Exception Handling

An exception is an unexpected event that occurs during program execution.

Python Logical Errors (Exceptions)

Errors that occur at runtime (after passing the syntax test) are called exceptions or logical errors.

For instance, they occur when we

• try to open a file(for reading) that does not exist (FileNotFoundError)
• try to divide a number by zero (ZeroDivisionError)
• try to import a module that does not exist (ImportError) and so on.
• Whenever these types of runtime errors occur, Python creates an exception object.

Error in Python can be of two types i.e. Syntax errors and Exceptions. Errors are the problems in a program due to which the program will stop the execution. On the other hand, exceptions are raised when some internal events occur which changes the normal flow of the program. When these exceptions occur, the Python interpreter stops the current process and passes it to the calling process until it is handled. If not handled, the program will crash. For example, let us consider a program where we have a function A that calls function B, which in turn calls function C. If an exception occurs in function C but is not handled in C, the exception passes to B and then to A. If never handled, an error message is displayed and our program comes to a sudden unexpected halt.

Try and Except Statement – Catching Exceptions

Try and except statements are used to catch and handle exceptions in Python. Statements that can raise exceptions are kept inside the try clause and the statements that handle the exception are written inside except clause.

Example: Let us try to access the array element whose index is out of bound and handle the corresponding exception.

# Python program to handle simple runtime error
#Python 3
a = [1, 2, 3]
try:
   print ("Second element = %d" %(a[1]))
   # Throws error since there are only 3 elements in array
   print ("Fourth element = %d" %(a[3]))
except:
   print ("An error occurred")