"""Exceptions
http://openbookproject.net/thinkcs/python/english2e/ch11.html#exceptions
See also:
http://docs.python.org/tutorial/errors.html
"""

# Whenever a runtime error occurs, it creates an exception. 
# The program stops running and Python prints out the traceback, 
# which ends with the exception that occured.

# Dividing by zero creates an exception:

print 1/0

# And so does accessing a nonexistent element in a list:

a = []
print a[5]

# As does trying to modify a string:

s = 'python'
s[0] = 'j'

# Sometimes we want to execute an operation that might cause an exception, 
# but we don't want the program to stop. We can handle the exception using 
# the try and except statements.

# For example:

file_name = raw_input('Enter a file name: ')
try:
    file_handle = open(file_name, "r")
except:
    print 'There is no file named', filename

def exists(file_name):
    try:
        f = open(file_name)
        f.close()
        return True
    except:
        return False

# Here's a snippet of code that prompts the user for a number and
# continues to ask for a number until a valid number is provided

while True:
    try:
        x = int(raw_input("Please enter an integer: "))
        break
    except ValueError:
        print "Oops!  That was no valid number.  Try again..."

# note the use of the break statement - it exits the loop in which
# it is put

# the 'raise' statement is used to create an exception:

age = input('Please enter your age: ')
if age < 0:
    raise ValueError, '%s is not a valid age' % age

# each exception has a type, that identifies the nature of the error
# A complete list of exceptions is found at:
# http://docs.python.org/library/exceptions.html