Thanks Steven. I was just confused on the execution of when Python destroys objects that are no long bound or referenced.
On Tue, May 5, 2015 at 2:00 PM, Steven D'Aprano <st...@pearwood.info> wrote: > On Tue, May 05, 2015 at 12:29:59AM -0400, Brandon D wrote: > > Hello tutors, > > > > I'm having trouble understanding, as well as visualizing, how object > > references work in the following situation. For demonstration purposes I > > will keep it at the most rudimentary level: > > > > x = 10 > > > > x = x ** x > > > In the first case statement, Python creates the integer object 10, and > binds it to the name 'x'. From this instant onwards, x will resolve as > the int 10. > > When the second line executes, Python first evaluates the right hand > side 'x ** x'. To do this, it looks up the name 'x', which resolves to > 10. It then evaluates 10**10, which creates the int 10000000000, and > binds that to the name 'x'. From this instant, x now resolves as the new > value 10000000000. > > Immediately afterwards, Python sees that the int 10 is no longer in use. > (Well, in principle -- in practice, things are more complicated.) Since > it is no longer in use, the garbage collector deletes the object, and > reclaims the memory. > > That, at least, is how it works in principle. In practice, Python may > keep a cache of small integers, so that they never get deleted. That > makes things a bit faster, at the cost of a tiny amount of extra memory. > But this will depend on the version and implementation of Python. For > example, some versions of Python cached integers 0 to 100, others -1 to > 255, very old versions might not cache any at all. As a Python > programmer, you shouldn't care about this, it is purely an optimization > to speed up the language. > > > > If my knowledge serves me correctly, Python destroys the value once > > reassigned. So, how does x = x + 1 work if it's destroyed before it can > > be referenced? The only solution I came up with is that the two operands > > are evaluated before storing it in the variable, > > Correct. Let's say x = 10 again, just for simplicity. > > Python first evaluates the right hand side: it looks up 'x', which > resolves to 10. Then it generates the int 1, and adds them together, > giving 11. Then it binds 11 to the name 'x', which frees up the > reference to 10, and (in principle) 10 will be deleted. > > The right hand side of the equals sign is always fully evaluated before > any assignments are done. This is why we can swap two variables like > this: > > a = 1 > b = 2 > a, b = b, a > > The third line evaluates b (giving 2) and a (giving 1), and Python then > does the assignment: > > a, b = 2, 1 > > which is equivalent to a=2, b=1, thus swapping the values. > > > -- > Steve > _______________________________________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > https://mail.python.org/mailman/listinfo/tutor > _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
