exceptions_handling

=Exceptions Handling= Python provides two very important features to handle any unexpected error in your Python programs and to add debugging capabilities in them: > > = 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 can't 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 would terminate and come out. = 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.
 *  **Exception Handling:** This would be covered in this tutorial.
 *  **Assertions:** This would be covered in [|Assertions in Python] tutorial.

Syntax:
Here is simple syntax of //try....except...else// blocks: code 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. code || 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.

Example:
Here is simple example which opens a file and writes the content in the file and comes out gracefully because there is no problem at all: code #!/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 code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This will produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">Written content in the file successfully code ||

Example:
<span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Here is one more simple example which tries to open a file where you do not have permission to write in the file so it raises an exception: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/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" code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This will produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">Error: can't find file or read data code || = The //except// clause with no exceptions: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You can also use the except statement with no exceptions defined as follows: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">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. code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">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. = The //except// clause with multiple exceptions: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You can also use the same //except// statement to handle multiple exceptions as follows: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">try: You do your 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. code || = Standard Exceptions: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Here is a list standard Exceptions available in Python: [|Standard Exceptions] = The try-finally clause: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">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: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">try: You do your operations here; ......................  Due to any exception, this may be skipped. finally: This would always be executed. ...................... code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Note that you can provide except clause(s), or a finally clause, but not both. You can not use //else//clause as well along with a finally clause.

Example:
code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python

try: fh = open("testfile", "w") fh.write("This is my test file for exception handling!!") finally: print "Error: can\'t find file or read data" code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">If you do not have permission to open the file in writing mode then this will produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">Error: can't find file or read data code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Same example can be written more cleanly as follows: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python

try: fh = open("testfile", "w") try: fh.write("This is my test file for exception handling!!") finally: fh.close except IOError: print "Error: can\'t find file or read data" code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">When an exception is thrown in the //try// block, the execution immediately passes to the //finally//block. After all the statements in the finally block are executed, the exception is raised again and is handled in the //except// statements if present in the next higher layer of the //try-except//statement. = Argument of an Exception: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">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: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">try: You do your operations here; ...................... except //ExceptionType, Argument//: You can print value of Argument here... code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">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. <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">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:
<span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Following is an example for a single exception: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">#!/usr/bin/python

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

temp_convert("xyz"); code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">This would produce following result: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">The argument does not contain numbers invalid literal for int with base 10: 'xyz' code || = Raising an exceptions: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">You can raise exceptions in several ways by using the raise statement. The general syntax for the **raise** statement.
 * 1) Call above function here.

Syntax:
code <span style="font-family: 'Courier New',monospace; font-size: 12px;">raise [Exception [, args [, traceback]]] code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">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. <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">The final argument, traceback, is also optional (and rarely used in practice), and, if present, is the traceback object used for the exception

Example:
<span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">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: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">def functionName( level ): if level < 1: raise "Invalid level!", level # The code below to this would not be executed # if we raise the exception code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">**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: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">try: Business Logic here... except "Invalid level!": Exception handling here... else: Rest of the code here... code || = User-Defined Exceptions: = <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">Python also allows you to create your own exceptions by deriving classes from the standard built-in exceptions. <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">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. <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">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. code <span style="font-family: 'Courier New',monospace; font-size: 12px;">class Networkerror(RuntimeError): def __init__(self, arg): self.args = arg code || <span style="background-color: #ffffff; font-family: verdana,helvetica,arial,sans-serif; font-size: 11px; text-align: justify;">So once you defined above class, you can raise your exception as follows: code <span style="font-family: 'Courier New',monospace; font-size: 12px;">try: raise Networkerror("Bad hostname") except Networkerror,e: print e.args code ||