Python Custom Exceptions

In this tutorial, you will learn how to define custom exceptions relying on your prerequisites with the help of examples.

Python has variously built-in exceptions that force your program to output an error when something in the program goes wrong.

However, sometimes you may need to make your own custom exceptions that serve your purpose.

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Making Custom Exceptions

In Python, users can define custom exemptions by making another class. This exception class must be inferred, either directly or indirectly, from the inherent Exception class. A large portion of the inherent exemptions is also derived from this class.

>>> class CustomError(Exception):
...     pass
...

>>> raise CustomError
Traceback (most recent call last):
...
__main__.CustomError

>>> raise CustomError("An error occurred")
Traceback (most recent call last):
...
__main__.CustomError: An error occurred

Here, we have made a client characterized exemption called CustomError which acquires from the Exception class. These new exceptions, as different exemptions, can be raised using the raise proclamation with a discretionary error message.

When we are developing up a huge Python program, it is a decent practice to put all the client characterized special cases that our program brings up in a different record. Numerous standard modules do this. They characterize their special cases independently as exceptions.py or errors.py (generally but not always).

User-defined exception class can actualize everything an ordinary class can do, yet we by and large make them basic and succinct. Most executions proclaim a custom base class and get others, special case classes, from this base class. This concept is made clearer in the following example.

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Example: User-Defined Exception in Python

In this example, we will represent how client characterized special cases can be utilized in a program to raise and catch blunders.

This program will request that the client enter a number until they guess a stored number accurately. To help them figure it out, an indication is given whether their guess is greater than or less than the stored number.

# define Python user-defined exceptions
class Error(Exception):
    """Base class for other exceptions"""
    pass


class ValueTooSmallError(Error):
    """Raised when the input value is too small"""
    pass


class ValueTooLargeError(Error):
    """Raised when the input value is too large"""
    pass


# you need to guess this number
number = 10

# user guesses a number until he/she gets it right
while True:
    try:
        i_num = int(input("Enter a number: "))
        if i_num < number:
            raise ValueTooSmallError
        elif i_num > number:
            raise ValueTooLargeError
        break
    except ValueTooSmallError:
        print("This value is too small, try again!")
        print()
    except ValueTooLargeError:
        print("This value is too large, try again!")
        print()

print("Congratulations! You guessed it correctly.")

Here is a sample run of this program.

Enter a number: 12
This value is too large, try again!

Enter a number: 0
This value is too small, try again!

Enter a number: 8
This value is too small, try again!

Enter a number: 10
Congratulations! You guessed it correctly.

We have defined a base class called Error.

The other two exceptions (ValueTooSmallError and ValueTooLargeError) that are actually raised by our program are derived from this class. This is the standard way to define user-defined exceptions in Python programming, but you are not limited to this way only.

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Customizing Exception Classes

We can further customize this class to acknowledge different contentions according to our requirements.

To find out about altering the Exception classes, you have to have the fundamental information on Object-Oriented programming.

Visit Python Object Oriented Programming to start learning about Object-Oriented programming in Python.

Let’s look at one example:

class SalaryNotInRangeError(Exception):
    """Exception raised for errors in the input salary.

    Attributes:
        salary -- input salary which caused the error
        message -- explanation of the error
    """

    def __init__(self, salary, message="Salary is not in (5000, 15000) range"):
        self.salary = salary
        self.message = message
        super().__init__(self.message)


salary = int(input("Enter salary amount: "))
if not 5000 < salary < 15000:
    raise SalaryNotInRangeError(salary)

Output

Enter salary amount: 2000
Traceback (most recent call last):
  File "<string>", line 17, in <module>
    raise SalaryNotInRangeError(salary)
__main__.SalaryNotInRangeError: Salary is not in (5000, 15000) range

Here, we have overridden the constructor of the Exception class to accept our own custom arguments salary and message. Then, the constructor of the parent Exception class is called manually with the self. message argument using super().

The custom self. salary attribute is defined to be used later.

The inherited __str__ method of the Exception class is then used to display the corresponding message when SalaryNotInRangeError is raised.

We can also customize the __str__ method itself by overriding it.

class SalaryNotInRangeError(Exception):
    """Exception raised for errors in the input salary.

    Attributes:
        salary -- input salary which caused the error
        message -- explanation of the error
    """

    def __init__(self, salary, message="Salary is not in (5000, 15000) range"):
        self.salary = salary
        self.message = message
        super().__init__(self.message)

    def __str__(self):
        return f'{self.salary} -> {self.message}'


salary = int(input("Enter salary amount: "))
if not 5000 < salary < 15000:
    raise SalaryNotInRangeError(salary)

Output

Enter salary amount: 2000
Traceback (most recent call last):
  File "/home/bsoyuj/Desktop/Untitled-1.py", line 20, in <module>
    raise SalaryNotInRangeError(salary)
__main__.SalaryNotInRangeError: 2000 -> Salary is not in (5000, 15000) range

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