On Tue, Apr 19, 2011 at 3:32 PM, <bod...@googlemail.com> wrote: > And presumably cleans up the leftover object with the value of 42 when it > changes to point at the 43 object? > > Or does it leave all changes in memory until the program exits? > > Bodsda. > Sorry for top posting, my phone won't let me change it > Sent from my BlackBerry® wireless device > > -----Original Message----- > From: Steven D'Aprano <st...@pearwood.info> > Sender: tutor-bounces+bodsda=ubuntu....@python.org > Date: Wed, 20 Apr 2011 04:24:03 > To: tutor<tutor@python.org> > Subject: [Tutor] Variables and constants [was Re: working with strings in > python3] > > Rance Hall wrote: > > > Variables are variable, that's why we call them variable. > > Constants are constant, and that's why we call them constant. > > And Python has neither variables nor constants in the sense that (say) > Pascal, C or Fortran have, even though we often use the same words. > > The differences are quite deep, but they're also subtle. > > In classic programming languages with variables and/or constants, the > model is that names like "x" refer to *memory locations*. If the name is > a variable, the compiler will allow you to mutate the value stored at > that memory location; if the name is a constant, it won't. But once a > name "x" is associated with memory location (say) 123456, it can never > move. But note that the "variability" or "constantness" is associated > with the *name* (the memory location), not the value. > > In languages like Python, names are associated with values, without > reference to memory locations. In this case, the "variability" or > "constantness" is associated with the *value*, not the name. > > Consider x = 42; x = x+1. In Pascal, C or Fortran, this will actually > change a block of memory that had the value 42 into 43 instead: > > The name x points to a memory location with value 42. > Leave the name pointing to the same place, but change the value to 43 > instead. > > In Python, the situation is different: > > The name x points to an object with value 42. > Leave the object 42 alone, but change the name x to point to an object > with value 43 instead. > > > > -- > Steven > _______________________________________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor > _______________________________________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor >
If a value has no name bound to it, python figures that out and destroys it -- Joel Goldstick
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