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Raise keyword in Python with examples

Raise keyword in Python

In this tutorial we will learn about Raise keyword in Python .

What is raise keyword in Python ?

The raise keyword in Python is used to raise an custom Exception . We can also specify what kind of error and text raise keyword will show .

Syntax of raise keyword in Python

raise Exception(text)

Note :

  • If we don’t know what type of error we should raise then we simply use Exception .
  • If we know what type of error we should raise then we will use that specific error such as TypeError , IndexError etc .

Why to use raise keyword in Python ?

Suppose we have written a program which will accept only positive number . But if user gives negative number then the output of the program may go wrong . So after running this program we will get wrong output which is not a matter for a small program but for a big program it is completely waste of time and resources .

So to save the time and resources we can raise an error whenever an user provides a negative number .

# taking user input
number=int(input("Enter a positive number"))
if number<0:
    # raising exception if number is neagative       
    raise Exception("Number can't be negative")       
print(number)
First run :
Enter a positive number6
6
Second run:
Enter a positive number-4

---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
C:\Users\RAJKUM~1\AppData\Local\Temp/ipykernel_13352/1764935657.py in <module>
      3 if number<0:
      4     # raising exception if number is neagative
----> 5     raise Exception("Number can't be negative")
      6 print(number)

Exception: Number can't be negative
  • In the above code first we have taken an integer input from the user and stored it in number variable .
  • Then we have checked if the number is negative or not . If the number is negative then program will raise an Exception otherwise it will print the number .
  • After that we have executed the program twice .
  • In the first run as we give positive integer as input so the program just printed the positive integer .
  • But in the second run as we give negative integer as input so the program has raised an exception with type as Exception and text as Number can't be negative

Examples of raise keyword in Python

Example 1: Use general exception when we don’t know the type of error

To raise general exception in Python we will use Exception keyword .

raise Exception("Myerror")
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
C:\Users\RAJKUM~1\AppData\Local\Temp/ipykernel_13352/1353725599.py in <module>
----> 1 raise Exception("Myerror")

Exception: Myerror

Example 2: Use specified exception when we know the type of error

Raise TypeError in Python

If program need to use a string but we use an integer instead of that string then TypeError should be raised . In case of of any mismatch in type TypeError will be raised .

name=1
if type(name)!=str:
    raise TypeError("Only strings are allowed")
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
C:\Users\RAJKUM~1\AppData\Local\Temp/ipykernel_13352/1156069888.py in <module>
      1 name=1
      2 if type(name)!=str:
----> 3     raise TypeError("Only strings are allowed")

TypeError: Only strings are allowed
Raise ZeoDivisionError in Python

When we divide a number with another number then if the another number is 0 then ZeroDivisionError occurs .

a=10
b=0
if b==0:
    raise ZeroDivisionError("Value of divisor can't be 0")
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
C:\Users\RAJKUM~1\AppData\Local\Temp/ipykernel_13352/1594730086.py in <module>
      2 b=0
      3 if b==0:
----> 4     raise ZeroDivisionError("Value of divisor can't be 0")

ZeroDivisionError: Value of divisor can't be 0

Example 3: Use raise keyword in Python without any error

We can also use raise keyword in Python without specifying any error .

name="Rajkumar"
number=4

try:
    name+number
except:
    raise
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
C:\Users\RAJKUM~1\AppData\Local\Temp/ipykernel_13352/2094443883.py in <module>
      3 
      4 try:
----> 5     name+number
      6 except:
      7     raise

TypeError: can only concatenate str (not "int") to str

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