PYTHON EXCEPTIONS HANDLING
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 standard
Exceptions available in Python: Standard Exceptions.
. Assertions: This would be covered in Assertions in Python tutorial.
List of Standard Exceptions −
EXCEPTION NAME
Exception
Base class for all exceptions
StopIteration
Raised when the next method of an iterator does not point to any object.
SystemExit
Raised by the sys.exit function.
StandardError
Base class for all built-in exceptions except StopIteration and SystemExit.
ArithmeticError
Base class for all errors that occur for numeric calculation.
OverflowError
Raised when a calculation exceeds maximum limit for a numeric type.
FloatingPointError
Raised when a floating point calculation fails.
ZeroDivisonError
Raised when division or modulo by zero takes place for all numeric types.
AssertionError
Raised in case of failure of the Assert statement.
AttributeError
Raised in case of failure of attribute reference or assignment.
EOFError
Raised when there is no input from either the raw_input or input function and the end of file is reached.
ImportError
Raised when an import statement fails.
KeyboardInterrupt
Raised when the user interrupts program execution, usually by pressing Ctrl+c.
LookupError
Base class for all lookup errors.
IndexError
KeyError
Raised when an index is not found in a sequence.Raised when the specified key is not found in dictionary.
NameError
Raised when an identifier is not found in the local or global namespace
UnboundLocalError
Raised when trying to access a local variable in a function or method but no value has been assigned to it
IOError
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.
IOError
Raised for operating system-related errors.
SyntaxError
Raised when there is an error in Python syntax.
IndentationError
Raised when indentation is not specified properly.
SystemError
Raised when the interpreter finds an internal problem, but when this error is encountered the Python interpreter does not exit.
SystemExit
Raised when Python interpreter is quit by using the sys.exit function. If not handled in the code, causes the interpreter to exit.
ValueError
Raised when the built-in function for a data type has the valid type of arguments, but the arguments have invalid values specified.
RuntimeError
Raised when a generated error does not fall into any category.
NotImplementedError
Raised when an abstract method that needs to be implemented in an inherited class is not actually implemented.
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 a raise-if statement
ortobemoreaccurate, araise − if − notstatement. 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 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.
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 −
#!/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
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
If 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.
Syntax
Here 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 −
. A single try statement 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 generic except clause, which handles any exception.
. After the except clauses, you can include an else-clause. The code in the else-block executes
if the code in the try: block does not raise an exception.
. The else-block 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 −
#!/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
Example
This 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 Exceptions
You 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 tryexcept 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 Exceptions
You 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 Clause
You 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.
Note that you can provide except clauses, or a finally clause, but not both. You cannot use else
clause as well along with a finally clause.
Argument of an Exception
An 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.
Example
Following 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 numbersn”, 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 Exceptions
You can raise exceptions in several ways by using the raise statement. The general syntax for the
raise statement is as follows.
Syntax
raise [Exception [, args [, traceback]]
Here, Exception is the type of exception forexample, 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 andrarelyusedinpractice, 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 −
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 Exceptions
Python 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
