error handling in user input: is this natural or just laborious

sam python.sam at googlemail.com
Fri Oct 6 22:12:25 CEST 2006


hi all,

i'm starting to put together a program to simulate the performance of
an investment portfolio in a monte carlo manner doing x thousand
iterations and extracting data from the results.

i'm still in the early stages, and am trying to code something simple
and interactive to get the percentages of the portfolio in the five
different investment categories. i thought i'd get in with the error
handling early so if someone types in something wrong (like a word), or
the numbers don't add up to 100%, the error would be caught immediately
and the user sent back to the start of the loop. granting that there
may be better ways of doing this, if i decide that i do want to do it
like this (i.e. a single error requires all data to be re-entered, not
unreasonable for only five items), is this a good way of doing it or a
confusing way of doing it from the perspective of readability and
maintenance:

while True:

	cash, bond, blue, tech, dev = 0,0,0,0,0
	check=False

	try:
		cash=input('Please enter a cash percentage for the portfolio: ')
	except NameError:
		print 'That is not a number. Please start again and remember to enter
integers.'
	else:
		try:
			bond=input('Please enter a bond portfolio for the portfolio: ')

		except NameError:
			print 'That is not a number. Please start again and remember to
enter integers.'
		else:
			try:
				blue=input('Please enter a blue-chip percentage for the portfolio:
')
			except NameError:
				print 'That is not a number. Please start again and remember to
enter integers.'
			else:
				try:
					tech=input('Please enter a tech stocks percentage for the
portfolio: ')
				except NameError:
					print 'That is not a number. Please start again and remember to
enter integers.'
				else:
					try:
						dev=input('Please enter a developing countries index for the
portfolio: ')
						check=True
					except NameError:
						print 'That is not a number. Please start again and remember to
enter integers.'

	if cash+bond+blue+tech+dev==100:
		break
	if cash+bond+blue+tech+dev!=100 and check!= False:
		print 'Those numbers do not sum to 100. Please start again.'



i know it's a bit verbose, but it was the nested try clauses i was
really wondering about. is the code immediate legible to experienced
python users? or does it look like gibberish at first?

just hoped for some fresh eyes on it.

thanks,

sam

PS making check=True just saves the code from printing 'those numbers
don't sum to 100' if they haven't all been entered, which looks kind of
silly.




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