On Jan 12, 2008 8:51 AM, Jared Flatow <[EMAIL PROTECTED]> wrote: > > On Jan 11, 2008 5:21 PM, Raymond Hettinger <[EMAIL PROTECTED]> wrote: > >> When does it come-up that you want a third summed dict > >> while keeping the two originals around unchanged? Does > >> it matter that the addition is non-commutative? Would > >> a + b + c produce an intermediate a/b combo and then > >> another new object for a/b/c so that the entries in > >> a get copied twice and memory usage has to hold a, b, > >> a/b, c, and a/b/c in memory all at the same time? > > There is no way around it, this will be less efficient than the > inplace operation. If there were a precedent for calling a method > like update and returning itself instead of None, I would suggest > that, but since this is the way that has already been established for > lists, it seems a natural extension. > > On Jan 11, 2008, at 9:22 PM, Guido van Rossum wrote: > > It does suggest that we have two choices for the proposed operation: > > d1+d2 or d1|d2. I think the latter may be more appropriate: > > len(seq1+seq2) == len(seq1) ++len(seq2), but no such invariant exists > > for set union. > > This might be way out there but I suppose you could consider that a > dict is actually two different things, depending on what you are > doing, and that these operations might actually do different things. > Interpreted as a sequence, it is a sequence of key mappings. As > confirmed in another recent discussion, Python guarantees consistent > (read: repeatable) ordering of iteration through a dict, so in this > sense it really is a sequence. (On a side note, I frequently rely on > the repeatability of ordering when interacting with the Python shell). > > The notable sequence operation being + for concatenation, would > perform an update of the keys (thus for the sequence op the mappings > aren't guaranteed to be preserved, only the keys). > > The other interpretation of dict is obviously as a set of (key, > value) pairs. For sets, the four major operations could behave > exactly as any other set of (key, value) tuples (i.e. if you > transform it to a list and then apply the same ops you should get the > same result).
No, a dict is not a set of (key, value) pairs. The keys form a set, you cannot have duplicate keys. > jared > > p.s. If I were to get approval to implement some version of this, > which version of Python would be appropriate to work with? > > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org > http://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: > http://mail.python.org/mailman/options/python-dev/guido%40python.org > -- --Guido van Rossum (home page: http://www.python.org/~guido/) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
