bpo-40755: Add missing multiset operations to Counter()

Doc/library/collections.rst

@@ -290,6 +290,47 @@ For example::
  >>> sorted(c.elements())
  ['a', 'a', 'a', 'a', 'b', 'b']
 
+ .. method:: isdisjoint(other)
+
+ True if none of the elements in *self* overlap with those in *other*.
+ Negative or missing counts are ignored.
+ Logically equivalent to: ``not (+self) & (+other)``
+
+ .. versionadded:: 3.10
+
+ .. method:: isequal(other)
+
+ Test whether counts agree exactly.
+ Negative or missing counts are treated as zero.
+
+ This method works differently than the inherited :meth:`__eq__` method
+ which treats negative or missing counts as distinct from zero::
+
+ >>> Counter(a=1, b=0).isequal(Counter(a=1))
+ True
+ >>> Counter(a=1, b=0) == Counter(a=1)
+ False
+
+ Logically equivalent to: ``+self == +other``
+
+ .. versionadded:: 3.10
+
+ .. method:: issubset(other)
+
+ True if the counts in *self* are less than or equal to those in *other*.
+ Negative or missing counts are treated as zero.
+ Logically equivalent to: ``not self - (+other)``
+
+ .. versionadded:: 3.10
+
+ .. method:: issuperset(other)
+
+ True if the counts in *self* are greater than or equal to those in *other*.
+ Negative or missing counts are treated as zero.
+ Logically equivalent to: ``not other - (+self)``
+
+ .. versionadded:: 3.10
+
  .. method:: most_common([n])
 
  Return a list of the *n* most common elements and their counts from the

Lib/collections/__init__.py

@@ -712,12 +712,24 @@ def __repr__(self):
  # To strip negative and zero counts, add-in an empty counter:
  # c += Counter()
  #
- # Rich comparison operators for multiset subset and superset tests
- # are deliberately omitted due to semantic conflicts with the
- # existing inherited dict equality method. Subset and superset
- # semantics ignore zero counts and require that p≤q ∧ p≥q → p=q;
- # however, that would not be the case for p=Counter(a=1, b=0)
- # and q=Counter(a=1) where the dictionaries are not equal.
+ # When the multiplicities are all zero or one, multiset operations
+ # are guaranteed to be equivalent to the corresponding operations
+ # for regular sets.
+ # Given counter multisets such as:
+ # cp = Counter(a=1, b=0, c=1)
+ # cq = Counter(c=1, d=0, e=1)
+ # The corresponding regular sets would be:
+ # sp ={'a', 'c'}
+ # sq ={'c', 'e'}
+ # All of the following relations would hold:
+ # set(cp + cq) == sp | sq
+ # set(cp - cq) == sp - sq
+ # set(cp | cq) == sp | sq
+ # set(cp & cq) == sp & sq
+ # cp.isequal(cq) == (sp == sq)
+ # cp.issubset(cq) == sp.issubset(sq)
+ # cp.issuperset(cq) == sp.issuperset(sq)
+ # cp.isdisjoint(cq) == sp.isdisjoint(sq)
 
  def __add__(self, other):
  '''Add counts from two counters.
@@ -876,6 +888,92 @@ def __iand__(self, other):
  self[elem] = other_count
  return self._keep_positive()
 
+ def isequal(self, other):
+ ''' Test whether counts agree exactly.
+
+ Negative or missing counts are treated as zero.
+
+ This is different than the inherited __eq__() method which
+ treats negative or missing counts as distinct from zero:
+
+ >>> Counter(a=1, b=0).isequal(Counter(a=1))
+ True
+ >>> Counter(a=1, b=0) == Counter(a=1)
+ False
+
+ Logically equivalent to: +self == +other
+ '''
+ if not isinstance(other, Counter):
+ other = Counter(other)
+ for elem in set(self) | set(other):
+ left = self[elem]
+ right = other[elem]
+ if left == right:
+ continue
+ if left < 0:
+ left = 0
+ if right < 0:
+ right = 0
+ if left != right:
+ return False
+ return True
+
+ def issubset(self, other):
+ '''True if the counts in self are less than or equal to those in other.
+
+ Negative or missing counts are treated as zero.
+
+ Logically equivalent to: not self - (+other)
+ '''
+ if not isinstance(other, Counter):
+ other = Counter(other)
+ for elem, count in self.items():
+ other_count = other[elem]
+ if other_count < 0:
+ other_count = 0
+ if count > other_count:
+ return False
+ return True
+
+ def issuperset(self, other):
+ '''True if the counts in self are greater than or equal to those in other.
+
+ Negative or missing counts are treated as zero.
+
+ Logically equivalent to: not other - (+self)
+ '''
+ if not isinstance(other, Counter):
+ other = Counter(other)
+ return other.issubset(self)
+
+ def isdisjoint(self, other):
+ '''True if none of the elements in self overlap with those in other.
+
+ Negative or missing counts are ignored.
+
+ Logically equivalent to: not (+self) & (+other)
+ '''
+ if not isinstance(other, Counter):
+ other = Counter(other)
+ for elem, count in self.items():
+ if count > 0 and other[elem] > 0:
+ return False
+ return True
+
+ # Rich comparison operators for multiset subset and superset tests
+ # have been deliberately omitted due to semantic conflicts with the
+ # existing inherited dict equality method. Subset and superset
+ # semantics ignore zero counts and require that p⊆q ∧ p⊇q ⇔ p=q;
+ # however, that would not be the case for p=Counter(a=1, b=0)
+ # and q=Counter(a=1) where the dictionaries are not equal.
+
+ def _omitted(self, other):
+ raise TypeError(
+ 'Rich comparison operators have been deliberately omitted. '
+ 'Use the isequal(), issubset(), and issuperset() methods instead.')
+
+ __lt__ = __le__ = __gt__ = __ge__ = __lt__ = _omitted
+
 
