On Sun, Apr 19, 2015 at 12:24 AM, Cameron Simpson <c...@zip.com.au> wrote: > On 19Apr2015 15:09, Cameron Simpson <c...@zip.com.au> wrote: >> >> On 18Apr2015 23:26, boB Stepp <robertvst...@gmail.com> wrote: >>> >>> On Sat, Apr 18, 2015 at 11:08 PM, Cameron Simpson <c...@zip.com.au> wrote:
[...] >>> "Two problems often exist with deep copy operations that don’t exist >>> with shallow copy operations: >>> >>> Recursive objects (compound objects that, directly or indirectly, >>> contain a reference to themselves) may cause a recursive loop. >>> Because deep copy copies everything it may copy too much, e.g., >>> administrative data structures that should be shared even between >>> copies." >>> >>> If I am understanding things correctly, should not that last sentence >>> read instead: >>> >>> "...structures that should *not* be shared even between copies." ??? >> >> >> No, the text is correct. > > > Um, my explaination was incomplete. [...] > The second sentence "Because deep copy copies everything it may copy too > much, e.g., administrative data structures that should be shared even > between copies" is an issuewhich is addressed lower down when mentioning the > __copy_ and __deepcopy__ methods. And it is not necessarily obvious. > > Suppose you've got a data structure of objects, which should broadly be > copied. However, _internally_, these objects may use some external > facility. Further, suppose that facility can't be copied; perhaps it is a > reference to a databse or something built around something like that. Like > this: > > G -> A > -> B > -> _open_database > > When you deepcopy that data sctructure you want to copy everything, but > _not_ copy the external facility object. So for the above example, after the > deepcopy you want this: > > Gcopy -> Acopy > -> Bcopy > -> _open_database > > i.e. the same reference to the database, but copies of "A" and "B". Ah! This example clarifies things for me. This provides a specific context for the phrase, "...that should be shared even between copies..." That now makes sense. > If you give "G"'s class a .__deepcopy__ method, then that will be called by > deepcopy() to make a copy of "G" by calling "G.__deepcopy__(memodict)". And > G's class will define its __deepcopy__ method to copy "A" and "B" but not > "_open_database". > > Most classes do not need this and deepcopy() just copies everything. > > Does this clarify things? Very much! Thanks! -- boB _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
