{"id":24,"date":"2023-02-16T06:07:00","date_gmt":"2023-02-16T06:07:00","guid":{"rendered":""},"modified":"2024-01-28T09:05:29","modified_gmt":"2024-01-28T09:05:29","slug":"python-exceptions-handling","status":"publish","type":"post","link":"https:\/\/pythonpower.in\/index.php\/2023\/02\/16\/python-exceptions-handling\/","title":{"rendered":"PYTHON EXCEPTIONS HANDLING"},"content":{"rendered":"<\/p>\n
<\/a><\/div>\n

 <\/p>\n

PYTHON EXCEPTIONS HANDLING<\/b><\/span><\/p>\n

Python provides two very important features to handle any unexpected error in your Python
\nprograms and to add debugging capabilities in them \u2212<\/p>\n

. Exception Handling: <\/b><\/span>This would be covered in this tutorial. Here is a list standard
\nExceptions available in Python: Standard Exceptions.<\/p>\n

. Assertions: <\/b><\/span>This would be covered in Assertions in Python tutorial.<\/p>\n

List of Standard Exceptions \u2212<\/p>\n

EXCEPTION NAME                                            <\/b><\/span> <\/b><\/p>\n

Exception <\/b> <\/p>\n

Base class for all exceptions                                                                        <\/p>\n

StopIteration<\/b><\/p>\n

Raised when the next method of an iterator does not point to any object.   <\/b>                                                                                               <\/p>\n

SystemExit<\/b><\/p>\n

Raised by the sys.exit function.   <\/b>                                                                    <\/p>\n

StandardError <\/b><\/p>\n

Base class for all built-in exceptions except StopIteration and SystemExit.    <\/b>                                                                           <\/p>\n

ArithmeticError <\/b><\/p>\n

<\/b> Base class for all errors that occur for numeric                                                                                                  calculation.                                                                                                           <\/p>\n

OverflowError<\/b><\/p>\n

Raised when a calculation exceeds maximum limit  for a numeric type. <\/p>\n

FloatingPointError<\/b><\/p>\n

Raised when a floating point calculation fails.                                   <\/b><\/p>\n

   <\/b>                                                                                                                      <\/p>\n

ZeroDivisonError <\/b><\/p>\n

 Raised when division or modulo by zero takes  place for all numeric types. <\/p>\n

AssertionError <\/b><\/p>\n

Raised in case of failure of the Assert statement.     <\/b>               <\/p>\n

  <\/b>                                                                                                                  <\/p>\n

AttributeError <\/b><\/p>\n

Raised in case of failure of attribute reference or                                                                                              assignment.                                                                                                                    <\/p>\n

EOFError <\/b><\/p>\n

Raised when there is no input from either the  raw_input or input function and the end of file is                    reached.    <\/b>                                                                                                                        <\/p>\n

ImportError <\/b><\/p>\n

 Raised when an import statement fails.  <\/b>                                                                 <\/p>\n

KeyboardInterrupt <\/b><\/p>\n

Raised when the user interrupts program execution, usually by pressing Ctrl+c.     <\/b>                                                                             <\/p>\n

LookupError<\/b>  <\/p>\n

Base class for all lookup errors.                                                                                   <\/p>\n

IndexError
\nKeyError <\/b><\/p>\n

Raised when an index is not found in a sequence.Raised when the specified key is not found in dictionary.<\/p>\n

NameError  <\/b><\/p>\n

Raised when an identifier is not found in the local or   global namespace  <\/b>                                                        <\/p>\n

UnboundLocalError<\/b><\/p>\n

Raised when trying to access a local variable in a function or method but no value has been assigned         to it                                                                                        <\/p>\n

 IOError <\/b><\/p>\n

Raised when an input\/ output operation fails, such   as the print statement or the open function when          trying to open a file that does not exist.                 <\/b>                                                                                                                            <\/p>\n

IOError <\/b> <\/p>\n

Raised for operating system-related errors.                                                                         <\/p>\n

SyntaxError   <\/b><\/p>\n

Raised when there is an error in Python syntax. <\/p>\n

IndentationError  <\/b> <\/p>\n

Raised when indentation is not specified properly.                                                                                                                          <\/p>\n

