On 09/05/07, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: > try: > print "The fridge contains %s" %fridge[food_sought] > except (KeyError): > print "The fridge does not contain %s"%food_sought [...] > Is the same true of Python? Or is ok to use Exception handling like the book > suggests?
This general debate is called "look before you leap" vs "easier to ask forgiveness than permission". If you google for "python eafp lbyl" you will get a zillion pages of people debating it. Here's a quote from a post by Alex Martelli (http://en.wikipedia.org/wiki/Alex_Martelli): # from http://mail.python.org/pipermail/python-list/2003-May/205182.html """ There are umpteen good reasons why EAFP is vastly superior to LBYL. For example, we can just focus on the fact that these days we work mostly on multiprogrammed machines. While script A is running, some other programs B, C, D, ... are typically also running -- and they might be mucking with the same directories and/or files that A is working with. So, if A's structure is: if iswhatiwant(thefile): useappropriately(thefile) else: dealwithwrongness() then A is buggy. That is because, between the moment in which the test 'iswhatiwant' runs (and returns a true value), and the later moment in which procedure 'useappropriately' runs, *just about anything may have happened* -- in particular, some other program Z might have removed or modified 'thefile' so that it's NOT what A wants any more. I.e., A may lose control of the CPU between the moment it tests and the later time in which it uses the result of that test. This is known as a "race condition" and it's among the hardest problems you may run into. A may seem to be running just fine 99 times and then the 100th time BOOM -- because of accidents of timing between A and other stuff that may be running "at the same time"... a "race", so to speak, whence the name whereby this horrid condition is known. Fortunately, in a language with good support for exceptions such as Python, you are NOT doomed to enter the hell of race conditions -- just use EAFP instead of LBYL: try: useappropriately(thefile) except ItWasWrong, howwasitwrong: dealwithwrongness() See how deeply simpler this is? 'useappropriately' just ASSUMES the file is 'what A wants' and raises an ItWasWrong exception if the assumption proves to be unfounded. You don't have to code a separate 'iswhatiwant' test -- what you DO want is determined inherently by what 'useappropriately' tries to do. No race conditions, no code that must duplicate the set of conditions to be checked for, no duplicate work at runtime in terms of system calls to determine if a condition holds followed by system calls to take advantaqe of that condition. This risks leaving the impression that EAFP is a panacea - it isn't, and it has its own issues to watch for -- it's simply heads and shoulders above LBYL in most practical cases. Please see my more detailed discussions of this in the Cookbook and the Nutshell for something more about error-checking strategies. """ -- John. _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
