Computing without errors

Wouldn't it be a lot more fun to write code if you could guarantee that those pesky end users would never enter the wrong type of data? Not to mention, shouldn't it be their responsibility to know the bounds of whatever is entered or passed to the program. After all isn't it enough that some helpful soul is writing code to make every one's life easier, why should I care if the "File does not exist". Well we all know that errors happen (sometimes even the program isn't perfect). One helpful tool when unexpected problems arise is the try: - except: block. In simple terms anything that falls within a try: block and causes a potentially fatal error, should cause execution to jump directly to the except: block.

#try-except.py
print '\nTesting try:'
try:
..print "divide 1 by 3 and it's ok",1.0/3.0
except:
..print 'it worked so this never prints'
try:
..print "This prints until it does the division of 1 by 'a' then ",1.0/'a'
except:
..print '\nThere is an error that would normally end the program\nbut instead jumps to the exception'
print '\n'

Should print.
divide 1 by 3 and it's ok 0.333333333333
This prints until it does the division of 1 by 'a' then
There is an error that would normally end the program
but instead jumps to the exception

try-except.py
print '\nTesting try:'
try:
..print "divide 1 by 3 and it's ok",1.0/3.0
Anything that should happen as part of normal execution goes inside the "try" block, probably including a proper exit code if you are a good programmer and this is a non-Vr program.
except:
..print 'it worked so this never prints'
Anything that should be done if there is any kind of error encountered in the try block should go in the "except" block. This is a good place to inform the user that something went wrong. In this example this line never gets printed because there is no error in the code.
try:
..print "This prints until it does the division of 1 by 'a' then ",1.0/'a'
Here the error is that there is a division by a string, notice that if it had worked there would have been some characters printed directly after the word "then", since the error is in the division the error comes at this point and execution would normally print out a bunch of cryptic python like error statements.
except:
..print '\nThere is an error that would normally end the program\nbut instead jumps to the exception'
print '\n'
However since we "trapped the exception", execution continues with the first statement in the except block.

I don't use try for every line of code I write, mostly because I'm lazy and they are for personal or in-house usage. The more likely an important program is to leave the building, the more likely I am to worry about exceptions. Within VrPython I tend to use them where there is a reasonable expectation that the same problem could come up on a regular basis.