Lecture - 2 - Exceptions in Python2.pdf

SHAQRA UNIVERSITY College of Computing and Information Technology Programming Language (2) CSC 213 Exception Handling in python programming  In this Chapter » Introduction » Syntax Errors » Exceptions » Built-in Exceptions » Raising Exceptions » Handling Exceptions » Fin...

SHAQRA UNIVERSITY College of Computing and Information Technology Programming Language (2) CSC 213 Exception Handling in python programming  In this Chapter » Introduction » Syntax Errors » Exceptions » Built-in Exceptions » Raising Exceptions » Handling Exceptions » Finally Claus 1.1 INTRODUCTION Sometimes while executing a Python program, the program does not execute at all or the program executes but generates unexpected output or behaves abnormally. These occur when there are syntax errors, runtime errors or logical errors in the code. In Python, exceptions are errors that get triggered automatically. However, exceptions can be forcefully triggered and handled through program code. In this chapter, we will learn about exception handling in Python programs. 1.2 SYNTAX ERRORS Syntax errors are detected when we have not followed the rules of the particular programming language while writing a program. These errors are also known as parsing errors. On encountering a syntax error, the interpreter does not execute the program unless we rectify the errors, save and rerun the program. When a syntax error is encountered while working in shell mode, Python displays the name of the error and a small description about the error as shown in Figure 1.1. Figure 1.1: A syntax error displayed in Python shell mode So, a syntax error is reported by the Python interpreter giving a brief explanation about the error and a suggestion to rectify it. Similarly, when a syntax error is encountered while running a program in script mode as shown in Figure 1.2, a dialog box specifying the name of the error (Figure 1.3) and a small description about the error is displayed. 1.3 EXCEPTIONS Even if a statement or expression is syntactically correct, there might arise an error during its execution. For example, trying to open a file that does not exist, division by zero and so on. Such types of errors might disrupt the normal execution of the program and are called exceptions. An exception is a Python object that represents an error. When an error occurs during the execution of a program, an exception is said to have been raised. Such an exception needs to be handled by the programmer so that the program does not terminate abnormally. Therefore, while designing a program, a programmer may anticipate such erroneous situations that may arise during its execution and can address them by including appropriate code to handle that exception. It is to be noted that SyntaxError shown at Figures 1.1 and 1.3 is also an exception. But, all other exceptions are generated when a program is syntactically correct. 1.4 BUILT-IN EXCEPTIONS Commonly occurring exceptions are usually defined in the compiler/interpreter. These are called built-in exceptions. Python’s standard library is an extensive collection of built-in exceptions that deals with the commonly occurring errors (exceptions) by providing the standardized solutions for such errors. On the occurrence of any built-in exception, the appropriate exception handler code is executed which displays the reason along with the raised exception name. The programmer then has to take appropriate action to handle it. Some of the commonly occurring built-in exceptions that can be raised in Python are explained in Table 1.1 In Python, every exception is class, and these classes are the derived class of the BaseException class. Thus BaseException class is the topmost class in the hierarchy of exception classes. Figure 1.4 shows the built-in exceptions viz, ZeroDivisionError, NameEError, and TypeError raised by the Python interpreter in different situations. A programmer can also create custom exceptions to suit one’s requirements. These are called user-defined exceptions. We will learn how to handle exceptions in the next section. 1.5 RAISING EXCEPTIONS Each time an error is detected in a program, the Python interpreter raises (throws) an exception. Exception handlers are designed to execute when a specific exception is raised. Programmers can also forcefully raise exceptions in a program using the raise and assert statements. Once an exception is raised, no further statement in the current block of code is executed. So, raising an exception involves interrupting the normal flow execution of program and jumping to that part of the program (exception handler code) which is written to handle such exceptional situations. 1.5.1 The raise Statement The raise statement can be used to throw an exception. The syntax of raise statement is: raise exception-name[(optional argument)] The argument is generally a string that is displayed when the exception is raised. For example, when an exception is raised as shown in Figure 1.5, the message “OOPS : An Exception has occurred” is displayed along with a brief description of the error. The error detected may be a built-in exception or may be a user-defined one. Consider the example given in Figure 1.6 that uses the raise statement to raise a built-in exception called IndexError. In Figure 1.6, since the value of variable length is greater than the length of the list numbers, an IndexError exception will be raised. The statement following the raise statement will not be executed. So the message “NO EXECUTION” will not be displayed in this case. 1.5.2 The assert Statement An assert statement in Python is used to test an expression in the program code. If the result after testing comes false, then the exception is raised. This statement is generally used in the beginning of the function or after a function call to check for valid input. The syntax for assert statement is : assert Expression[,arguments] On encountering an assert statement, Python evaluates the expression given immediately after the assert keyword. If this expression is false, an AssertionError exception is raised which can be handled like any other exception. Consider the code given in Program 1-1. Program 1-1 Use of assert statement In the code, the assert statement checks for the value of the variable number. In case the number gets a negative value, AssertionError will be thrown, and subsequent statements will not be executed. Hence, on passing a negative value (-350) as an argument, it results in AssertionError and displays the message “OOPS…. Negative Number”. The output of the code is shown in Figure 1.7. 