New submission from nyoshimizu:
The non-operator versions of set comparisons (intersection(), union(), etc.)
exhibit
inconsistent type checking. They only check the first input before deciding
whether or not to raise a TypeError exception.
Therefore, it's possible to pass a set first, then other objects (e.g. lists,
dicts, tuples) and a correct 'intersection' is returned (apparently by ignoring
ordering and using the keys in dicts). I've attached demonstrative example for
Python 3.5, although Python 2.7 appears to exhibit the same behavior.
I'm not sure what the intended behavior was (whether or not to require sets).
8.7.1 Set Objects states: "Note, the non-operator versions of union(),
intersection(), difference(), and symmetric_difference() will accept any
iterable as an argument."
Note that #8743 and #17626 appear to confirm that the operator versions should
not accept non-sets and matches the observed behavior. As the latter issue
points out, it's documented -- again in 8.7.1 -- that "...their operator based
counterparts require their arguments to be sets."
Is this behavior necessary but just not documented? The documentation states
that "This precludes error-prone constructions like Set('abc') & 'cbs' in favor
of the more readable Set('abc').intersection('cbs')." In the second example, a
first set is needed to do the intersection, then 'cbs' gets typecast into a set
(although I guess so was 'abc'). Then should the first inputs also be typecast
as sets?
----------
files: test_comparison.py
messages: 282036
nosy: nyoshimizu
priority: normal
severity: normal
status: open
title: Type checking in set comparisons.
type: behavior
versions: Python 2.7, Python 3.5
Added file: http://bugs.python.org/file45694/test_comparison.py
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue28834>
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