empty set difference does not check types of values

[asottile]

BPO	37219
Nosy	@rhettinger, @asottile
PRs	bpo-37219: Remove errorneous optimization for differencing an empty set #13965 [3.8] bpo-37219: Remove erroneous optimization for differencing an empty set (GH-13965) #13967 [3.7] bpo-37219: Remove erroneous optimization for differencing an empty set (GH-13965) #13968

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2019-06-11.09:01:44.022>
created_at = <Date 2019-06-10.22:01:36.483>
labels = ['3.8', '3.7', 'library', '3.9']
title = 'empty set difference does not check types of values'
updated_at = <Date 2019-06-11.09:01:44.021>
user = 'https://github.com/asottile'

bugs.python.org fields:

activity = <Date 2019-06-11.09:01:44.021>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2019-06-11.09:01:44.022>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2019-06-10.22:01:36.483>
creator = 'Anthony Sottile'
dependencies = []
files = []
hgrepos = []
issue_num = 37219
keywords = ['patch']
message_count = 5.0
messages = ['345147', '345181', '345199', '345200', '345205']
nosy_count = 2.0
nosy_names = ['rhettinger', 'Anthony Sottile']
pr_nums = ['13965', '13967', '13968']
priority = 'high'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue37219'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[asottile]

This is a regression from python2.x behaviour:

$ python2
Python 2.7.15rc1 (default, Nov 12 2018, 14:31:15) 
[GCC 7.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> x = set()
>>> x.difference(123)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable


$ python3.8
Python 3.8.0b1 (default, Jun  6 2019, 03:44:52) 
[GCC 7.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> x = set()
>>> x.difference(123)
set()

The regression appears to be introduced in this patch: 4103e4d

4103e4dfbce

[rhettinger]

Nice catch. Thanks.

[rhettinger]

New changeset 1f11cf9 by Raymond Hettinger in branch 'master':
bpo-37219: Remove erroneous optimization for differencing an empty set (GH-13965)
1f11cf9

[rhettinger]

New changeset 583ff84 by Raymond Hettinger (Miss Islington (bot)) in branch '3.8':
bpo-37219: Remove erroneous optimization for differencing an empty set (GH-13965) (GH-13967)
583ff84

[rhettinger]

New changeset 1b615b2 by Raymond Hettinger in branch '3.7':
[3.7] bpo-37219: Remove erroneous optimization for differencing an empty set (GH-13965) (GH-13968)
1b615b2