Issue #20478: avoid special casing Counter in statistics

Passing Counter objects to the Counter constructor is
special cased, going through iter() firsts ensures they
are handled the same way as any other iterable.

(Committing on Steven's behalf as I don't believe his
SSH key is registered yet)

Lib/statistics.py

@@ -268,9 +268,7 @@ def _coerce_types(T1, T2):
 
 def _counts(data):
     # Generate a table of sorted (value, frequency) pairs.
-    if data is None:
-        raise TypeError('None is not iterable')
-    table = collections.Counter(data).most_common()
+    table = collections.Counter(iter(data)).most_common()
     if not table:
         return table
     # Extract the values with the highest frequency.

Lib/test/test_statistics.py

@@ -1355,6 +1355,14 @@ class TestMode(NumericTestCase, AverageMixin, UnivariateTypeMixin):
         # collections.Counter, which accepts None and returns an empty dict.
         self.assertRaises(TypeError, self.func, None)
 
+    def test_counter_data(self):
+        # Test that a Counter is treated like any other iterable.
+        data = collections.Counter([1, 1, 1, 2])
+        # Since the keys of the counter are treated as data points, not the
+        # counts, this should raise.
+        self.assertRaises(statistics.StatisticsError, self.func, data)
+
+
 
 # === Tests for variances and standard deviations ===
 

Misc/NEWS

@@ -24,6 +24,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #20478: the statistics module now treats collections.Counter inputs
+  like any other iterable.
+
 - Issue #17369: get_filename was raising an exception if the filename
   parameter's RFC2231 encoding was broken in certain ways.  This was
   a regression relative to python2.