Python provides two very important features to handle any unexpected error in your Python programs and to add debugging capabilities in them − Show
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 <module> 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 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 are common exceptions in Python?Python Built-in Exceptions. What are the 3 major exception types in Python?There are mainly three kinds of distinguishable errors in Python: syntax errors, exceptions and logical errors.
How do I get a list of exceptions in Python?Another way to catch all Python exceptions when it occurs during runtime is to use the raise keyword. It is a manual process wherein you can optionally pass values to the exception to clarify the reason why it was raised. if x <= 0: raise ValueError(“It is not a positive number!”)
What is the top most Python exception?The top‑most Python exception is named:
BaseException.
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