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