Python's Exception Hierarchy

Every Exception in Python is a class.

All exception classes are child classes of BaseException.i.e every exception class extends

BaseException either directly or indirectly. Hence BaseException acts as root for Python

Exception Hierarchy.

Most of the times being a programmer we have to concentrate Exception and its child

classes.

Customized Exception Handling by using try-except:

It is highly recommended to handle exceptions.

The code which may raise exception is called risky code and we have to take risky code

inside try block. The corresponding handling code we have to take inside except block.

Attribute

Error

Arithmetic

Error

EOF

Error

Name

Error

Lookup

Error

OS

Error

Type

Error

Value

Error

BaseException

Exception KeyboardInterrupt SystemExit GeneratorExit

ZeroDivision

Error

FloatingPoint

Error

Overflow

Error

Index

Error

Key

Error

FileNotFound

Error

Interrupted

Error

Permission

Error

TimeOut

Error

try:

Risky Code

except XXX:

Handling code/Alternative Code

without try-except:

4.

• Output
• stmt-1
• ZeroDivisionError: division by zero Abnormal termination/Non-Graceful Termination with try-except:

• try:

3. print(10/0)

• except ZeroDivisionError:

7.

• Output
• stmt-1
• 5.0
• stmt-3 Normal termination/Graceful Termination Control Flow in try-except: try: stmt-1 stmt-2 stmt-3 except XXX: stmt-4 stmt-5 case-1: If there is no exception 1,2,3,5 and Normal Termination case-2: If an exception raised at stmt-2 and corresponding except block matched 1,4,5 Normal Termination case-3: If an exception raised at stmt-2 and corresponding except block not matched 1, Abnormal Termination case-4: If an exception raised at stmt-4 or at stmt-5 then it is always abnormal termination. Conclusions:
• within the try block if anywhere exception raised then rest of the try block wont be executed eventhough we handled that exception. Hence we have to take only risky code inside try block and length of the try block should be as less as possible.
• In addition to try block,there may be a chance of raising exceptions inside except and finally blocks also.
• If any statement which is not part of try block raises an exception then it is always abnormal termination. How to print exception information: try:

• except ZeroDivisionError as msg:

4.

• Output exception raised and its description is: division by zero try with multiple except blocks: The way of handling exception is varied from exception to exception.Hence for every exception type a seperate except block we have to provide. i.e try with multiple except blocks is possible and recommended to use. Eg: try:
• ------
• ------
• ------ except ZeroDivisionError: perform alternative arithmetic operations except FileNotFoundError: use local file instead of remote file If try with multiple except blocks available then based on raised exception the corresponding except block will be executed. Eg:

9)

10) D:\Python_classes>py test.py
11) Enter First Number: 10
12) Enter Second Number: 2
13) 5.0

14)

15) D:\Python_classes>py test.py
16) Enter First Number: 10
17) Enter Second Number: 0
18) Can't Divide with Zero

19)

20) D:\Python_classes>py test.py
21) Enter First Number: 10
22) Enter Second Number: ten
23) please provide int value only

If try with multiple except blocks available then the order of these except blocks is

important .Python interpreter will always consider from top to bottom until matched

except block identified.

Eg:

1) try:
2) x=int(input("Enter First Number: "))
3) y=int(input("Enter Second Number: "))
4) print(x/y)
5) except ArithmeticError :
6) print("ArithmeticError")
7) except ZeroDivisionError:
8) print("ZeroDivisionError")

9)

10) D:\Python_classes>py test.py
11) Enter First Number: 10
12) Enter Second Number: 0
13) ArithmeticError

Single except block that can handle multiple exceptions:

We can write a single except block that can handle multiple different types of exceptions.

except (Exception1,Exception2,exception3,..): or

except (Exception1,Exception2,exception3,..) as msg :

Parenthesis are mandatory and this group of exceptions internally considered as tuple.

Eg:

1) try:
2) x=int(input("Enter First Number: "))
3) y=int(input("Enter Second Number: "))
4) print(x/y)
5) except (ZeroDivisionError,ValueError) as msg:
6) print("Plz Provide valid numbers only and problem is: ",msg)

7)

8) D:\Python_classes>py test.py
9) Enter First Number: 10
10) Enter Second Number: 0
11) Plz Provide valid numbers only and problem is: division by zero

12)

13) D:\Python_classes>py test.py
14) Enter First Number: 10
15) Enter Second Number: ten
16) Plz Provide valid numbers only and problem is: invalid literal for int() with b
17) ase 10: 'ten'

Default except block:

We can use default except block to handle any type of exceptions.

In default except block generally we can print normal error messages.

Syntax:

except:

statements

Eg:

1) try:
2) x=int(input("Enter First Number: "))
3) y=int(input("Enter Second Number: "))
4) print(x/y)
5) except ZeroDivisionError:
6) print("ZeroDivisionError:Can't divide with zero")
7) except:
8) print("Default Except:Plz provide valid input only")

9)

10) D:\Python_classes>py test.py
11) Enter First Number: 10
12) Enter Second Number: 0
13) ZeroDivisionError:Can't divide with zero

14)

15) D:\Python_classes>py test.py
16) Enter First Number: 10
17) Enter Second Number: ten
18) Default Except:Plz provide valid input only

• **Note: If try with multiple except blocks available then default except block should be last,otherwise we will get SyntaxError. Eg:

1) try:
2) print(10/0)
3) except:
4) print("Default Except")
5) except ZeroDivisionError:
6) print("ZeroDivisionError")

7)

8) SyntaxError: default 'except:' must be last

Note:

The following are various possible combinations of except blocks

• except ZeroDivisionError:
• except ZeroDivisionError as msg:
• except (ZeroDivisionError,ValueError) :
• except (ZeroDivisionError,ValueError) as msg:
• except : finally block:
• It is not recommended to maintain clean up code(Resource Deallocating Code or Resource Releasing code) inside try block because there is no guarentee for the execution of every statement inside try block always.
• It is not recommended to maintain clean up code inside except block, because if there is no exception then except block won't be executed.