gh-149259: Fix median_grouped() exception chaining and error messages

Split the single try/except block into two, add `from exc` chaining so
the original ValueError is visible in tracebacks, and improve the error
messages to identify whether `interval` or the data value failed the
float conversion.

Author: htjworld

Lib/statistics.py

@@ -453,9 +453,16 @@ def median_grouped(data, interval=1.0):
     # Coerce to floats, raising a TypeError if not possible
     try:
         interval = float(interval)
+    except ValueError as exc:
+        raise TypeError(
+            f'interval must be a real number, got {interval!r}'
+        ) from exc
+    try:
         x = float(x)
-    except ValueError:
-        raise TypeError(f'Value cannot be converted to a float')
+    except ValueError as exc:
+        raise TypeError(
+            f'data sequence must contain real numbers, found {x!r}'
+        ) from exc
 
     # Interpolate the median using the formula found at:
     # https://www.cuemath.com/data/median-of-grouped-data/

Lib/test/test_statistics.py

@@ -1799,6 +1799,25 @@ def test_data_type_error(self):
         interval = b""
         self.assertRaises(TypeError, self.func, data, interval)
 
+    def test_exception_chaining_and_messages(self):
+        # TypeError raised for bad interval should chain the original ValueError
+        # and identify the problematic parameter.
+        data = [1, 2, 3]
+        with self.assertRaises(TypeError) as cm:
+            self.func(data, interval="bad")
+        exc = cm.exception
+        self.assertIsInstance(exc.__cause__, ValueError)
+        self.assertIn("interval", str(exc))
+
+        # TypeError raised for non-numeric data should chain the original ValueError
+        # and identify that the data is the problem.
+        data = ["a", "b", "c"]
+        with self.assertRaises(TypeError) as cm:
+            self.func(data)
+        exc = cm.exception
+        self.assertIsInstance(exc.__cause__, ValueError)
+        self.assertIn("data", str(exc))
+
 
 class TestMode(NumericTestCase, AverageMixin, UnivariateTypeMixin):
     # Test cases for the discrete version of mode.

Misc/NEWS.d/next/Library/2026-05-02-11-32-35.gh-issue-149259.rJ6MXk.rst

@@ -0,0 +1,3 @@
+Fix :func:`statistics.median_grouped` to chain the original :exc:`ValueError`
+when coercing *interval* or the median data value to ``float`` fails, and
+improve the error message to identify which parameter caused the failure.