1.6 HANDLING EXCEPTIONS Each and every exception has to be handled by the programmer to avoid the program from crashing abruptly. This is done by writing additional code in a program to give proper messages or instructions to the user on encountering an exception. This process is known as exception handling. 1.6.1 Need for Exception Handling Exception handling is being used not only in Python programming but in most programming languages like C++, Java, Ruby, etc. It is a useful technique that helps in capturing runtime errors and handling them so as to avoid the program getting crashed. Following are some of the important points regarding exceptions and their handling: Following are some of the important points regarding exceptions and their handling: Python categorises exceptions into distinct types so that specific exception handlers (code to handle that particular exception) can be created for each type. Exception handlers separate the main logic of the program from the error detection and correction code. The segment of code where there is any possibility of error or exception, is placed inside one block. The code to be executed in case the exception has occurred, is placed inside another block. These statements for detection and reporting the exception do not affect the main logic of the program. The compiler or interpreter keeps track of the exact position where the error has occurred. Exception handling can be done for both user-defined and built-in exceptions 1.6.2 Process of Handling Exception When an error occurs, Python interpreter creates an object called the exception object. This object contains information about the error like its type, file name and position in the program where the error has occurred. The object is handed over to the runtime system so that it can find an appropriate code to handle this particular exception. This process of creating an exception object and handing it over to the runtime system is called throwing an exception. It is important to note that when an exception occurs while executing a particular program statement, the control jumps to an exception handler, abandoning execution of the remaining program statements. The runtime system searches the entire program for a block of code, called the exception handler that can handle the raised exception. It first searches for the method in which the error has occurred and the exception has been raised. If not found, then it searches the method from which this method (in which exception was raised) was called. This hierarchical search in reverse order continues till the exception handler is found. This entire list of methods is known as call stack. When a suitable handler is found in the call stack, it is executed by the runtime process. This process of executing a suitable handler is known as catching the exception. If the runtime system is not able to find an appropriate exception after searching all the methods in the call stack, then the program execution stops. The flowchart in Figure 1.8 describes the exception handling process. 1.6.3 Catching Exceptions An exception is said to be caught when a code that is designed to handle a particular exception is executed. Exceptions, if any, are caught in the try block and handled in the except block. While writing or debugging a program, a user might doubt an exception to occur in a particular part of the code. Such suspicious lines of codes are put inside a try block. Every try block is followed by an except block. The appropriate code to handle each of the possible exceptions (in the code inside the try block) are written inside the except clause. While executing the program, if an exception is encountered, further execution of the code inside the try block is stopped and the control is transferred to the except block. The syntax of try … except clause is as follows try: [ program statements where exceptions might occur] except [exception-name]: [ code for exception handling if the exception-name error is encountered]  Consider the Program 1-2 given below: Program 1-2 Using try..except block Here program terminated successfully. We can also print the error message by modifying the code as: Example 2:  Example 2: In the above code, the first line executed successfully and “Shaqra" will be printed. When the python interpreter executes the print statement "(10/0)" an exception is raised, and a message given in the print statement will be printed with the name of the exception "division by zero". After that, the interpreter executes the last statement, and it will print "Welcome to the University." If there are more than one except block written in the try block, then the try block will execute the except block which is responsible for the code. Example 3: Try block with more than one except block. 1.6.4 try...except…else clause We can put an optional else clause Program 1-3 Use of else clause along with the try...except clause. An except block will be executed only if some exception is raised in the try block. But if there is no error then none of the except blocks will be executed. In this case, the statements inside the else clause will be executed. Program 1-3 along with its output explains the use of else block with the try...except block. 1.7 FINALLY CLAUSE The try statement in Python can also have an optional finally clause. The statements inside the finally block are always executed regardless of whether an exception has occurred in the try block or not. It is a common practice to use finally clause while working with files to ensure that the file object is closed. If used, finally should always be placed at the end of try clause, after all except blocks and the else block. In the following program, the message “OVER AND OUT” will be displayed irrespective of whether an exception is raised or not. 1.8 Nested try-except block If a try block is written inside another try block, then it is called as nested. The code having higher risk must be defined in the innermost try block. If there is any exception raised by the innermost try block, then it will be handled by the innermost except block. If the innermost except block is not able to handle the error then except block defined outside the inner try block will handle the exception. Example 4: Implementation of the nested try block A single except block can be used to handle multiple exceptions. Consider the given an example: Example 5: Single except block handling multiple exceptions. In case of different types of error, we can use a default except for block. This block is generally used to display normal error messages. The default except block must be the last block of all except blocks. Example 06: Implementation of default except for block.