Allow passing of method keyword to numpy.percentile

lib/iris/analysis/__init__.py

@@ -1296,6 +1296,10 @@ def _calc_percentile(data, percent, fast_percentile_method=False, **kwargs):
     Calculate percentiles along the trailing axis of a 1D or 2D array.
 
     """
+    # Get hold of method keyword to pass it to numpy.percentile, or just remove it
+    # from kwargs so it's ignored for scipy mquantiles.  Note that kwargs always
+    # contains alphap and betap so we can't pass it to numpy.percentile.
+    method = kwargs.pop("method", "linear")
     if fast_percentile_method:
         if kwargs.pop("error_on_masked", False):
             msg = (
@@ -1309,7 +1313,7 @@ def _calc_percentile(data, percent, fast_percentile_method=False, **kwargs):
                 "ignore",
                 "Warning: 'partition' will ignore the 'mask' of the MaskedArray.",
             )
-            result = np.percentile(data, percent, axis=-1)
+            result = np.percentile(data, percent, axis=-1, method=method)
         result = result.T
     else:
         quantiles = percent / 100.0
@@ -1346,7 +1350,7 @@ def _percentile(data, percent, fast_percentile_method=False, **kwargs):
 
     **kwargs
         passed to scipy.stats.mstats.mquantiles if fast_percentile_method is
-        False
+        False.  Otherwise "method" is extracted and passed to numpy.percentile.
 
     """
     if not isinstance(percent, Iterable):
@@ -1993,6 +1997,9 @@ def interp_order(length):
     betap are ignored. An exception is raised if the data are masked and the
     missing data tolerance is not 0.
     Defaults to False.
+* method (str):
+    Passed to :func:`numpy.percentile`, if fast_percentile_method is True.
+    Otherwise ignored.  Defaults to "linear".
 
 **For example**:
 

lib/iris/tests/unit/analysis/test_PERCENTILE.py

@@ -239,6 +239,23 @@ def test_numpy_percentile_called(self, mocked_percentile):
         self.agg_method(data, axis=0, percent=42, fast_percentile_method=True)
         mocked_percentile.assert_called_once()
 
+    @mock.patch("numpy.percentile")
+    def test_chosen_kwarg_passed(self, mocked_percentile):
+        data = np.arange(5)
+        percent = [42, 75]
+        axis = 0
+
+        self.agg_method(
+            data,
+            axis=axis,
+            percent=percent,
+            fast_percentile_method=True,
+            method="nearest",
+        )
+        self.assertEqual(
+            mocked_percentile.call_args.kwargs["method"], "nearest"
+        )
+
 
 class MultiAxisMixin:
     """
@@ -336,6 +353,26 @@ def test_numpy_percentile_called(self, mocked_percentile):
         as_concrete_data(result)
         mocked_percentile.assert_called()
 
+    @mock.patch("numpy.percentile")
+    def test_chosen_kwarg_passed(self, mocked_percentile):
+        data = da.arange(5)
+        percent = [42, 75]
+        axis = 0
+
+        result = self.agg_method(
+            data,
+            axis=axis,
+            percent=percent,
+            fast_percentile_method=True,
+            method="nearest",
+        )
+
+        self.assertTrue(is_lazy_data(result))
+        as_concrete_data(result)
+        self.assertEqual(
+            mocked_percentile.call_args.kwargs["method"], "nearest"
+        )
+
 
 class Test_lazy_aggregate(
     tests.IrisTest, AggregateMixin, ScipyAggregateMixin, MultiAxisMixin