[New-bugs-announce] [issue26478] dict views don't implement subtraction correctly
Josh Rosenberg
report at bugs.python.org
Thu Mar 3 18:20:42 EST 2016
New submission from Josh Rosenberg:
Don't know when the problem was introduced, but dictviews_sub is doing:
tmp = _PyObject_CallMethodId(result, &PyId_difference_update, "O", other);
to implement subtraction (after creating result as a set of the keys in question). That's violating the CallMethod contract though (which states the format string should produce a tuple), and while it looks like CallMethod does fixups when the contract is violated, this creates some very odd behaviors.
With a list, everything is fine:
>>> d = {0: 'zero', 1: 'one', 2: 'two', 3: 'three'}
>>> d.keys() - [0, 2]
{1, 3}
>>> d.keys() - (0, 2)
TypeError: 'int' object is not iterable
Basically, the fix up doesn't get applied when you subtract a tuple, so it's as if it's trying to call:
result.difference_update(*(0, 2)) # Unpacking used to illustrate, effect is result.difference_update(0, 2)
With the list, it's wrapping to make a one element tuple, so it behaves like:
result.difference_update(*([0, 2],)) # Unpacking used to illustrate, effect is result.difference_update([0, 2])
For more details, see http://stackoverflow.com/q/35784258/364696
Fix should be to change call line to:
tmp = _PyObject_CallMethodObjArgsId(result, &PyId_difference_update, other, NULL);
(assuming _PyObject_CallMethodObjArgsId is a thing), or if it's not a thing, to fix the format string to force tuple wrapping:
tmp = _PyObject_CallMethodId(result, &PyId_difference_update, "(O)", other);
----------
messages: 261180
nosy: josh.r
priority: normal
severity: normal
status: open
title: dict views don't implement subtraction correctly
versions: Python 3.5, Python 3.6
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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26478>
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