Terry J. Reedy <[email protected]> added the comment:
In English, calling someone or something 'degenerate' is a major insult. In
means that x is not really human or what it seems or what it was. I remember
being initially startled or puzzled at the mathematical usage, but it actually
is similar, just without the strong insult. A line is a degenerate parabola; a
line segment is a degenerate ellipse; crossed lines constitute a degenerate
hyperbola; a line segment and a point on the segment constitute a degenerate
triangle. There are all limit cases, but they are simplified 'degenerated'
limit cases that lack the curvature or dimensionality of the members of the
sets that they are limits of. Note that straight lines are only degenerate as a
limit of parabolas; they are fine figures in their only right.
0/0 as the limit of 0/x for small x is degenerate in that it looses the
essential property of being a well defined real number.
'Degenerate' should not be casually applied to limits, edge cases, corner
cases, or oddball cases in general. One could claim that empty counts and
collections, including sequences, are 'degenerate' limits in that they loose
the 'essential' property of somethingness. But if one means that, then they
should be denied existence, as was done for 0 and empty sets, or put in
separate classes. (For 0 and {}, there really is a sense of insult that does
not apply to lines. We still see it with 'imaginary' numbers.) By including 0
and empty collections in their respective classes, Python denies that they are
degenerate. So I do not think any slice should be called 'degenerate'.
Certainly, the contrast between all slices working and only some indexes
working has nothing to do with 'degeneracy'.
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<http://bugs.python.org/issue14097>
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