When an error occurs, or exception as we call it, Python will normally stop and generate an error message.

These exceptions can be handled using the try statement

• The try block lets you test a block of code for errors.
• The except block lets you handle the error.
• The finally block lets you execute code, regardless of the result of the try- and except blocks.

Example:This statement will raise an error, because x is not defined

print(x)

Traceback (most recent call last):
  File "<pyshell#26>", line 1, in <module>
    print(x)
NameError: name 'x' is not defined

The try block will generate an exception, because x is not defined.Since the try block raises an error, the except block will be executed.

try:
  print(x)
except:
  print("An exception occurred")

An exception occurred

Many Exceptions

You can define as many exception blocks as you want, e.g. if you want to execute a special block of code for a special kind of error:

Example:Print one message if the try block raises a NameError and another for other errors:

try:
  print(x)
except NameError:
  print("Variable x is not defined")
except:
  print("Something else went wrong")

Variable x is not defined

Else Block

You can use the else keyword to define a block of code to be executed if no errors were raised:

Example:In this example, the try block does not generate any error:

try:
    x=int(input('enter any number:'))
    y=100/x       
except ValueError:
    print('Nothing entered')
except ZeroDivisionError:
    print('enter any number other then Zero')
else:
    print(y)

enter any number:10
10.0

Finally Block

The finally block, if specified, will be executed regardless if the try block raises an error or not.

try:
    x=int(input('enter any number:'))
    y=100/x   
except ValueError:
    print('Nothing entered')
except ZeroDivisionError:
    print('enter any number other then Zero')
else:
    print(y)
finally:
    print('the try : except is finished')

enter any number:0
enter any number other then Zero
the try : except is finished

Raise an exception

As a Python developer you can choose to throw an exception if a condition occurs.To throw (or raise) an exception, use the raise keyword.

Example:Raise an error and stop the program if x is lower than 0

x = -1
if x < 0:
  raise Exception("Sorry, no numbers below zero")

Traceback (most recent call last):
  File "C:/Users/Pallavi/AppData/Local/Programs/Python/Python36/my programs/new module.py", line 3, in <module>
    raise Exception("Sorry, no numbers below zero")
Exception: Sorry, no numbers below zero

The raise keyword is used to raise an exception.

You can define what kind of error to raise, and the text to print to the user.

Example:Raise a TypeError if x is not a string

x = 35
if type(x)!=str:
    raise Exception("Only strings are allowed")

Traceback (most recent call last):
  File "C:/Users/Pallavi/AppData/Local/Programs/Python/Python36/my programs/new module.py", line 3, in <module>
    raise Exception("Only strings are allowed")
Exception: Only strings are allowed