Python TypeError: exceptions must derive from BaseException is a common error that may occur when you work with exceptions.
This tutorial helps you understand why this error happens and how to resolve it.
Why TypeError: exceptions must derive from BaseException occurs
The TypeError: exceptions must derive from BaseException occurs when you try to raise an error without passing a class that inherits from the BaseException class.
For example, suppose you want to raise an error when the x variable is not a string like this:
x = 9
if type(x) is not str:
raise "Error! x must be a string" # ❌
Because the raise keyword above is not followed with a class that inherits from the BaseException class, Python will show the following error message:
Traceback (most recent call last):
File ...
raise "Error! x must be a string"
TypeError: exceptions must derive from BaseException
To fix this error, you need to pass a built-in exception or your custom exception class.
How to fix TypeError: exceptions must derive from BaseException
Depending on the nature of your error message, you can choose from a wide selection of exception classes provided by Python.
When you have a wrong type as in the above example, you would use the TypeError class like this:
x = 9
if type(x) is not str:
raise TypeError("Error! x must be a string") # ✅
Other common exception classes are AssertionError, ValueError, or RuntimeError.
You can use any exception class as shown in this Exception hierarchy.
You can even define your own custom exception class in Python. To do so, you need to create a class that extends from the Exception class.
Suppose you want to create an exception class named MyException. Here’s how you do it:
class MyException(Exception):
def __init__(self, message):
self.message = message
def __str__(self):
return self.message
Once defined, you can use the class when raising an error as follows:
class MyException(Exception):
def __init__(self, message):
self.message = message
def __str__(self):
return self.message
x = 9
if type(x) is not str:
raise MyException("Error! x must be a string") # ✅
But creating your own exception classes will increase the complexity of your code.
I’d recommend you only create a customized exception class when the built-in ones don’t cover the error you need to raise.
Should I use Exception or BaseException?
Python developers are recommended to create a custom exception class using the Exception class instead of BaseException.
This is because the BaseException class include errors that must not be caught by the try ... except Exception block, such as SystemExit.
When you create your own custom exceptions, you want those exceptions to be caught and show a traceback to help you identify the error.
This is the reason why you’re recommended to use the Exception class.
Conclusion
To conclude, the TypeError: exceptions must derive from BaseException occurs when you try to raise an error without defining the exception class.
To fix this error, you need to pass a built-in or custom exception class that’s relevant to the error message you want to raise.
Good work resolving this error! See you in other tutorials 👍