On Thu, Jul 25, 2019 at 10:15:15AM +1200, Greg Ewing wrote:
> What I'm getting from this thread is that there are a variety of
> possible behaviours for dict values comparison, any of which could
> be considered "correct" depending on what the programmer is trying
> to do.
Can you elaborate on these varieties of behaviour? Aside from "change
nothing" and "raise an exception".
Speaking for myself, its taken a few iterations to nail down *precisely*
how equality ought to work in detail. But the basic semantics hasn't
really changed: two (multi)sets of values are equal if they have the
same individual values, regardless of order.
values {1, 2, 2, [], "abc", 3} and {2, 1, "abc", 3, [], 2} are equal
since each have the same elements and counts:
1 occurs once in both;
2 occurs twice in both;
3 occurs once in both;
[] occurs once in both;
"abc" occurs once in both.
So there's a 1:1 correspondence of elements in one values view to
elements in the other.
(Ignore the fact that lists are unhashable so cannot be inserted into
efficient, hash-based Python sets. I'm talking abstract multisets.)
I'll admit it took me a few attempts to get the details right (assuming
they are right now...), one of my earliest attempts included a fall back
to compare lists which was a bug.
If there is any other behaviour[1] that makes sense, I haven't seen
anyone suggest it.
[1] Again, setting aside the current behaviour inherited from object,
and raising an exception.
--
Steven
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