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Python Exception

What is an Exception?

An exception is an event, which occurs during the execution of a program that disrupts the normal flow of the program's instructions.

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

An exception is a Python object that represents an error.

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

Handling an exception

If you have some suspicious code that may raise an exception, you can defend your program by placing the suspicious code in a try: block. After the try: block, include an except: statement, followed by a block of code which handles the problem as elegantly as possible.

The except Clause with Multiple Exceptions

You can also use the same except statement to handle multiple exceptions as follows:

The finally Clause

You can use a finally: block along with a try: block.

The finally: block is a place to put any code that must execute, whether the try: block raised an exception or not.

Argument of an Exception

An exception can have an argument, which is a value that gives additional information about the problem.

The contents of the argument vary by exception.

You capture an exception's argument by supplying a variable in the except clause.

Python Standard Exceptions

Here is a list of Standard Exceptions available in Python:

Exception Description
Exception Base class for all exceptions
StopIteration Raised when the next() method of an iterator does not point to any object
SystemExit Raised by the sys.exit() function
StandardError Base class for all built-in exceptions except StopIteration and SystemExit
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
ZeroDivisonError Raised when division or modulo by zero takes place for all numeric types
AssertionError Raised in case of failure of the Assert statement
AttributeError Raised in case of failure of attribute reference or assignment
EOFError Raised when there is no input from either the raw_input() or input() 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
LookupError Base class for all lookup errors
IndexError Raised when an index is not found in a sequence
KeyError Raised when the specified key is not found in the dictionary
NameError Raised when an identifier is not found in the local or global namespace
UnboundLocalError Raised when trying to access a local variable in a function or method but no value has been assigned to it
EnvironmentError Base class for all exceptions that occur outside the Python environment
IOError Raised when an input/ output operation fails, such as the print statement or the open() function when trying to open a file that does not exist
OSError Raised for operating system-related errors
SyntaxError Raised when there is an error in Python syntax
IndentationError Raised when indentation is not specified properly
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
NotImplementedError Raised when an abstract method that needs to be implemented in an inherited class is not actually implemented