SystemError <\/b> <\/p>\n

Raised when the interpreter finds an internal   problem, but when this error is encountered the                      Python interpreter does not exit.                                                                  <\/p>\n

SystemExit <\/b> <\/p>\n

Raised when Python interpreter is quit by using the  sys.exit function. If not handled in the code, causes the interpreter to exit.                                                                                                                      <\/p>\n

ValueError <\/b><\/p>\n

Raised when the built-in function for a data type has the  valid type of arguments, but the arguments have  invalid values specified.                                     <\/p>\n

<\/b> RuntimeError <\/b><\/p>\n

<\/b>Raised when a generated error does not fall into any  category.                                                                                                                    <\/p>\n

NotImplementedError <\/b><\/p>\n

Raised when an abstract method that needs to be                                                                                          implemented in an inherited class is not actually implemented.<\/p>\n

<\/p>\n

<\/b>                                    <\/p>\n

Assertions in Python<\/b><\/span><\/p>\n

An assertion is a sanity-check that you can turn on or turn off when you are done with your testing
\nof the program. <\/p>\n

The easiest way to think of an assertion is to liken it to a raise-if statement
\nortobemoreaccurate, araise \u2212 if \u2212 notstatement. An expression is tested, and if the result comes up false, an
\nexception is raised. <\/p>\n

Assertions are carried out by the assert statement, the newest keyword to Python, introduced in
\nversion 1.5.
\nProgrammers often place assertions at the start of a function to check for valid input, and after a
\nfunction call to check for valid output.<\/p>\n

The assert Statement<\/b><\/span><\/p>\n

When it encounters an assert statement, Python evaluates the accompanying expression, which is
\nhopefully true. If the expression is false, Python raises an AssertionError exception.<\/p>\n

The syntax for assert is \u2212<\/p>\n

assert Expression[, Arguments]<\/p>\n

If the assertion fails, Python uses ArgumentExpression as the argument for the AssertionError.
\nAssertionError exceptions can be caught and handled like any other exception using the try-except
\nstatement, but if not handled, they will terminate the program and produce a traceback.<\/p>\n

Example<\/b><\/span><\/p>\n

Here is a function that converts a temperature from degrees Kelvin to degrees Fahrenheit. Since
\nzero degrees Kelvin is as cold as it gets, the function bails out if it sees a negative temperature \u2212<\/p>\n

#!\/usr\/bin\/python <\/p>\n

def KelvinToFahrenheit(Temperature): <\/p>\n

assert (Temperature >= 0),”Colder than absolute zero!” <\/p>\n

return ((Temperature-273)*1.8)+32 <\/p>\n

print KelvinToFahrenheit(273) <\/p>\n

print int(KelvinToFahrenheit(505.78)) <\/p>\n

print KelvinToFahrenheit(-5)<\/p>\n

When the above code is executed, it produces the following result \u2212<\/p>\n

32.0 <\/p>\n

451 <\/p>\n

Traceback (most recent call last): <\/p>\n

File “test.py”, line 9, in<\/p>\n

 print KelvinToFahrenheit(-5) <\/p>\n

File “test.py”, line 4, in KelvinToFahrenheit <\/p>\n

assert (Temperature >= 0),”Colder than absolute zero!” <\/p>\n

AssertionError: Colder than absolute zero!<\/p>\n

What is Exception?<\/b><\/span><\/p>\n

An exception is an event, which occurs during the execution of a program that disrupts the normal
\nflow of the program’s instructions. In general, when a Python script encounters a situation that it
\ncannot cope with, it raises an exception. An exception is a Python object that represents an error.<\/p>\n

When a Python script raises an exception, it must either handle the exception immediately
\notherwise it terminates and quits.<\/p>\n

Handling an exception<\/b><\/span><\/p>\n

If you have some suspicious code that may raise an exception, you can defend your program by
\nplacing the suspicious code in a try:<\/b> block. After the try: block, include an except:<\/b> statement,
\nfollowed by a block of code which handles the problem as elegantly as possible.<\/p>\n

