Python: Exceptions

Exceptions

Errors are normally referred as bugs in the program. They are almost always the fault of the programmer. The process of finding and eliminating errors is called debugging.

 

There are mainly two types of errors:

1. Syntax errors: The python finds the syntax errors when it parses the source program. Once it find a syntax error, the python will exit the program without running anything. Commonly occurring syntax errors are :

i) Putting a keyword at wrong place.

ii) Misspelling the keyword.

iii) Incorrect indentation.

iv) Forgetting the symbols such as comma, brackets, quotes.

v) Empty block.

2. Run time errors: If a program is syntactically correct ‒ that is, free of syntax errors ‒ it will be run by the python interpreter. However, the program may exit unexpectedly during execution if it encounters a runtime error. The run‒time errors are not detected while parsing the source program, but will occur due to some logical mistake.

Examples of runtime error are :

i) Trying to access the a file which does not exists.

ii) Performing the operation of incompatible type elements.

iii) Using an identifier which is not defined.

iv) Division by zero.

Such type of errors are handled using exception handling mechanism.

1. Handling Exceptions

Definition of exception: An exception is an event which occurs during the execution of a program that interrupts the normal flow of the program.

• In general, when a python script encounters a situation that it cannot cope with, it raises an exception.

• When a python script raises an exception, it must either handle the exception immediately otherwise it terminates and quits.

• The exception handling mechanism using the try...except...else blocks.

• The suspicious code is placed in try block.

• After try block place the except block which handles the exception elegantly.

• If there is no exception then the else block statements get executed.

Syntax of try...except...else

try:

write the suspicious code here

except Exception 1:

If Exception 1 occurs then execute this code

except Exception 2:

If Exception 2 occurs then execute this code

…

…

else:

If there is no exception then execute this code.

Example

Suppose programmer wants some integer value and some character value is entered then python will raise error. This scenario can be illustrated by following screenshot

In [1]: n=int(input("Enter some number"))

Enter some number x

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                                                                       Traceback (most recent call last)

ValueError

Cell In[1], line 1

‒‒‒‒> 1 n=int(input("Enter some number"))

ValueError: invalid literal for int() with base 10: 'x'

Such situation can be gracefully handled using exception handling mechanism as follows :

Step 1: Create a python script as follows:

Step 2: Now run the above code for both valid and invalid inputs.

Output(Run1: Execution of except block)

In [2]:

%run ExceptionDemo.py

Enter some number a

You have entered wrong data

Output(Run2: Execution of else block)

In [3]: %run ExceptionDemo.py

Enter some number10

You have entered: 10

• A single try can have multiple except statements. We can specify standard exception names for handling specific type of exception. For example

 

Example:1

Write a Python program to perform division of two numbers. Raise the exception if the wrong input(other than integer) is entered by the user. Also raise an exception when divide by zero occurs.

Solution :

try:

    a=int(input("Enter value of a: "))

    b=int(input("Enter value of b: "))

    c=a/b

except ValueError:

    print("You have entered wrong data")

except ZeroDivisionError:

    print("Divide by Zero Error!!!")

else:

print("The result: ",c)

Output(Run1)

Enter value of a: 10

Enter value of b: a

You have entered wrong data

Output(Run2)

Enter value of a: 10

Enter value of b: 0

Divide by Zero Error!!!

Output(Run3)

Enter value of a: 10

Enter value of b: 5

The result: 2.0

 

Example: 2

Write a program to read the contents of the file. If the file does not exist then raise appropriate exception.

Solution :

try:

    inFile=open("myfile.txt","rt")

except IOError:

    print("Error:File Not found")

else:

    print(inFile.read()) #displaying contents of file on getting file

Output

Error: File Not found

 

Example:3

Write a Python program to open a file having no write permission but trying to write the data. Handle this situation using exception handling mechanism.

Solution :

try:

    FileObj=open("myfile.txt",'rt')

    FileObj.write("This is my data")

except IOError:

    print("Error:File does not have write permission!!!")

else:

    print("Contents are written Successfully!!!")

Output

Error: File does not have write permission!!!

 

Standard exceptions in Python

Name and Purpose

Exception ‒ Base class for all exceptions.

ArithmeticError ‒ Base class for all errors that occur for numeric calculation.

OverflowError ‒ Raised when a calculation exceeds maximum limit for a numeric type.

FloatingPointError ‒ Raised when a floating point calculation fails.

ZeroDivisionError ‒ Raised when division or modulo by zero takes place for all numeric types.

EOFError ‒ Raised when there is no input from either the raw_input() or inputO function and the end of file is reached.

ImportError ‒ Raised when an import statement fails.

KeyboardInterrupt ‒ Raised when the user interrupts program execution, usually by pressing Ctrl+c.

NameError ‒ Raised when an identifier is not found in the local or global namespace.

IOError ‒ Raised when an input/ output operation fails.

SystemError ‒ Raised when the interpreter finds an internal problem, but when this error is encountered the python interpreter does not exit.

SystemExit ‒ Raised when python interpreter is quit by using the sys.exit() function. If not handled in the code, causes the interpreter to exit.

TypeError ‒ Raised when an operation or function is attempted that is invalid for the specified data type.

ValueError ‒ Raised when the built‒in function for a data type has the valid type of arguments, but the arguments have invalid values specified.

RuntimeError ‒ Raised when a generated error does not fall into any category.

 

Use of finally

The finally clause will be executed at the end of the try‒except block no matter what ‒ if there is no exception, if an exception is raised and handled, if an exception is raised and not handled and even if we exit the block using break, continue or return. We can use the finally clause for cleanup code that we always want to be executed.

For example:

try:

    age=int(input("Enter your age"))

except ValueError:

    print("Invalid age")

else:

    print("Your age is: ",age)

finally:

    print("Good Bye")

Output

Enter your age26

Your age is: 26

Good Bye

Output

Enter your age ten

Invalid age

Good Bye

 

Review Questions

1. Appraise use of try block and except block in Python with syntax.

2. Describe how exceptions are handled in Python with necessary examples.

3. What are exceptions? Explain the methods to handle them with example.