Simplify explanation of multiset operations by removing restrictions on negative...

diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index dc1a70e4b358a77c9281657d417cd72c9bbda3c6..a2af6b5309c8d94154b2a82455c175c0acc4a970 100644 (file)
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -265,16 +265,18 @@ Common patterns for working with :class:`Counter` objects::
     c.items()                     # convert to a list of (elem, cnt) pairs
     Counter(dict(list_of_pairs))  # convert from a list of (elem, cnt) pairs
     c.most_common()[:-n:-1]       # n least common elements
+    c += Counter()                # remove zero and negative counts
 
 Several multiset mathematical operations are provided for combining
-:class:`Counter` objects.  Multisets are like regular sets but allowed to
+:class:`Counter` objects.  Multisets are like regular sets but are allowed to
 contain repeated elements (with counts of one or more).  Addition and
 subtraction combine counters by adding or subtracting the counts of
 corresponding elements.  Intersection and union return the minimum and maximum
-of corresponding counts::
+of corresponding counts.  All four multiset operations exclude results with
+zero or negative counts::
 
-    >>> c = Counter({'a': 3, 'b': 1})
-    >>> d = Counter({'a': 1, 'b': 2})
+    >>> c = Counter(a=3, b=1)
+    >>> d = Counter(a=1, b=2)
     >>> c + d                           # add two counters together:  c[x] + d[x]
     Counter({'a': 4, 'b': 3})
     >>> c - d                           # subtract (keeping only positive counts)
@@ -284,16 +286,6 @@ of corresponding counts::
     >>> c | d                           # union:  max(c[x], d[x])
     Counter({'a': 3, 'b': 2})
 
-All four multiset operations produce only positive counts (negative and zero
-results are skipped). If inputs include negative counts, addition will sum
-both counts and then exclude non-positive results.  The other three operations
-are undefined for negative inputs::
-
-    >>> e = Counter(a=8, b=-2, c=0)
-    >>> e += Counter()                  # remove zero and negative counts
-    >>> e
-    Counter({'a': 8})
-
 .. seealso::
 
     * `Bag class <http://www.gnu.org/software/smalltalk/manual-base/html_node/Bag.html>`_

diff --git a/Lib/collections.py b/Lib/collections.py
index a853076cb295e3844227c8abce760912f0db83cf..eb13f4d2fd860de4700f7f26f53f99c1a2f5d53b 100644 (file)
--- a/Lib/collections.py
+++ b/Lib/collections.py
@@ -312,8 +312,8 @@ class Counter(dict):
         if not isinstance(other, Counter):
             return NotImplemented
         result = Counter()
-        for elem, count in self.iteritems():
-            newcount = count - other[elem]
+        for elem in set(self) | set(other):
+            newcount = self[elem] - other[elem]
             if newcount > 0:
                 result[elem] = newcount
         return result

diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index ddf83985e0ea265472fa2789bdc1143a9d4f9722..47b093e9dd68a2e9857e0116559f4deb29c7ab18 100644 (file)
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -457,18 +457,19 @@ class TestCounter(unittest.TestCase):
         for i in range(1000):
             # test random pairs of multisets
             p = Counter(dict((elem, randrange(-2,4)) for elem in elements))
+            p.update(e=1, f=-1, g=0)
             q = Counter(dict((elem, randrange(-2,4)) for elem in elements))
-            for counterop, numberop, defneg in [
-                (Counter.__add__, lambda x, y: x+y if x+y>0 else 0, True),
-                (Counter.__sub__, lambda x, y: x-y if x-y>0 else 0, False),
-                (Counter.__or__, max, False),
-                (Counter.__and__, min, False),
+            q.update(h=1, i=-1, j=0)
+            for counterop, numberop in [
+                (Counter.__add__, lambda x, y: max(0, x+y)),
+                (Counter.__sub__, lambda x, y: max(0, x-y)),
+                (Counter.__or__, lambda x, y: max(0,x,y)),
+                (Counter.__and__, lambda x, y: max(0, min(x,y))),
             ]:
                 result = counterop(p, q)
                 for x in elements:
-                    # all except __add__ are undefined for negative inputs
-                    if defneg or (p[x] >= 0 and q[x] >= 0):
-                        self.assertEqual(numberop(p[x], q[x]), result[x])
+                    self.assertEqual(numberop(p[x], q[x]), result[x],
+                                     (counterop, x, p, q))
                 # verify that results exclude non-positive counts
                 self.assert_(x>0 for x in result.values())