'''
    exceptions.py

    Jeff Ondich, 21 September 2009

    This program gives a brief illustration of the use of
    exceptions arguments in Python.  To test the exceptions,
    try entering a non-integer at the first prompt, or
    the name of a non-existent file at the second prompt.
'''

import math

# Here, an exception may be raised when we try to convert
# a string into a real number (float).  Thus, we put our
# dangerous operation in a "try" block, and handle the
# exception, if it occurs, in the "except" block.
numberString = raw_input('Number, please: ')
try:
    number = float(numberString)
except Exception, e:
    print numberString, 'is not a number.'
    print 'Here is what the Python interpreter says:', e
    print '(See how my error message is more helpful (and more helpful) than Python\'s?)'
    exit()

# Here, we might get an exception when we try to compute
# a square root.
try:
    squareRoot = math.sqrt(number)
except Exception, e:
    print 'Something about', number, 'made sqrt() sad.  I bet it was negative.'
    print 'Alternatively, here is what Python says:', e
    exit()

print 'The square-root of', number, 'is', squareRoot

# Finally, if you try to open a file that doesn't exist, an
# exception will be raised. 
fileName = raw_input('Name a file, please: ')
try:
    f = open(fileName)
except Exception, e:
    print 'There has been some trouble opening', fileName, '.  Did you get the name right?'
    print 'Or, if you want the official story:', e
    exit()

n = 0
for line in f:
    n = n + 1
f.close()
print 'There are', n, 'lines in', fileName