Syntax<\/b><\/span><\/p>\n

Here is simple syntax of try….except…else blocks \u2212<\/p>\n

try:
\nYou do your operations here; <\/p>\n

     …………………. <\/p>\n

except ExceptionI: <\/p>\n

If there is ExceptionI, then execute this block. <\/p>\n

except ExceptionII: <\/p>\n

If there is ExceptionII, then execute this block.<\/p>\n

     …………………. <\/p>\n

else: <\/p>\n

If there is no exception then execute this block.<\/p>\n

Here are few important points about the above-mentioned syntax \u2212<\/p>\n

. <\/b><\/span>A single try statement can have multiple except statements. This is useful when the try block
\ncontains statements that may throw different types of exceptions.<\/p>\n

. <\/b>You can also provide a generic except clause, which handles any exception.<\/p>\n

.<\/b><\/span> After the except clauses, you can include an else-clause. The code in the else-block executes
\nif the code       in the try: block does not raise an exception.<\/p>\n

. <\/b><\/span>The else-block is a good place for code that does not need the try: block’s protection.<\/p>\n

Example<\/b><\/span><\/p>\n

This example opens a file, writes content in the, file and comes out gracefully because there is no
\nproblem at all \u2212<\/p>\n

#!\/usr\/bin\/python<\/p>\n

 try: <\/p>\n

fh = open(“testfile”, “w”) <\/p>\n

fh.write(“This is my test file for exception handling!!”)<\/p>\n

 except IOError: <\/p>\n

print “Error: can’t find file or read data” <\/p>\n

else: <\/p>\n

print “Written content in the file successfully”
\nfh.close()<\/p>\n

This produces the following result \u2212<\/p>\n

Written content in the file successfully<\/b><\/p>\n

Example<\/b><\/span><\/p>\n

This example tries to open a file where you do not have write permission, so it raises an exception-<\/p>\n

#!\/usr\/bin\/python <\/p>\n

try: <\/p>\n

fh = open(“testfile”, “r”) <\/p>\n

fh.write(“This is my test file for exception handling!!”) <\/p>\n

except IOError: <\/p>\n

print “Error: can’t find file or read data” <\/p>\n

else: <\/p>\n

print “Written content in the file successfully”<\/p>\n

This produces the following result \u2212<\/p>\n

Error: can’t find file or read data<\/b><\/p>\n


<\/b><\/p>\n

The except Clause with No Exceptions<\/b><\/span><\/p>\n

You can also use the except statement with no exceptions defined as follows \u2212<\/p>\n

try: <\/p>\n

You do your operations here; <\/p>\n

     …………………. <\/p>\n

except: <\/p>\n

If there is any exception, then execute this block.<\/p>\n

 …………………. <\/p>\n

else: <\/p>\n

If there is no exception then execute this block.<\/p>\n

This kind of a try-except<\/b> statement catches all the exceptions that occur. Using this kind of tryexcept statement is not considered a good programming practice though, because it catches all
\nexceptions but does not make the programmer identify the root cause of the problem that may
\noccur.<\/p>\n

The except Clause with Multiple Exceptions<\/b><\/span><\/p>\n

You can also use the same except statement to handle multiple exceptions as follows<\/p>\n

try: <\/p>\n

   You do your operations here; <\/p>\n

     …………………. <\/p>\n

except(Exception1[, Exception2[,…ExceptionN]]]): <\/p>\n

        If there is any exception from the given exception list,
\nthen execute this block. <\/p>\n

…………………. <\/p>\n

else: <\/p>\n

    If there is no exception then execute this block.<\/p>\n

The try-finally Clause<\/b><\/span><\/p>\n

You can use a finally:<\/b> block along with a try: <\/b>block. The finally block is a place to put any code
\nthat must execute, whether the try-block raised an exception or not. The syntax of the try-finally
\nstatement is this \u2212<\/p>\n

try: <\/p>\n

You do your operations here;<\/p>\n

 …………………. <\/p>\n

Due to any exception, this may be skipped. <\/p>\n

finally: <\/p>\n

This would always be executed.<\/p>\n

Note that you can provide except clauses, or a finally clause, but not both. You cannot use else
\nclause as well along with a finally clause.<\/p>\n

