Forward port r68792 and r68789 putting Counter in __all__ and adding Counter buildouts.
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index 816d814..0984751 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -177,6 +177,7 @@
>>> c = Counter() # a new, empty counter
>>> c = Counter('gallahad') # a new counter from an iterable
>>> c = Counter({'red': 4, 'blue': 2}) # a new counter from a mapping
+ >>> c = Counter(spam=8, eggs=1) # a new counter from keyword args
The returned object has a dictionary style interface except that it returns
a zero count for missing items (instead of raising a :exc:`KeyError` like a
@@ -207,7 +208,7 @@
Elements are returned in arbitrary order. If an element's count has been
set to zero or a negative number, :meth:`elements` will ignore it.
- >>> c = Counter({'a': 4, 'b': 2, 'd': 0, 'e': -2})
+ >>> c = Counter(a=4, b=2, c=0, d=-2)
>>> list(c.elements())
['a', 'a', 'a', 'a', 'b', 'b']
@@ -232,10 +233,10 @@
.. method:: update([iterable-or-mapping])
- Like :meth:`dict.update` but adds-in counts instead of replacing them.
-
Elements are counted from an *iterable* or added-in from another
- *mapping* (or counter)::
+ *mapping* (or counter). Like :meth:`dict.update` but adds-in counts
+ instead of replacing them, and the *iterable* is expected to be a
+ sequence of elements, not a sequence of ``(key, value)`` pairs::
>>> c = Counter('which')
>>> c.update('witch') # add elements from another iterable
@@ -255,6 +256,34 @@
Counter(dict(list_of_pairs)) # convert from a list of (elem, cnt) pairs
c.most_common()[:-n:-1] # n least common elements
+Several multiset mathematical operations are provided for combining
+:class:`Counter` objects. Multisets are like regular sets but 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::
+
+ >>> 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)
+ Counter({'a': 2})
+ >>> c & d # interection: min(c[x], d[x])
+ Counter({'a': 1, 'b': 1})
+ >>> 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})
+
**References**:
* Wikipedia entry for `Multisets <http://en.wikipedia.org/wiki/Multiset>`_