[Python-Dev] PySequence_Concat for dicts

[Guido van Rossum]

On Jan 12, 2008 8:51 AM, Jared Flatow <jflatow at northwestern.edu> wrote:
> > On Jan 11, 2008 5:21 PM, Raymond Hettinger <python at rcn.com> 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?
>
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--Guido van Rossum (home page: http://www.python.org/~guido/)