What will happen if an exception occurs but is not handled by the program in Python?
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The The The Exception HandlingWhen an error occurs, or exception as we call it, Python will normally stop and generate an error message. These exceptions can be handled
using the ExampleThe try: Try it Yourself » Since the try block raises an error, the except block will be executed. Without the try block, the program will crash and raise an error: ExampleThis statement will raise an error, because print(x) Try it Yourself » Many ExceptionsYou can define as many exception blocks as you want, e.g. if you want to execute a special block of code for a special kind of error: ExamplePrint one message if the try block raises a
try: Try it Yourself » ElseYou can use the ExampleIn this
example, the try: Try it Yourself » FinallyThe Example try: Try it Yourself » This can be useful to close objects and clean up resources: ExampleTry to open and write to a file that is not writable: try: Try it Yourself » The program can continue, without leaving the file object open. Raise an exceptionAs a Python developer you can choose to throw an exception if a condition occurs. To throw (or raise) an exception, use the ExampleRaise an error and stop the program if x is lower than 0: x = -1 if x < 0: Try it Yourself » The You can define what kind of error to raise, and the text to print to the user. ExampleRaise a TypeError if x is not an integer: x = "hello" if not type(x) is int: Try it Yourself » Python provides two very important features to handle any unexpected error in your Python programs and to add debugging capabilities in them −
List of Standard Exceptions −
Assertions in PythonAn 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 StatementWhen 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. ExampleHere 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 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 exceptionIf 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. SyntaxHere 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 −
ExampleThis 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 ExampleThis example tries to open a file where you do not have write permission, so it raises an exception − #!/usr/bin/python try: fh = open("testfile", "r") 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" This produces the following result − Error: can't find file or read data The except Clause with No ExceptionsYou 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. The except Clause with Multiple ExceptionsYou can also use the same except statement to handle multiple exceptions as follows − 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. The try-finally ClauseYou 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: You do your operations here; ...................... Due to any exception, this may be skipped. finally: This would always be executed. ...................... You cannot use else clause as well along with a finally clause. Example#!/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" If you do not have permission to open the file in writing mode, then this will produce the following result − Error: can't find file or read data Same example can be written more cleanly as follows − #!/usr/bin/python try: fh = open("testfile", "w") try: fh.write("This is my test file for exception handling!!") finally: print "Going to close the file" fh.close() except IOError: print "Error: can\'t find file or read data" 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 ExceptionAn 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 write 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 receives 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. ExampleFollowing 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 produces the following result − The argument does not contain numbers invalid literal for int() with base 10: 'xyz' Raising an ExceptionsYou can raise exceptions in several ways by using the raise statement. The general syntax for the raise statement is as follows. Syntaxraise [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. ExampleAn 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 the except clause as follows − try: Business Logic here... except "Invalid level!": Exception handling here... else: Rest of the code here... User-Defined ExceptionsPython 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 the exception as follows − try: raise Networkerror("Bad hostname") except Networkerror,e: print e.args What happens if exception is not handled in Python?If an exception occurs during execution of the try clause, the exception may be handled by an except clause. If the exception is not handled by an except clause, the exception is re-raised after the finally clause has been executed.
What will happen if an exception occurs but is not handled by the program?When an exception occurred, if you don't handle it, the program terminates abruptly and the code past the line that caused the exception will not get executed.
What happens when an exception occurs in Python?Python has many built-in exceptions that are raised when your program encounters an error (something in the program goes wrong). When these exceptions occur, the Python interpreter stops the current process and passes it to the calling process until it is handled. If not handled, the program will crash.
What happens if an exception occurs inside a try block in Python?While the except block is executed if the exception occurs inside the try block, the else block gets processed if the try block is found to be exception free.
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