Python – Exceptions Handling

• 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.

Assertions in Python

The assert Statement

assert Expression[, Arguments]

Example

#!/usr/bin/python
def KelvinToFahrenheit(Temperature):
   assert (Temperature >= ),"Colder than absolute zero!"
   return ((Temperature-273)*1.8)+32
print KelvinToFahrenheit(273)
print int(KelvinToFahrenheit(505.78))
print KelvinToFahrenheit(-5)

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?

Handling an exception

Syntax

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.

• 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 clause(s), 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

#!/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()

Written content in the file successfully

Example

#!/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"

Error: can't find file or read data

The except Clause with No Exceptions

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.

The except Clause with Multiple Exceptions

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

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

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"

Error: can't find file or read data

#!/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"

Argument of an Exception

try:
   You do your operations here;
   ......................
except ExceptionType, Argument:
   You can print value of Argument here...

Example

#!/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");

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

Raising an Exceptions

Syntax

raise [Exception [, args [, traceback]]]

Example

def functionName( level ):
   if level < 1:
      raise "Invalid level!", level
      # The code below to this would not be executed
      # if we raise the exception

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

User-Defined Exceptions

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

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