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Add tests and design notes for Counter subset/superset operations. #17625

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Merged
rhettinger merged 1 commit into from
Dec 16, 2019
Merged

Add tests and design notes for Counter subset/superset operations. #17625

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23 changes: 23 additions & 0 deletions Lib/test/test_collections.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2064,6 +2064,29 @@ def test_multiset_operations(self):
set_result = setop(set(p.elements()), set(q.elements()))
self.assertEqual(counter_result, dict.fromkeys(set_result, 1))

def test_subset_superset_not_implemented(self):
# Verify that multiset comparison operations are not implemented.

# These operations were intentionally omitted because multiset
# comparison semantics conflict with existing dict equality semantics.

# For multisets, we would expect that if p<=q and p>=q are both true,
# then p==q. However, dict equality semantics require that p!=q when
# one of sets contains an element with a zero count and the other
# doesn't.

p = Counter(a=1, b=0)
q = Counter(a=1, c=0)
self.assertNotEqual(p, q)
with self.assertRaises(TypeError):
p < q
with self.assertRaises(TypeError):
p <= q
with self.assertRaises(TypeError):
p > q
with self.assertRaises(TypeError):
p >= q

def test_inplace_operations(self):
elements = 'abcd'
for i in range(1000):
Expand Down