Add support for unary plus and unary minus to collections.Counter()
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index f344b5a..df231f8 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -264,7 +264,7 @@
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
+ +c # remove zero and negative counts
Several mathematical operations are provided for combining :class:`Counter`
objects to produce multisets (counters that have counts greater than zero).
@@ -284,6 +284,18 @@
>>> c | d # union: max(c[x], d[x])
Counter({'a': 3, 'b': 2})
+Unary addition and substraction are shortcuts for adding an empty counter
+or subtracting from an empty counter.
+
+ >>> c = Counter(a=2, b=-4)
+ >>> +c
+ Counter({'a': 2})
+ >>> -c
+ Counter({'b': 4})
+
+.. versionadded:: 3.3
+ Added support for unary plus and unary minus.
+
.. note::
Counters were primarily designed to work with positive integers to represent
diff --git a/Lib/collections/__init__.py b/Lib/collections/__init__.py
index 03bd2b3..2007e68 100644
--- a/Lib/collections/__init__.py
+++ b/Lib/collections/__init__.py
@@ -672,6 +672,17 @@
result[elem] = newcount
return result
+ def __pos__(self):
+ 'Adds an empty counter, effectively stripping negative and zero counts'
+ return self + Counter()
+
+ def __neg__(self):
+ '''Subtracts from an empty counter. Strips positive and zero counts,
+ and flips the sign on negative counts.
+
+ '''
+ return Counter() - self
+
########################################################################
### ChainMap (helper for configparser and string.Template)
diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index eb6d4d7..04c4d97 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -943,6 +943,11 @@
c.subtract('aaaabbcce')
self.assertEqual(c, Counter(a=-1, b=0, c=-1, d=1, e=-1))
+ def test_unary(self):
+ c = Counter(a=-5, b=0, c=5, d=10, e=15,g=40)
+ self.assertEqual(dict(+c), dict(c=5, d=10, e=15, g=40))
+ self.assertEqual(dict(-c), dict(a=5))
+
def test_helper_function(self):
# two paths, one for real dicts and one for other mappings
elems = list('abracadabra')
diff --git a/Misc/NEWS b/Misc/NEWS
index d257632..704e5e7 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -252,6 +252,8 @@
- Issue #12540: Prevent zombie IDLE processes on Windows due to changes
in os.kill().
+- Add support for unary plus and unary minus to collections.Counter().
+
- Issue #12683: urlparse updated to include svn as schemes that uses relative
paths. (svn from 1.5 onwards support relative path).