 ########################################################################
 ### ChainMap

Lib/test/test_collections.py

@@ -7,6 +7,7 @@
 import operator
 import pickle
 from random import choice, randrange
+from itertools import product, chain, combinations
 import string
 import sys
 from test import support
@@ -2219,6 +2220,64 @@ def test_helper_function(self):
  self.assertTrue(c.called)
  self.assertEqual(dict(c),{'a': 5, 'b': 2, 'c': 1, 'd': 1, 'r':2 })
 
+ def test_multiset_operations_equivalent_to_set_operations(self):
+ # When the multiplicities are all zero or one, multiset operations
+ # are guaranteed to be equivalent to the corresponding operations
+ # for regular sets.
+ s = list(product(('a', 'b', 'c'), range(2)))
+ powerset = chain.from_iterable(combinations(s, r) for r in range(len(s)+1))
+ counters = [Counter(dict(groups)) for groups in powerset]
+ for cp, cq in product(counters, repeat=2):
+ sp = set(cp.elements())
+ sq = set(cq.elements())
+ self.assertEqual(set(cp + cq), sp | sq)
+ self.assertEqual(set(cp - cq), sp - sq)
+ self.assertEqual(set(cp | cq), sp | sq)
+ self.assertEqual(set(cp & cq), sp & sq)
+ self.assertEqual(cp.isequal(cq), sp == sq)
+ self.assertEqual(cp.issubset(cq), sp.issubset(sq))
+ self.assertEqual(cp.issuperset(cq), sp.issuperset(sq))
+ self.assertEqual(cp.isdisjoint(cq), sp.isdisjoint(sq))
+
+ def test_multiset_equal(self):
+ self.assertTrue(Counter(a=3, b=2, c=0).isequal('ababa'))
+ self.assertFalse(Counter(a=3, b=2).isequal('babab'))
+
+ def test_multiset_subset(self):
+ self.assertTrue(Counter(a=3, b=2, c=0).issubset('ababa'))
+ self.assertFalse(Counter(a=3, b=2).issubset('babab'))
+
+ def test_multiset_superset(self):
+ self.assertTrue(Counter(a=3, b=2, c=0).issuperset('aab'))
+ self.assertFalse(Counter(a=3, b=2, c=0).issuperset('aabd'))
+
+ def test_multiset_disjoint(self):
+ self.assertTrue(Counter(a=3, b=2, c=0).isdisjoint('cde'))
+ self.assertFalse(Counter(a=3, b=2, c=0).isdisjoint('bcd'))
+
+ def test_multiset_predicates_with_negative_counts(self):
+ # Multiset predicates run on the output of the elements() method,
+ # meaning that zero counts and negative counts are ignored.
+ # The tests below confirm that we get that same results as the
+ # tests above, even after a negative count has been included
+ # in either *self* or *other*.
+ self.assertTrue(Counter(a=3, b=2, c=0, d=-1).isequal('ababa'))
+ self.assertFalse(Counter(a=3, b=2, d=-1).isequal('babab'))
+ self.assertTrue(Counter(a=3, b=2, c=0, d=-1).issubset('ababa'))
+ self.assertFalse(Counter(a=3, b=2, d=-1).issubset('babab'))
+ self.assertTrue(Counter(a=3, b=2, c=0, d=-1).issuperset('aab'))
+ self.assertFalse(Counter(a=3, b=2, c=0, d=-1).issuperset('aabd'))
+ self.assertTrue(Counter(a=3, b=2, c=0, d=-1).isdisjoint('cde'))
+ self.assertFalse(Counter(a=3, b=2, c=0, d=-1).isdisjoint('bcd'))
+ self.assertTrue(Counter(a=3, b=2, c=0, d=-1).isequal(Counter(a=3, b=2, c=-1)))
+ self.assertFalse(Counter(a=3, b=2, d=-1).isequal(Counter(a=2, b=3, c=-1)))
+ self.assertTrue(Counter(a=3, b=2, c=0, d=-1).issubset(Counter(a=3, b=2, c=-1)))
+ self.assertFalse(Counter(a=3, b=2, d=-1).issubset(Counter(a=2, b=3, c=-1)))
+ self.assertTrue(Counter(a=3, b=2, c=0, d=-1).issuperset(Counter(a=2, b=1, c=-1)))
+ self.assertFalse(Counter(a=3, b=2, c=0, d=-1).issuperset(Counter(a=2, b=1, c=-1, d=1)))
+ self.assertTrue(Counter(a=3, b=2, c=0, d=-1).isdisjoint(Counter(c=1, d=2, e=3, f=-1)))
+ self.assertFalse(Counter(a=3, b=2, c=0, d=-1).isdisjoint(Counter(b=1, c=1, d=1, e=-1)))
+
 
 ################################################################################
 ### Run tests

Misc/NEWS.d/next/Library/2020-05-23-18-24-13.bpo-22533.k64XGo.rst

@@ -0,0 +1,2 @@
+Add multiset comparison methods to collections.Counter(): isequal(),
+issubset(), issuperset(), and isdisjoint().