[issue4090] Documenting set comparisons and operations

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

[Raymond]
> I don't think this is necessary.

I disagree.  I think some sort of warning is necessary;  it doesn't need
to be particularly prominent, but it should be there.

Almost *all* expectations are broken for sets in the absence of
transitivity of equality for the set elements.  Consider the following
(Python 2.6) snippet involving a set s:

>>> s.remove(17)
>>> 17 in s
True

An element is removed from a set s, and yet it's still present after the
removal!  Doesn't this deserve an explanation somewhere?

In case you haven't guessed, here's what s is:

>>> s
set([Fraction(17, 1), Decimal('17')])

Regardless of whether one wants to call this a bug or not, I think it's
sufficiently unintuitive and surprising that it should be documented.

Terry's suggestion and wordings for the reference and library warnings
look good to me.

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nosy: +marketdickinson

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