Raymond Hettinger <[email protected]> added the comment:
It's true the concrete set API differs in some ways from the Set abstract base
class followed by dictviews. The concrete set-to-set operators are restricted
to only work with other sets, leaving the named set methods (union,
intersection, difference, etc) to accept any iterable. In contrast, the Set
abstract base class only has operators and those are specifically allowed to
accept any iterable.
It may not seem harmonious, but those were intentional and long-standing design
decisions. The restriction on concrete set operators to only work with other
sets can be traced back to bad experiences with the += operator for lists
accepting any iterable (allowing mistakes like s+='abc' when s.append('abc')
was intended).
Different choices were made in the design of the abstract Set API. In order to
be useful, that API can't make as strong of a restriction, so it allows any
iterable to be used as inputs to the operators. Also note that the abstract
Set API doesn't have the named set methods (union, intersection, difference,
etc), so the burden of falls on the operators to support iterables. IIRC, the
reason that the named set methods were omitted was to make it easier to
implement conforming classes that could interoperate with one another. For
more details on the design of the collections ABCs, see Guido's PEP on the
subject (that's where he explains was problem the abstract classes where
intended to solve and some of design considerations).
One can argue with those design decisions, but that ship sailed a long time ago
and it would no longer be possible to change either set or Set without breaking
existing code. The existing behaviors are intentional, venerable, tested, and
guaranteed.
----------
nosy: +rhettinger
resolution: -> not a bug
stage: -> resolved
status: open -> closed
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Python tracker <[email protected]>
<https://bugs.python.org/issue33874>
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