3. Exception Handling

In any programming language there are 2 types of errors are possible.

1. Syntax Errors
2. Runtime Errors

3.1 Syntax Errors

The errors which occurs because of invalid syntax are called syntax errors.

x=10
if x==10
print("Hello")

# SyntaxError: invalid syntax

Programmer is responsible to correct these syntax errors. Once all syntax errors are corrected then only program execution will be started.

3.2 Runtime Errors

AKA exceptions. While executing the program if something goes wrong because of end user input or programming logic or memory problems etc then we will get Runtime Errors.

print(10/0)
# ZeroDivisionError: division by zero

print(10/"ten")
# TypeError: unsupported operand type(s) for /: 'int' and 'str

3.3 Exception

An unwanted and unexpected event that disturbs normal flow of program is called exception.

It is highly recommended to handle exceptions. The main objective of exception handling is Graceful Termination of the program

To handle exception and proper execution of program we put Risky code inside try-except block

try-except

try:
    Risky Code
except XYZ:
    Handling code/Alternative Code

• Default except block - handles all exception

try:
    Risky Code
except:
    Handling code/Alternative Code

• try with multiple except blocks

try:
    Risky Code
except ZeroDivisionError:
    Handling code/Alternative Code
except FileNotFoundError:
    Handling code/Alternative Code

• Single except block that can handle multiple exceptions

try:
    Risky Code
except (Exception1,Exception2,exception3,..):
    Handling code/Alternative Code

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

try-except-finally

finally block is executed always irrespective of whether exception raised or not raised and whether exception handled or not handled.

try:
    Risky Code
except XYZ:
    Handling Code
finally:
    Cleanup code

There is only one situation where finally block won’t be executed ie whenever we are using os._exit(0) function.

3.4 Custom Exception

We can define custom exception by using "raise" keyword

age = -1
if age < 1:
    raise RuntimeError("Wrong age")
else:
    print("Done")

class NegativeAgeException(Exception):
    def __init__(self,arg):
        self.msg=arg

age = -1
if age < 1:
    raise NegativeAgeException("Age can't be Negative")
else:
    print("Valid Age")

3.5 List Of General Use Exceptions

try :
    a = 10 
    10/0
except ZeroDivisionError as e :
    print(e)
# division by zero

try :
    int("sudh")
except (ValueError , TypeError) as e :
    print(e)
# invalid literal for int() with base 10: 'sudh'

# Not good practice
# Shoulld use always a specific exception

try :
    int("sudh")
except  :
    print("this will catch an error")
# this will catch an error


try :
    with open("test.txt" , 'r') as f :
        f.read()
except Exception as e :
    print("test " , e)
except FileNotFoundError as e :
    print("this is my file not found type error " , e)

# test  [Errno 2] No such file or directory: 'test.txt'
# last `except block` will never get executed

try :
    import sudh
except ImportError as e : 
    print(e)
# No module named 'sudh'

try :
    d = {1: [3,4,5,6] , "key" :"sudh"}
    d["key10"]
except KeyError as e : 
    print(e)
# 'key10'

try :
    "sudh".test()
except AttributeError as e :
    print(e)
# 'str' object has no attribute 'test'

try :
    l = [1,2,3,3]
    l[10]
except IndexError as e :
    print(e)
# list index out of range

try :
    123 + "sudh"
except TypeError as e :
    print(e)
# unsupported operand type(s) for +: 'int' and 'str'

try :
    with open("test.txt" , 'r') as f :
        f.read()
except FileNotFoundError as e :
    print(e)
# [Errno 2] No such file or directory: 'test.txt'

3.6 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.

# Print Python Exception Hierarchy
def treeClass(cls, ind=0):
    # print name of the class
    print('-' * ind, cls.__name__)

    # iterating through subclasses
    for i in cls.__subclasses__():
        treeClass(i, ind + 3)

print("Hierarchy for Built-in exceptions is : ")
treeClass(BaseException)