Argument of an Exception<\/b><\/span><\/p>\n

An exception can have an argument, which is a value that gives additional information about the
\nproblem. The contents of the argument vary by exception. You capture an exception’s argument
\nby supplying a variable in the except clause as follows \u2212<\/p>\n

try: <\/p>\n

You do your operations here; <\/p>\n

…………………. <\/p>\n

except ExceptionType, Argument: <\/p>\n

You can print value of Argument here…<\/p>\n

If you write the code to handle a single exception, you can have a variable follow the name of the
\nexception in the except statement. If you are trapping multiple exceptions, you can have a
\nvariable follow the tuple of the exception.<\/p>\n

This variable receives the value of the exception mostly containing the cause of the exception. The
\nvariable can receive a single value or multiple values in the form of a tuple. This tuple usually
\ncontains the error string, the error number, and an error location.<\/p>\n

Example<\/b><\/span><\/p>\n

Following is an example for a single exception \u2212<\/p>\n

#!\/usr\/bin\/python <\/p>\n

# Define a function here. <\/p>\n

def temp_convert(var): <\/p>\n

     try: <\/p>\n

        return int(var) <\/p>\n

             except ValueError, Argument: <\/p>\n

                        print “The argument does not contain numbersn”,  Argument <\/p>\n

# Call above function here. <\/p>\n

temp_convert(“xyz”);<\/p>\n

This produces the following result \u2212<\/p>\n

The argument does not contain numbers <\/b><\/p>\n

invalid literal for int() with base 10: ‘xyz’<\/b><\/p>\n

Raising an Exceptions<\/b><\/span><\/p>\n

You can raise exceptions in several ways by using the raise<\/b> statement. The general syntax for the
\nraise statement is as follows.<\/p>\n

Syntax<\/b><\/span><\/p>\n

raise [Exception [, args [, traceback]]<\/p>\n

Here, Exception is the type of exception forexample, NameError and argument is a value for the
\nexception argument. The argument is optional; if not supplied, the exception argument is None<\/p>\n

The final argument, traceback, is also optional andrarelyusedinpractice, and if present, is the traceback
\nobject used for the exception.<\/p>\n

Example<\/b><\/span><\/p>\n

An exception can be a string, a class or an object. Most of the exceptions that the Python core
\nraises are classes, with an argument that is an instance of the class. Defining new exceptions is
\nquite easy and can be done as follows \u2212<\/p>\n

def functionName( level ):<\/p>\n

if level < 1: <\/p>\n

         raise “Invalid level!”, level <\/p>\n

         # The code below to this would not be executed <\/p>\n

          # if we raise the exception<\/p>\n

Note: <\/b><\/span>In order to catch an exception, an “except” clause must refer to the same exception thrown
\neither class object or simple string. For example, to capture above exception, we must write the
\nexcept clause as follows \u2212<\/p>\n

try: <\/p>\n

      Business Logic here… <\/p>\n

except “Invalid level!”: <\/p>\n

      Exception handling here… <\/p>\n

else: <\/p>\n

      Rest of the code here…<\/p>\n

User-Defined Exceptions<\/b><\/span><\/p>\n

Python also allows you to create your own exceptions by deriving classes from the standard built-in
\nexceptions.<\/p>\n

Here is an example related to RuntimeError. Here, a class is created that is subclassed from
\nRuntimeError. This is useful when you need to display more specific information when an
\nexception is caught.<\/p>\n

In the try block, the user-defined exception is raised and caught in the except block. The variable e
\nis used to create an instance of the class Networkerror.<\/p>\n

class Networkerror(RuntimeError): <\/p>\n

        def __init__(self, arg): <\/p>\n

         self.args = arg<\/p>\n

<\/p>\n

So once you defined above class, you can raise the exception as follows \u2212<\/p>\n

try: <\/p>\n

     raise Networkerror(“Bad hostname”) <\/p>\n

except Networkerror,e: <\/p>\n

           print e.args <\/p>\n","protected":false},"excerpt":{"rendered":"

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