Exceptions in Python

In this part of the Python programming tutorial, we will talk about exceptions in Python.
Errors detected during execution are called exceptions. During the execution of our application, many things might go wrong. A disk might get full and we cannot save our file. A internet connection might go down and our application tries to connect to a site. All these might result in a crash of our application. To prevent happening this, we must cope with all possible errors that might occur. For this, we can use the exception handling.

>>> 3 / 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

It is not possible to divide by zero. If we try to do this, a ZeroDivisionError is raised and the script is interrupted.

$ cat zerodivision.py 
#!/usr/bin/python

# zerodivision.py


def input_numbers():
    a = float(raw_input("Enter first number:"))
    b = float(raw_input("Enter second number:"))
    return a, b

x, y = input_numbers()
print "%d / %d is %f" % (x, y, x/y)

In this script, we get two numbers from the console. We divide these two numbers. If the second number is zero, we get an exception.

$ ./zerodivision.py 
Enter first number:2
Enter second number:0
Traceback (most recent call last):
  File "./zerodivision.py", line 12, in <module>
    print "%d / %d is %f" % (x, y, x/y)
ZeroDivisionError: float division

We could handle this in two ways.

#!/usr/bin/python

# zerodivision2.py


def input_numbers():
    a = float(raw_input("Enter first number:"))
    b = float(raw_input("Enter second number:"))
    return a, b


x, y = input_numbers()

while True:
    if y != 0:
        print "%d / %d is %f" % (x, y, x/y)
        break
    else:
       print "Cannot divide by zero"
       x, y = input_numbers() 

First, we simply check that y value is not zero. If the y value is zero, we print a warning message and repeat the input cycle again. This way, we handled the error and the script is not interrupted.

$ ./zerodivision2.py 
Enter first number:4
Enter second number:0
Cannot divide by zero
Enter first number:5
Enter second number:0
Cannot divide by zero
Enter first number:5
Enter second number:6
5 / 6 is 0.833333

The other way is to use exceptions.

#!/usr/bin/python

# zerodivision3.py


def input_numbers():
    a = float(raw_input("Enter first number:"))
    b = float(raw_input("Enter second number:"))
    return a, b


x, y = input_numbers()

while True:
    try:
        print "%d / %d is %f" % (x, y, x/y)
        break
    except ZeroDivisionError:
       print "Cannot divide by zero"
       x, y = input_numbers() 

After try keyword, we put the code, where we expect an exception. The except keyword catches the exception, if it is raised. We specify, what kind of exception we are looking for.

---

except ValueError:
   pass
except (IOError, OSError):
   pass

To handle more exceptions, we can either use more except keywords or place the exception names inside a tuple.

Second argument of the except keyword

If we provide a second argument for the except keyword, we get a reference to the exception object.

#!/usr/bin/python

# zero.py

try:
    3/0
except ZeroDivisionError, e:
    print "Cannot divide by zero"
    print "Message:", e.message
    print "Class:", e.__class__

From the exception object, we can get the error message or the class name.

$ ./zero.py 
Cannot divide by zero
Message: integer division or modulo by zero
Class: <type 'exceptions.ZeroDivisionError'>

The hierarchy of exceptions

The exceptions are organized in a hierarchy, being Exception the parent of all exceptions.

#!/usr/bin/python

# interrupt.py

try:
    while True:
       pass
except KeyboardInterrupt:
   print "Program interrupted"

The script starts and endless cycle. If we press Ctrl + C, we interrupt the cycle. Here, we caught the KeyboardInterrupt exception.

Exception
  BaseException
    KeyboardInterrupt

This is the hierarchy of the KeyboardInterrupt exception.

#!/usr/bin/python

# interrupt.py

try:
    while True:
       pass
except BaseException:
   print "Program interrupted"

This example works too. The BaseException also catches the keyboard interruption. Among other exceptions.

User defined exceptions

We can create our own exceptions, if we want. We do it by defining a new exception class.

#!/usr/bin/python

# b.py


class BFoundError(Exception):
   def __init__(self, value):
      print "BFoundError: b character found at position %d" % value

string = "You make me want to be a better man."


pos = 0
for i in string:
   if i == 'b':
      raise BFoundError, pos
   pos = pos + 1

In our code example, we have created a new exception. The exception is derived from the base Exception class. Say we hate the b letter. And if we find any occurence of that letter in a string, we raise our exception.

$ ./b.py 
BFoundError: b character found at position 20
Traceback (most recent call last):
  File "./b.py", line 16, in <module>
    raise BFoundError, pos 
__main__.BFoundError

The cleanup

There is a finally keyword, which is always executed. No matter if the exception is raised or not. It is often used to do some cleanup of resources in a program.

#!/usr/bin/python

# cleanup.py

f = None

try:
   f = file('indaclub', 'r')
   contents = f.readlines()
   for i in contents:
      print i,
except IOError:
   print 'Error opening file'
finally:
   if f:
      f.close()

In our example, we try to open a file. If we cannot open the file, an IOError is raised. In case we opened the file, we want to close the file handler. For this, we use the finally keyword. In the finally block we check if the file is opened or not. If it is opened, we close it. This is a common programming construct when we work with databases. There we similarly cleanup the opened database connections.
In this chapter, we have covered exceptions in Python.