From 82b41f10688d32b316d3b3c14d955c40c3091852 Mon Sep 17 00:00:00 2001
From: Stephen Moseley <stephen.moseley@metoffice.gov.uk>
Date: Tue, 30 Sep 2025 09:40:52 +0100
Subject: [PATCH] Adds call_object_method.

---
 improver/api/__init__.py                      |  1 +
 improver/utilities/call_object_method.py      | 31 ++++++++++
 .../utilities/test_call_object_method.py      | 57 +++++++++++++++++++
 3 files changed, 89 insertions(+)
 create mode 100644 improver/utilities/call_object_method.py
 create mode 100644 improver_tests/utilities/test_call_object_method.py

diff --git a/improver/api/__init__.py b/improver/api/__init__.py
index 53dd1ff5b8..96472449ee 100644
--- a/improver/api/__init__.py
+++ b/improver/api/__init__.py
@@ -30,6 +30,7 @@
     "BaseNeighbourhoodProcessing": "improver.nbhood.nbhood",
     "CalculateForecastBias": "improver.calibration.simple_bias_correction",
     "CalibratedForecastDistributionParameters": "improver.calibration.emos_calibration",
+    "call_object_method": "improver.utilities.call_object_method",
     "ChooseDefaultWeightsLinear": "improver.blending.weights",
     "ChooseDefaultWeightsNonLinear": "improver.blending.weights",
     "ChooseDefaultWeightsTriangular": "improver.blending.weights",
diff --git a/improver/utilities/call_object_method.py b/improver/utilities/call_object_method.py
new file mode 100644
index 0000000000..a072cd73c9
--- /dev/null
+++ b/improver/utilities/call_object_method.py
@@ -0,0 +1,31 @@
+# (C) Crown Copyright, Met Office. All rights reserved.
+#
+# This file is part of 'IMPROVER' and is released under the BSD 3-Clause license.
+# See LICENSE in the root of the repository for full licensing details.
+
+"""module to give access to callable methods on objects."""
+
+
+def call_object_method(obj: object, method_name: str, **kwargs):
+    """
+    Calls a method on an object with the supplied arguments.
+
+    This method allows us to construct a callable method for DAGRunner to execute
+    where the method to be called is on an object that comes from another plugin.
+
+    e.g. cube.collapsed("height", iris.analysis.SUM) becomes
+         call_object_method(cube, "collapsed", coords="height", aggregator=iris.analysis.SUM)
+
+    Args:
+        obj:
+            The object containing the method to be called.
+        method_name:
+            The name of the method to be called.
+        **kwargs:
+            The keyword arguments to be passed to the method.
+
+    Returns:
+        The return value from the called method.
+    """
+    method = getattr(obj, method_name)
+    return method(**kwargs)
diff --git a/improver_tests/utilities/test_call_object_method.py b/improver_tests/utilities/test_call_object_method.py
new file mode 100644
index 0000000000..012306e0a8
--- /dev/null
+++ b/improver_tests/utilities/test_call_object_method.py
@@ -0,0 +1,57 @@
+# (C) Crown Copyright, Met Office. All rights reserved.
+#
+# This file is part of 'IMPROVER' and is released under the BSD 3-Clause license.
+# See LICENSE in the root of the repository for full licensing details.
+"""Tests for the call_object_method utility function."""
+
+import iris.analysis
+import numpy as np
+import pytest
+from iris.cube import Cube
+
+from improver.synthetic_data.set_up_test_cubes import set_up_variable_cube
+from improver.utilities.call_object_method import call_object_method
+
+
+@pytest.fixture(name="cube")
+def cube_fixture() -> Cube:
+    """Set up a cube of data"""
+    data = np.array([[[1, 2], [3, 4]], [[5, 6], [7, 8]]], dtype=np.float32)
+    cube = set_up_variable_cube(
+        data,
+        name="test_variable",
+        units="m/s",
+        vertical_levels=[1000, 2000],
+        height=True,
+    )
+    return cube
+
+
+def test_call_object_method(cube):
+    """Test that call_object_method correctly calls a method on an object."""
+    result = call_object_method(
+        cube, "collapsed", coords="height", aggregator=iris.analysis.SUM
+    )
+    expected_data = np.array([[6.0, 8.0], [10.0, 12.0]], dtype=np.float32)
+    assert np.array_equal(result.data, expected_data)
+    assert result.name() == "test_variable"
+    assert result.units == "m/s"
+
+
+def test_call_object_method_invalid_method(cube):
+    """Test that call_object_method raises an AttributeError for an invalid method."""
+    with pytest.raises(AttributeError):
+        call_object_method(cube, "non_existent_method")
+
+
+def test_call_object_method_invalid_args(cube):
+    """Test that call_object_method raises a TypeError for invalid arguments."""
+    with pytest.raises(TypeError):
+        call_object_method(cube, "collapsed", non_existent_arg=True)
+
+
+def test_call_object_method_no_args(cube):
+    """Test that call_object_method works with no additional arguments."""
+    result = call_object_method(cube, "copy")
+    assert result == cube
+    assert result is not cube  # Ensure it's a copy, not the same object