#                                                            exceptions.py

# Exceptions

# Whenever a runtime error occurs, it creates an exception.  
# Usually, the program stops and Python prints an error message.

# For example, dividing by zero creates an exception:

# >>> print 55/0
# ZeroDivisionError: integer division or modulo

# So does accessing a nonexistent list item:
# 
# >>> a = []
# >>> print a[5]
# IndexError: list index out of range
# 
# Or accessing a key that isn't in the dictionary:
# 
# >>> b = {}
# >>> print b['what']
# KeyError: what

# In each case, the error message has two parts:
#
#  1. the type of error before the colon, and
#  2. specifics about the error after the colon.
#
# Normally Python also prints a traceback of where the program was at
# the time of error, but we have omitted that from the examples.
# 
# Sometimes we want to execute an operation that could 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, we might prompt the user for the name of a file and
# then try to open it.  If the file doesn't exist, we don't want
# the program to crash; we want to handle the exception.  Here is one
# way to handle the situation:

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

# The try statement executes the statements in the first block. 
# If no exceptions occur, it ignores the except statement.  If any
# exception occurs, it executes the statements in the except branch
# and then continues.

# We can encapsulate this capability in a function: exists takes a
# filename and returns True if the file exists, False if it doesn't:
# 
def exists(filename):
    try:
        f = open(filename)
        f.close()
        return 1
    except:
        return 0