----------------------------- On Tue, May 5, 2015 8:00 PM CEST 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 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. It will probably only add confusion, but I thought it'd be nice to mention the weakref module: http://pymotw.com/2/weakref/ (the only thing I *might* ever use is WeakValueDictionary or WeakKeyDictionary) _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
