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Changing global variables in tkinter/pmw callback

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"Brian Elmegaard" <be at mek.dtu.dk> wrote in messagenews:3ACD5D99.47AED2BE at mek.dtu.dk...    [snip]> > ("normal variables", like integers, floats, strings, belong to> > this category.  if you want a new integer/float/string value,> > you have to create a new object)>> I think it's more the methodology in bindings that confused me, and then> lambdas also.You never HAVE to use a lambda: it's supposed to be a convenience,but, whenever you can use a lambda, you can just as well use a(named) local function object if that seems clearer to you (andoften it WILL be clearer to most readers).> > (when you type "s[0] = 10", you ask whatever "s" points to> > to replace item zero with a new value.  in your case, you ask> > the list "s" to modify it's first item.  anyone else referring to> > the same list object will see this change)>> And someone are. Could the same be done with the 10 in s=10? If not then> why?Numbers of all kinds are among the _immutable_ objects (like tuplesand strings).  This is VERY much what you want, believe me... theonly programming system I ever heard about which let numbers changevalue (by accident!-) was a Fortran-II compiler that, when youcalled a procedure:    CALL FOO(3)would just pass the address of the memory word where it had storedthe number 3.  If FOO internally did (e.g.) N=N+1 on its integerargument N... then 3 now had a value of four!!!  If some placeafter the call you had for example a loop such as    DO 10, I=1,3then the loop's body would 'of course' now be executed four times,for example.We _don't_ really want this, do we?  Numbers *had better* becompletely immutable objects -- programming becomes a funnygame indeed if the actual values of numbers start changingfrom under our feet.It's unclear, on the other hand, if there NEED to be 'mutable'objects at all; Haskell, Erlang, and other functional programminglanguages, seem to do pretty well by only having immutable data.Still, that is a BIG conceptual break from the programmingparadigms we're most familiar with (procedural, relational,object-oriented), which all hinge on having SOME mutable data(and some re-bindable references, too, in a sense).So, Python hits a pragmatical compromise: some of the keylow-level objects (numbers, strings, tuples) are immutable;most kinds of objects at a somewhat higher 'structuring'level (lists, dictionaries, instances) are mutable.  Andit turns out that just about ALL references in Python arere-bindable -- items in tuples being the signal exception.Alex

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