gh-131723: Support compatible Decimal inputs in weighted fmean

Doc/library/statistics.rst

@@ -160,7 +160,7 @@ However, for reading convenience, most of the examples show sorted sequences.
 
 .. function:: fmean(data, weights=None)
 
-   Convert *data* to floats and compute the arithmetic mean.
+   Compute the arithmetic mean as a :class:`float`.
 
    This runs faster than the :func:`mean` function and it always returns a
    :class:`float`.  The *data* may be a sequence or iterable.  If the input
@@ -183,7 +183,10 @@ However, for reading convenience, most of the examples show sorted sequences.
       87.6
 
    If *weights* is supplied, it must be the same length as the *data* or
-   a :exc:`ValueError` will be raised.
+   a :exc:`ValueError` will be raised. The values in *data* and *weights*
+   must have compatible types for multiplication. For example,
+   :class:`~decimal.Decimal` values cannot be mixed with :class:`float`
+   or :class:`~fractions.Fraction` values.
 
    .. versionadded:: 3.8
 

Lib/statistics.py

@@ -181,14 +181,17 @@ def mean(data):
 
 
 def fmean(data, weights=None):
-    """Convert data to floats and compute the arithmetic mean.
+    """Compute the arithmetic mean as a float.
 
     This runs faster than the mean() function and it always returns a float.
     If the input dataset is empty, it raises a StatisticsError.
 
     >>> fmean([3.5, 4.0, 5.25])
     4.25
 
+    If provided, weights must have the same length as data. The values in
+    data and weights must have compatible types for multiplication.
+    Decimals cannot be mixed with floats or fractions.
     """
     if weights is None:
 
@@ -220,7 +223,7 @@ def fmean(data, weights=None):
     if not den:
         raise StatisticsError('sum of weights must be non-zero')
 
-    return num / den
+    return float(num) / den
 
 
 def geometric_mean(data):

Lib/test/test_statistics.py

@@ -1932,6 +1932,18 @@ def test_weights(self):
         with self.assertRaises(StatisticsError):
             fmean(iter([10, 20]), iter([-1, 1]))    # sum of weights is zero
 
+    def test_decimal_weights(self):
+        for data, weights in [
+            ([Decimal(2), Decimal(4), Decimal(8)], [1, 2, 1]),
+            ([2, 4, 8], [Decimal(1), Decimal(2), Decimal(1)]),
+            ([Decimal(2), Decimal(4), Decimal(8)],
+             [Decimal(1), Decimal(2), Decimal(1)]),
+        ]:
+            with self.subTest(data=data, weights=weights):
+                result = statistics.fmean(data, weights)
+                self.assertIs(type(result), float)
+                self.assertEqual(result, 4.5)
+
 
 # === Tests for variances and standard deviations ===
 

Misc/NEWS.d/next/Library/2026-10-20-02-30-00.gh-issue-131723.aH8nY4.rst

@@ -0,0 +1,3 @@
+Fix :func:`statistics.fmean` with compatible
+:class:`~decimal.Decimal` data and weights. Clarify that weighted inputs must
+have compatible types for multiplication.