[Tutor] Object references and garbage collection confusion
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 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, consequently replacing the original value of 0. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Object references and garbage collection confusion
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 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 100, and > binds that to the name 'x'. From this instant, x now resolves as the new > value 100. > > 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
Re: [Tutor] Object references and garbage collection confusion
> > This is what was also confusing as well. I assumed that python stored > objects rather than simply assigning them. > On Tue, May 5, 2015 at 4:48 AM, Alan Gauld wrote: > On 05/05/15 05:29, 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 >> > > Its good to use a simple example but in this case your > example bears no relation to your comments below! > > >> If my knowledge serves me correctly, Python destroys the value once >> reassigned. >> > > Eventually. > Or more accurately Python marks the object for deletion > once all references to the object have been lost. Remember > variables in Python are not storage locations, they are > names attached to objects. When there are no more names > attached to an object it can be deleted. But that does > not necessarily happen immediately, Python gets around > to it when it has the time. > > So, how does x = x + 1 work if it's destroyed before it can >> be referenced? >> > > Its not. This says assign x to the object on the right. > So it has to work out what "the object on the right" is > before it can stick the label x onto the result. > > The only solution I came up with is that the two operands >> are evaluated before storing it in the variable, consequently >> > > replacing the original value of 0. > > Remember, its the name that moves, not the objects. Python > doesn't store the objects in the variables, it assigns > the variable names to the objects. There is a big difference. > > HTH > -- > Alan G > Author of the Learn to Program web site > http://www.alan-g.me.uk/ > http://www.amazon.com/author/alan_gauld > Follow my photo-blog on Flickr at: > http://www.flickr.com/photos/alangauldphotos > > > ___ > 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
