Exception Handling In Python

Definition

Exception Handling In Python is one of the features of OOPS. It is mainly designed to handle runtime errors and the exceptions that corrupt the flow of the program. The general format is the usage of “try” and “except” keywords. There may be some block of code that may get an error during the runtime so we run that code inside the try block and if the code gets interrupted by any errors, it leaves the block and goes to the except block.

try:
    statements
except:
    statements

try:
    a = 5
    b = 0
    print(a/b)
except:
    print("ZeroDivision Error")

ZeroDivision Error

Explanation:

In the try block, the number is divided by zero which faces an exception and moves to the except. Here, no specific error type is found, and prints the message passed in the except block.

Keywords used with Exception Handling

There are 5 keywords that are mainly used in exception handling–

• Try
• Except
• Else
• Finally
• raise

Handling Common Errors

Some of the common types of errors that can be easily handled in Python are–

• Arithmetic error – occurs when the arithmetic calculations fail.
• Overflow error – occurs when the input size exceeds the initialized size of the variable.
• Floating-point error – occurs when the calculation of floating-point values fails.
• Zero division error – occurs when the number is divided by zero.
• Import error – occurs when the import file is wrong or mismatched.
• Key error – occurs when the key is not found or mismatched in the dictionary.
• Name error –  occurs when the variable name is not found or mismatched.
• Syntax error – occurs when the syntax for the logic used is wrong.
• Type Error – occurs when the data types of the variables do not match.
• IO error – occurs when the input or output acts as expected.
• Value error – occurs when the object created to store the value does not exist or fails to store the value.

try:
    a = 10
    b = "9"
    print(a + b)
except TypeError:
    print("Type error encountered")

Type error encountered

Explanation

The variables a and b are of not the same type i.e. a is numeric and b is a string. Thus, there is a difference in the data type of the two variables that’s why the code execution moves to the except block where the error is identified and prints the TypeError message.

try:
    a=10
    b='9'
    c=0
    print(a/c)
    print(a+b)
except TypeError:
    print('Type error encountered')
except ZeroDivisionError:
    print('Tried to divide the number by zero')

Tried to divide the number by zero

Explanation

Here, two statements are present, one is “dividing by zero” and “adding a number with a string“. In this case, when the first exception(a/c) is found in line 5, the flow of the program is broken and moves to the appropriate except section.

The else Keyword

The else keyword is used after the except block. The code flow is designed in such a way that, if the code does not face any exceptions/runtime errors then, it moves to the else block after executing the try block, otherwise it goes to the except block and halts the execution of the program.

try :
     statements
except:
     statements
else:
     statements

Now, let’s see examples–

try:
    a=10
    b=0
    c=4
    print(a/c)
except ZeroDivisionError:
    print('Tried to divide the number by zero')
else:
    print('No exception found.. Entered else part')

2.5
No exception found.. Entered else part

Explanation

The variables a and c are divided since the value of both are not 0 so it does not face exception and enter the else part.

try:
    a=10
    b=0
    c=4
    print(a/b)
except ZeroDivisionError:
    print('Tried to divide the number by zero')
else:
    print('No exception found.. Entered else part')

Tried to divide the number by zero

Explanation

Since the value of b is 0, it faces the zero division error and does not enter the else section.

The Finally Keyword

The finally keyword is placed at the end of the program. It will be executed no matter exception occurs or not.

try :
    statements
except:
    statements
finally:
    statements

Let’s see an example–

try:
    a=10
    b=0
    c=4
    print(a/b)
except ZeroDivisionError:
    print('Tried to divide the number by zero')
else:
    print('No exception found.. Entered else part')
finally:
    print('Entered finally.. End of the program')

Tried to divide the number by zero
Entered finally.. End of the program

Explanation

The program starts with the try block, since the number is divided by zero it faces the exception. Due to the exception, it does not move to the else block but executes the finally block.

The Raise Keyword

The built-in exceptions can be handled implicitly. But, Python also allows the user to trigger the exception i.e. the user can specify at what condition of the block the exception must be carried out.

try:
    condition
    raise exception
        statements
except Exception:
        statements

Let’s see an example–

try:
    a=10
    b=-2
    if b<0:
        raise ValueError
    else:
        print(a/b)
except ValueError:
    print(' Don’t divide by negative number... it will give negative result')

Don’t divide by negative number... it will give negative result

Explanation

In general, the value error occurs the object created to store the value does not exist or fails to store the value, but in this code, the ValueError is triggered when the value of b is negative.

Printing the errors

If we don't know the error type and want to know the error then we can use as keyword to print the error.

try:
    # some code
except Exception as e:
    print(e)

Now let's see some examples--

try:
  print(12/0)
except Exception as e:
  print(e)

division by zero

Explanation

In the second line, print(12/0) will give an error so, the program execution goes to except block where we first catch the error using as keyword and stored the error in the e variable, then we simply printed the e.

Now, what if we have not used try... except... blocks, let's see--

print(12/0)

Traceback (most recent call last):
  File "/tmp/sessions/cc324f5c8863ef98/main.py", line 1, in 
    print(12/0)
ZeroDivisionError: division by zero

Explanation

Here, we have tried to print the value which will occur due to 12/0 which is an error obviously, and the program terminates with an error, while in the above example, we have printed the error by using try-except block and as keyword.