Raise In Python + Examples

NITIN GUPTA

In this tutorial, we are going to learn the benefits and uses of Raise In Python. You all must remember that in tutorial#24 and tutorial#76, we learned a few ways to handle the exception. In tutorial #24, we learned about try and catch. In the try block, we write the code in which an exception might occur, and in except block, we write the code as a result if an exception occurs.

 

Figure1: Try and Except in Python

Moreover, in tutorial #76, we learned about else, and finally. We use an else keyword to print something in cases where no exception occurs. It is also known as code cleaner because it will perform its action, either an exception occurs or not.

 

Figure2: Else and Finally in Python

 If you have not watched them, go and watch them first or give their descriptions a read because they are important prerequisites to this tutorial.

First, let us briefly go over the meaning of the word exception. The exception is an error that halts the program's normal functioning and displays an error onto the screen. While the try and except block are for handling exceptions, the raise keyword, on the contrary, is to raise an exception.

Following is the syntax:

Syntax of raise keyword is:

if test_condition:
raise EXCEPTION_CLASS_NAME

Taking a simple usage example:

raise ZeroDivisionError

Python has a range of built-in exceptions that we can use for our benefit. We can learn and read about the exceptions by visiting https://docs.python.org/3/library/exceptions.html Python documentation of python site. A few of these exceptions include:

  • KeyError: Raised when a mapping key is not found in the set of existing keys.
  • ValueError: Raised when a function receives an argument with the right type but an inappropriate value.
  • EOFError (End Of File Error): Raised when the input() function hits an end-of-file condition without reading any data.
  • ImportError: Raised when the import statement has trouble trying to load a module.
  • NameError: Raised when a local or global name is not found.
  • ZeroDivisionError: Raised when the second argument of a division is zero.

These are only a few names. We will discuss few build-in exceptions in our code today, but you can search for more on the internet according to your requirement as nearly every possible exception is already out there. Covering so many of them in a single tutorial is impossible, so I will give you an idea about them with examples of the most frequently used exceptions. We can also make custom or user-defined exceptions that fulfills our purpose.

Example:-

Before moving on to their detailed work, let us cover the reason for their use. Suppose we have made a program in which we want a number that is greater than 10. Now the user is giving the input (x),  5. So in such a case, we can raise ValueError, returning an error to the user that the input is wrong. By doing this, we can save the program running time and prevent the program from storing the wrong input.

You can use the raise keyword to signal that the situation is exceptional to the normal flow. For example:

x = 5
if x < 10:
   raise ValueError('x should not be less than 10!')

Notice how you can write the error message with more information inside the parentheses. The example above gives the following output (by default, the interpreter will print a traceback and the error message):

>>>
Traceback (most recent call last):
File "C:/Python34/Scripts/raise1.py", line 3, in <module>
raise ValueError('x should not be less than 10!')
ValueError: x should not be less than 10!
>>>

Code as described/written in the video


# a = input("What is your name")
# b = input("How much do you earn")
# if int(b)==0:
#     raise ZeroDivisionError("b is 0 so stopping the program")
# if a.isnumeric():
#     raise Exception("Numbers are not allowed")
#
# print(f"Hello {a}")
# 1000 lines taking 1 hour

# Task - Write about 2 built in exception

c = input("Enter your name")
try:
    print(a)

except Exception as e:

    if c =="harry":
        raise ValueError("Harry is blocked he is not allowed")

    print("Exception handled")

 

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