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Python provides two very important features to handle any unexpected error in your Python programs and to add debugging capabilities in them −

• Exception Handling − This would be covered in this tutorial. Here is a list standard Exceptions available in Python: Standard Exceptions.
  

• Assertions − This would be covered in Assertions in Python tutorial.

List of Standard Exceptions −

Sr.No.	Exception Name & Description
1	Exception Base class for all exceptions
2	StopIteration Raised when the next() method of an iterator does not point to any object.
3	SystemExit Raised by the sys.exit() function.
4	StandardError Base class for all built-in exceptions except StopIteration and SystemExit.
5	ArithmeticError Base class for all errors that occur for numeric calculation.
6	OverflowError Raised when a calculation exceeds maximum limit for a numeric type.
7	FloatingPointError Raised when a floating point calculation fails.
8	ZeroDivisionError Raised when division or modulo by zero takes place for all numeric types.
9	AssertionError Raised in case of failure of the Assert statement.
10	AttributeError Raised in case of failure of attribute reference or assignment.
11	EOFError Raised when there is no input from either the raw_input() or input() function and the end of file is reached.
12	ImportError Raised when an import statement fails.
13	KeyboardInterrupt Raised when the user interrupts program execution, usually by pressing Ctrl+c.
14	LookupError Base class for all lookup errors.
15	IndexError Raised when an index is not found in a sequence.
16	KeyError Raised when the specified key is not found in the dictionary.
17	NameError Raised when an identifier is not found in the local or global namespace.
18	UnboundLocalError Raised when trying to access a local variable in a function or method but no value has been assigned to it.
19	EnvironmentError Base class for all exceptions that occur outside the Python environment.
20	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.
21	IOError Raised for operating system-related errors.
22	SyntaxError Raised when there is an error in Python syntax.
23	IndentationError Raised when indentation is not specified properly.
24	SystemError Raised when the interpreter finds an internal problem, but when this error is encountered the Python interpreter does not exit.
25	SystemExit Raised when Python interpreter is quit by using the sys.exit() function. If not handled in the code, causes the interpreter to exit.
26	TypeError Raised when an operation or function is attempted that is invalid for the specified data type.
27	ValueError Raised when the built-in function for a data type has the valid type of arguments, but the arguments have invalid values specified.
28	RuntimeError Raised when a generated error does not fall into any category.
29	NotImplementedError Raised when an abstract method that needs to be implemented in an inherited class is not actually implemented.

Assertions in Python

An assertion is a sanity-check that you can turn on or turn off when you are done with your testing of the program.

The easiest way to think of an assertion is to liken it to a raise-if statement (or to be more accurate, a raise-if-not statement). An expression is tested, and if the result comes up false, an exception is raised.

Assertions are carried out by the assert statement, the newest keyword to Python, introduced in version 1.5.

Programmers often place assertions at the start of a function to check for valid input, and after a function call to check for valid output.

The assert Statement

When it encounters an assert statement, Python evaluates the accompanying expression, which is hopefully true. If the expression is false, Python raises an AssertionError exception.

The syntax for assert is −

assert Expression[, Arguments]

If the assertion fails, Python uses ArgumentExpression as the argument for the AssertionError. AssertionError exceptions can be caught and handled like any other exception using the try-except statement, but if not handled, they will terminate the program and produce a traceback.

Example

Here is a function that converts a temperature from degrees Kelvin to degrees Fahrenheit. Since zero degrees Kelvin is as cold as it gets, the function bails out if it sees a negative temperature −

#!/usr/bin/python

def KelvinToFahrenheit(Temperature):

   assert (Temperature >= 0),"Colder than absolute zero!"

   return ((Temperature-273)*1.8)+32

print KelvinToFahrenheit(273)

print int(KelvinToFahrenheit(505.78))

print KelvinToFahrenheit(-5)

When the above code is executed, it produces the following result −

32.0
451
Traceback (most recent call last):
File "test.py", line 9, in 
print KelvinToFahrenheit(-5)
File "test.py", line 4, in KelvinToFahrenheit
assert (Temperature >= 0),"Colder than absolute zero!"
AssertionError: Colder than absolute zero!

What is 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.

Syntax

Here is simple syntax of try....except...else blocks −

try:
   You do your operations here;
   ......................
except ExceptionI:
   If there is ExceptionI, then execute this block.
except ExceptionII:
   If there is ExceptionII, then execute this block.
   ......................
else:
   If there is no exception then execute this block. 

Here are few important points about the above-mentioned syntax −

• A single try statement can have multiple except statements. This is useful when the try block contains statements that may throw different types of exceptions.

• You can also provide a generic except clause, which handles any exception.

• After the except clause(s), you can include an else-clause. The code in the else-block executes if the code in the try: block does not raise an exception.

• The else-block is a good place for code that does not need the try: block's protection.

Example

This example opens a file, writes content in the, file and comes out gracefully because there is no problem at all −

#!/usr/bin/python

try:

   fh = open("testfile", "w")

   fh.write("This is my test file for exception handling!!")

except IOError:

   print "Error: can\'t find file or read data"

else:

   print "Written content in the file successfully"

   fh.close()

This produces the following result −

Written content in the file successfully

The except Clause with No Exceptions

You can also use the except statement with no exceptions defined as follows −

try:
   You do your operations here;
   ......................
except:
   If there is any exception, then execute this block.
   ......................
else:
   If there is no exception then execute this block. 

This kind of a try-except statement catches all the exceptions that occur. Using this kind of try-except statement is not considered a good programming practice though, because it catches all exceptions but does not make the programmer identify the root cause of the problem that may occur.

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