class 6

Exception Handling in Python:

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:
   Do you 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 above 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.

The except clause with no exceptions:

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

try:
   Do you operations here;
   ......................
except:
   If there is any exception, then execute this block.
   ......................
else:
   If there is no exception then execute this block. 

The except clause with multiple exceptions:

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

try:
   Do you operations here;
   ......................
except(Exception1[, Exception2[,...ExceptionN]]]):
   If there is any exception from the given exception list, 
   then execute this block.
   ......................
else:
   If there is no exception then execute this block. 

Standard Exceptions:

Here is a list standard Exceptions available in Python:

Standard Exception

The try-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. The syntax of the try-finally statement is this:

try:
   Do you operations here;
   ......................
   Due to any exception, this may be skipped.
finally:
   This would always be executed.
   ......................

Example:

#!/usr/bin/python

try:
   fh = open("testfile", "w")
   fh_r = open("Break.py", "r")
   fh_r.write("Kalyan");
   fh.write("This is my test file for exception handling!!")
except ValueError: #IOError:
   print "Error: can\'t find file or read data"
except:
   print "error";
else:
   print "Written content in the file successfully"
finally:
   print ("In the finally block");
   fh.close()
   if fh_r :
        fh_r.close()
 

output:

error
In the finally block

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 as follows:

try:
   You do your operations here;
   ......................
except ExceptionType, Argument:
   You can print value of Argument here...

If you are writing the code to handle a single exception, you can have a variable follow the name of the exception in the except statement. If you are trapping multiple exceptions, you can have a variable follow the tuple of the exception.

This variable will receive the value of the exception mostly containing the cause of the exception. The variable can receive a single value or multiple values in the form of a tuple. This tuple usually contains the error string, the error number, and an error location.

Example:

Following is an example for a single exception:

#!/usr/bin/python

# Define a function here.
def temp_convert(var):
   try:
      return int(var)
   except ValueError, Argument:
      print "The argument does not contain numbers\n", Argument

# Call above function here.
temp_convert("xyz");

This would produce following result:

The argument does not contain numbers
invalid literal for int() with base 10: 'xyz'

Raising an exceptions:

You can raise exceptions in several ways by using the raise statement. The general syntax for the raise statement.

Syntax:

raise [Exception [, args [, traceback]]]

Here Exception is the type of exception (for example, NameError) and argument is a value for the exception argument. The argument is optional; if not supplied, the exception argument is None.

The final argument, traceback, is also optional (and rarely used in practice), and, if present, is the traceback object used for the exception

Example:

An exception can be a string, a class, or an object. Most of the exceptions that the Python core raises are classes, with an argument that is an instance of the class. Defining new exceptions is quite easy and can be done as follows:

def functionName( level ):
   if level < 1:
      raise "Invalid level!", level
      # The code below to this would not be executed
      # if we raise the exception

Note: In order to catch an exception, an "except" clause must refer to the same exception thrown either class object or simple string. For example to capture above exception we must write our except clause as follows:

try:
   Business Logic here...
except "Invalid level!":
   Exception handling here...
else:
   Rest of the code here...

User-Defined Exceptions:

Python also allows you to create your own exceptions by deriving classes from the standard built-in exceptions.

Here is an example related to RuntimeError. Here a class is created that is subclassed from RuntimeError. This is useful when you need to display more specific information when an exception is caught.

In the try block, the user-defined exception is raised and caught in the except block. The variable e is used to create an instance of the class Networkerror.

class Networkerror(RuntimeError):
   def __init__(self, arg):
      self.args = arg

So once you defined above class, you can raise your exception as follows:

try:
   raise Networkerror("Bad hostname")
except Networkerror,e:
   print e.args

Assertion 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 following result:

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