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PYTHON - Exceptions

Python - Exceptions Handling

Exception Handling in Python

Exception handling in Python refers to managing runtime errors that may occur during the execution of a program. In Python, exceptions are raised when errors or unexpected situations arise during program execution, such as division by zero, trying to access a file that does not exist, or attempting to perform an operation on incompatible data types.

Python provides two very important features to handle any unexpected error in your Python programs and to add debugging capabilities in them −

  • Exception Handling− This would be covered in this tutorial. Here is a list of standard Exceptions available in Python:Standard Exceptions.
  • Assertions− This would be covered inAssertions in Pythontutorial.

Assertions in Python

An 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 araise-ifstatement (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 Statement

When it encounters an assert statement, Python evaluates the accompanying expression, which is hopefully true. If the expression is false, Python raises anAssertionErrorexception.

Thesyntaxfor assert is −

PYTHON CODE
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 trace back.

Example

Here 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 −

PYTHON CODE
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 −

PYTHON CODE
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 Exception in Python

If you have some suspicious code that may raise an exception, you can defend your program by placing the suspicious code in atry: block. After thetry: block, include anexcept: statement, followed by a block of code which handles the problem as elegantly as possible.

  • Thetry: block contains statements which are susceptible for exception
  • If exception occurs, the program jumps to theexcept: block.
  • If no exception in thetry: block, theexcept: block is skipped.

Syntax

Here is the simple syntax oftry…except…elseblocks −

PYTHON 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.

Here are few important points about the above-mentioned syntax −

  • A singletrystatement 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 genericexceptclause, which handles any exception.
  • After the except clause(s), you can include anelseclause. The code in theelseblock executes if the code in the try: block does not raise an exception.
  • Theelseblock is a good place for code that does not need the try: block’s protection.

Example

This example opens a file, writes content in the file and comes out gracefully because there is no problem at all.

PYTHON CODE
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()

It will produce the followingoutput−

PYTHON CODE
Written content in the file successfully

However, change the mode parameter in open() function to “w”. If the testfile is not already present, the program encounters IOError in except block, and prints following error message −

PYTHON CODE
Error: can't find file or read data

Example

This example tries to open a file where you do not have write permission, so it raises an exception −

PYTHON CODE
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 −

PYTHON CODE
Error: can't find file or read data

TheexceptClause with No Exceptions

You can also use the except statement with no exceptions defined as follows −

PYTHON CODE
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 atry-exceptstatement 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.

TheexceptClause with Multiple Exceptions

You can also use the sameexceptstatement to handle multiple exceptions as follows −

PYTHON CODE
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 Clause

You can use afinally:block along with atry: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 −

PYTHON CODE
try:
   You do your operations here;
   ......................
   Due to any exception, this may be skipped.
finally:
   This would always be executed.
   ......................

You cannot useelseclause as well along with a finally clause.

Example

PYTHON CODE
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 −

PYTHON CODE
Error: can't find file or read data

Same example can be written more cleanly as follows −

PYTHON CODE
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 thetryblock, the execution immediately passes to thefinallyblock. After all the statements in thefinallyblock are executed, the exception is raised again and is handled in theexceptstatements if present in the next higher layer of thetry-exceptstatement.

Argument of an Exception

An exception can have anargument, 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 −

PYTHON CODE
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.

Example

Following is an example for a single exception −

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

# Call above function here.
temp_convert("xyz")

This produces the following result −

PYTHON CODE
The argument does not contain numbers
invalid literal for int() with base 10: 'xyz'

Raising an Exceptions

You can raise exceptions in several ways by using the raise statement. The general syntax for theraisestatement is as follows.

Syntax

PYTHON CODE
raise [Exception [, args [, traceback]]]

Here,Exceptionis the type of exception (for example, NameError) andargumentis a value for the exception argument. The argument is optional; if not supplied, the exception argument is None.

The final argument, trace back, is also optional (and rarely used in practice), and if present, is the traceback object used for the exception.

Example

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 −

PYTHON CODE
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 −

PYTHON CODE
try:
   Business Logic here...
except "Invalid level!":
   Exception handling here...
else:
   Rest of the code here...

User-Defined Exceptions

Python also allows you to create your own exceptions by deriving classes from the standard built-in exceptions.

Here is an example related toRuntimeError. Here, a class is created that is subclassed fromRuntimeError. 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 classNetworkerror.

PYTHON CODE
class Networkerror(RuntimeError):
   def __init__(self, arg):
      self.args = arg

So once you defined above class, you can raise the exception as follows −

PYTHON CODE
try:
   raise Networkerror("Bad hostname")
except Networkerror,e:
   print (e.args)

Standard Exceptions

Here is a list of Standard Exceptions available in Python −

Sr.No.Exception Name & Description1ExceptionBase class for all exceptions2StopIterationRaised when the next() method of an iterator does not point to any object.3SystemExitRaised by the sys.exit() function.4StandardErrorBase class for all built-in exceptions except StopIteration and SystemExit.5ArithmeticErrorBase class for all errors that occur for numeric calculation.6OverflowErrorRaised when a calculation exceeds maximum limit for a numeric type.7FloatingPointErrorRaised when a floating point calculation fails.8ZeroDivisionErrorRaised when division or modulo by zero takes place for all numeric types.9AssertionErrorRaised in case of failure of the Assert statement.10AttributeErrorRaised in case of failure of attribute reference or assignment.11EOFErrorRaised when there is no input from either the raw_input() or input() function and the end of file is reached.12ImportErrorRaised when an import statement fails.13KeyboardInterruptRaised when the user interrupts program execution, usually by pressing Ctrl+c.14LookupErrorBase class for all lookup errors.15IndexErrorRaised when an index is not found in a sequence.16KeyErrorRaised when the specified key is not found in the dictionary.17NameErrorRaised when an identifier is not found in the local or global namespace.18UnboundLocalErrorRaised when trying to access a local variable in a function or method but no value has been assigned to it.19EnvironmentErrorBase class for all exceptions that occur outside the Python environment.20IOErrorRaised 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.21IOErrorRaised for operating system-related errors.22SyntaxErrorRaised when there is an error in Python syntax.23IndentationErrorRaised when indentation is not specified properly.24SystemErrorRaised when the interpreter finds an internal problem, but when this error is encountered the Python interpreter does not exit.25SystemExitRaised when Python interpreter is quit by using the sys.exit() function. If not handled in the code, causes the interpreter to exit.26TypeErrorRaised when an operation or function is attempted that is invalid for the specified data type.27ValueErrorRaised when the built-in function for a data type has the valid type of arguments, but the arguments have invalid values specified.28RuntimeErrorRaised when a generated error does not fall into any category.29NotImplementedErrorRaised when an abstract method that needs to be implemented in an inherited class is not actually implemented.

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