Great explanation Alan. I am a newbie at Python but descriptions like this really help me better understand.
Thanks again. > To: tutor@python.org > From: alan.ga...@btinternet.com > Date: Thu, 18 Oct 2012 08:32:41 +0100 > Subject: Re: [Tutor] Objects, object references, object values and > memory addresses > > On 18/10/12 04:41, boB Stepp wrote: > > From Programming in Python 3, 2nd edition (p. 22-23): > > > >>>> a = ["Retention", 3, None] > >>>> b = ["Retention", 3, None] > >>>> a is b > > False > >>>> b = a > >>>> a is b > > True > > > > My current understanding is as follows: On the first two lines, two > > separate objects are defined, stored in two separate blocks of memory. > > Two separate list objects are created. In Python it is rarely helpful to > think about memory usage. Python is abstracted so far from the physical > machine that the connection usually depends on the creator of the > interpreter rather than the language. > > > These two objects just happen to have the same value, ["Retention", 3, > > None], stored in two separate locations. > > Yes, but note that its the fact that they are two separate lists that > matters. Even if the contents were the same objects they would still be > two lists: > > >>> x = [42] > >>> y = 'spam' > >>> z = None > >>> a = [x,y,z] > >>> b = [x,y,z] # different list, exact same content > >>> a is b > False > >>> a == b > True > >>> a = b > >>> a is b > True > >>> a == b > True > >>> > > > a and b, object references (Variables are what I used to call these. > > And most folks still do... > > > I ask: Which implementations of Python do this? In trying to make any > > code I write portable across as many platforms as possible, should I > > avoid using the identity operator, is (and its opposite, is not), > > except when I wish to compare to None? > > The point is that you don't know. And, even if you did, the very next > release might change it so you cannot rely on it. That's the real > message - do not assume a particular implementation technique because it > is not guaranteed to be that way or to stay that way. > > -- > Alan G > Author of the Learn to Program web site > http://www.alan-g.me.uk/ > > _______________________________________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor
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