On Thu, 21 Feb 2008 16:37:56 +0000, tinnews wrote: > Aahz <[EMAIL PROTECTED]> wrote: >> In article <[EMAIL PROTECTED]>, >> <[EMAIL PROTECTED]> wrote: >> >Aahz <[EMAIL PROTECTED]> wrote: >> >> In article >> >> <[EMAIL PROTECTED]>, >> >> <[EMAIL PROTECTED]> wrote: >> >>> >> >>>I had a global variable holding a count. One source Google found >> >>>suggested that I wouldn't need the global if I used an object. So I >> >>>created a Singleton class that now holds the former global as an >> >>>instance attribute. Bye, bye, global. >> >>> >> >>>But later I thought about it. I cannot see a single advantage to the >> >>>object approach. Am I missing something? Or was the original global >> >>>a better, cleaner solution to the "I need a value I can read/write >> >>>from several places" problem? >> >> >> >> The advantage of the global singleton is that it is a container; >> >> therefore, its contents are mutable and you don't need to keep using >> >> the ``global`` statement. >> > >> >.... but you do keep having to use a longer reference to the value so >> >what have you won? >> >> Clarity, simplicity, robustness > > Clarity - why is it clearer?
Consider two function calls: x = ham(arg, counter) y = spam(arg) Both do exactly the same thing: ham() takes an explicit "counter" argument, while spam() uses a global variable. Which one makes it clear that it uses a counter, and which does not? > Simplicity - no, you've added an extra layer. Consider trying to run ham() and spam() twice, independently: x1 = ham(arg, counter) x2 = ham(arg, another_counter) y1 = spam(arg) saved_counter = counter # save the global variable counter = 0 # reset it to zero y2 = spam(arg) another_counter = counter counter = saved_counter Which is simpler? > Robustness - how? If you avoid the use of globals, this code will work as expected: x = ham(arg, counter) assert counter.n = 5 # or whatever value ham() sets it to... function() another_function() yet_another_function() At the end of this block of function calls, you can be confident that counter still has the same value. Now consider using globals: x = spam(arg) assert counter = 5 # or whatever value spam() sets it to... function() another_function() yet_another_function() What value will counter have? The only way to tell is to carefully read through function(), another_function() and yet_another_function() and see whether or not they modify the global counter. Maybe they do, maybe they don't, who can tell? -- Steven -- http://mail.python.org/mailman/listinfo/python-list
