# This module demonstrates additional Python exception handling.

# In this example, we not only look for a TypeError but also a ValueError. 
# A type error will be raised automatically if we try to do operations that are
# not permitted for a certain type (like adding a string to an integer). A ValueError
# is not necessarily raised automatically--in our case below, we will raise one if one
# of our data elements is outside of a permissible range for our problem. You can
# handle more than one type of exception within a single try block--you just follow
# the try section with multiple except sections. 

ages = [18, 20, 'fred', 23, -1, 45] # our input list with 2 bad values ('fred' and -1) 
values = len(ages) # this will be the divisor we use to compute the average

sumAges = 0        # the running sum of ages in the list
for age in ages:   # for each age in the list,
	print age      # print it out
	try:
		if age < 0:             # now we also protect against bad numeric age values
			raise ValueError()  # if we get here, then we jump to except ValueError: below
		# If we got this far, then we don't have a bad integer value, but we still might
		# have non-numeric data in age. If that happens, then a TypeError exception will
		# be automatically generated by the following line of code:
		sumAges += age # try incrementing sumAges by age
	except TypeError:
		# We get here if some statement in the try block does something inappropriate with
		# specific data types. In this sample program, adding an integer and a string generates
		# a TypeError exception. We handle it here and then continue the for loop.
		print 'got a bad value, skipping ' + str(age)
		values -= 1 # decrement number of valid ages by 1
	except ValueError:
		# We get here when the if statement in the try block finds an age value less than 0.
		# When that happens, we manually generate a ValueError exception
		# by using the raise ValueError() statement. As soon as the exception is generated, 
		# we end up here.
		values -= 1 # decrement number of valid ages by 1
		print 'got a bad value: ' + str(age) + ' is not greater than 0'

try:
	# To be safe, we also check to make sure that we do not have a divide by zero problem. This
	# can happen if our input list is empty or if all the values in the list are bad (strings or
	# integer values less than or equal to 0). In either case, values will be 0. 
	# Looks picky, but it can happen!  Protect yourself!!
	print 'the average age is ' + str(sumAges / float(values)) # watch out for integer division!!
except ZeroDivisionError:
	print 'Cannot compute an average age because there was no valid input'