cause__ = cause
raise ex
Code language: Python (python)
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')
Code language: Python (python)
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 exdivide(10, 0)
Code language: Python (python)
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 zeroDuring 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
Code language: Python (python)
The import message is:
During handling of the above exception, another exception occurred:
Code language: Python (python)
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 exdivide(10, 0)
Code language: Python (python)
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 zeroThe 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
Code language: Python (python)
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 extry:
divide(10, 0)
except ValueError as ex:
print('cause:', ex.__cause__)
print('exception:', ex)
Code language: Python (python)
Output:
cause: division by zero
exception: b must not be zero
Code language: Python (python)
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
Code language: Python (python)
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 Nonetry:
divide(10, 0)
except ValueError as ex:
print('cause:', ex.__cause__)
print('exception:', ex)
Code language: Python (python)
Output:
cause: None
exception: b must not be zero
Code language: Python (python)
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 Nonedivide(10, 0)
Code language: Python (python)
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
Code language: Python (python)
Summary
- Use the Python
raise from
statement to modify and forward an existing exception. - Use the
raise exception from None
statement to hide the cause of the exception.