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What is an Exception in Python?

Exception in Python language is an unusual exceptional event(describing an error), which occurs only during the runtime of a program.

During the execution of a program, when an exception occurs, it is said to be thrown by the Python interpreter.

If this thrown exception is not handled, it stops the normal execution of the program and it ends abruptly. That's why we should handle an exception when it is thrown by the program. The process of handling an exception is called Exception Handling.

Some situations due to which an Exception may arise :

• When an integer number is divided by zero, it raises an exception.
• When we try to access an array with an out-of-range value, an exception is thrown.
• When we are trying to read a file which doesn't exist.

What happens when an Exception is thrown?

Exceptions are thrown at the runtime of a program. If an exception, which is thrown during the runtime of a program, is not handled, the program terminates right away without executing the remaining statements. Let us understand this by an example here.



Looking at the diagram above, it is easy to understand that if an exception is thrown when statement2 is executed, the program ends abruptly without executing any statement after it. Hence, statement3 and statement4 will not be executed as a result of an exception at statement2.

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Some important types of Exceptions in Python:

In the table below, we have given a short preview about some important exceptions in Python.

Exception Class	Description
BaseException	This is a base class for all built-in exceptions.
KeyboardInterrupt	This type of exception is raised when a user pressed CTRL+C to interrupt a program's execution.
SystemExit	This type of exception is raised by sys.exit() function.
Exception	This is a base class for all the built-in exceptions, except system exiting exceptions. User defined exceptions should be derived from this class.
StandardError	This is a base class for all the built-in exceptions except StopIteration, GeneratorExit, KeyboardInterrupt and SystemExit classes.
BufferError	This type of exception is raised during failure of buffer related processing.
ArithmeticError	This is a base class for all built-in exceptions that are raised for arithmetic errors
FloatingPointError	This type of exception is raised during a floating-point error.
OverflowError	This type of exception is raised when result of an arithmetic expression is too large to be displayed.
ZeroDivisionError	This type of exception is raised when a number is divided by a zero.
AssertionError	This type of exception is raised during the failure of an assert statement.
AttributeError	This type of exception is raised during the failure of an attribute reference or assignment.
EnvironmentError	This type of exception can occur outside the Python environment system.
IOError	This type of exception is raised during the failure of an I/O operation.
OSError	This type of exception is raised during an operating system related error.
EOFError	This type of exception is raised when EOF(end-of-file) is reached during the call to one of the built-in functions - input() or raw_input() functions, without reading any data.
ImportError	This type of exception is raised during the failure of import statement to find and import a module's definition.
LookupError	This is a base class for all exceptions raised due an invalid key or index used to mapping.
IndexError	This type of exception is raised when a searched index is not found or is out-of-range.
KeyError	This type of exception is raised when a searched key is not found.
MemoryError	This type of exception is raised when an operation runs out of memory.
NameError	This type of exception is raised when a local or global name is not found or its value is not defined.
UnboundLocalError	This type of exception is raised when a local variable is referenced in a function but there is no value bound to it.
RuntimeError	This type of exception is raised when a detected error does not fall in any category.
NotImplementedError	This type of exception is raised when a function of abstract class is not implemented in a derived class.
SyntaxError	This type of exception is raised when there is a syntax error in Python's script/program.
IndentationError	This type of exception is raised during an erroneous indentation in a program.
TabError	This type of exception is raised during an incorrect use of tab and spaces in indentation.
SystemError	This type of exception is raised when Python's interpreter find an internal error but still does not quit or exit.
TypeError	This type of exception is raised when an operation or function is executed over an object of an invalid data type.
ValueError	This type of exception is raised when an operation function recieves an argument whose value does not match with its specified type.
UnicodeError	This type of exception is raised during an error in Unicode coding/decoding.
KeyError	This type of exception is raised when a searched key is not found.

In the next few articles, we will discuss some very important keywords which are used to handle exceptions - try, except, finally and raise.

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