TST: Add tests for shepard_fallback in test_function_grid.py (#879)

* Add tests for shepard_fallback in test_function_grid.py

Co-authored-by: aZira371 <99824864+aZira371@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: aZira371 <99824864+aZira371@users.noreply.github.com>

Author: 2 people

tests/unit/mathutils/test_function_grid.py

@@ -1,5 +1,7 @@
 """Unit tests for Function.from_grid() method and grid interpolation."""
 
+import warnings
+
 import numpy as np
 import pytest
 
@@ -137,3 +139,141 @@ def test_from_grid_backward_compatibility():
     # Test callable function
     func3 = Function(lambda x: x**2)
     assert func3(2) == 4
+
+
+def test_shepard_fallback_warning():
+    """Test that shepard_fallback is triggered and emits a warning.
+
+    When linear_grid interpolation is set but no grid interpolator is available,
+    the Function class should fall back to shepard interpolation and emit a warning.
+    """
+    # Create a 2D function with scattered points (not structured grid)
+    source = [(0, 0, 0), (1, 0, 1), (0, 1, 2), (1, 1, 3)]
+    func = Function(
+        source=source, inputs=["x", "y"], outputs="z", interpolation="shepard"
+    )
+
+    # Now manually change interpolation to linear_grid without setting up the grid
+    # This simulates the fallback scenario
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        func.set_interpolation("linear_grid")
+
+        # Check that a warning was issued
+        assert len(w) == 1
+        assert "falling back to shepard interpolation" in str(w[0].message)
+
+
+def test_shepard_fallback_2d_interpolation():
+    """Test that shepard_fallback produces correct interpolation for 2D data.
+
+    This test verifies the fallback interpolation works correctly when
+    linear_grid is set without a grid interpolator.
+    """
+    # Create a 2D function: z = x + y
+    source = [
+        (0, 0, 0),  # f(0, 0) = 0
+        (1, 0, 1),  # f(1, 0) = 1
+        (0, 1, 1),  # f(0, 1) = 1
+        (1, 1, 2),  # f(1, 1) = 2
+    ]
+
+    # First, create with shepard to get baseline results
+    func_shepard = Function(
+        source=source, inputs=["x", "y"], outputs="z", interpolation="shepard"
+    )
+
+    # Create another function and trigger the fallback
+    func_fallback = Function(
+        source=source, inputs=["x", "y"], outputs="z", interpolation="shepard"
+    )
+
+    # Trigger fallback
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore")  # Suppress warnings for this test
+        func_fallback.set_interpolation("linear_grid")
+
+    # Test that both produce the same results at exact points
+    assert func_fallback(0, 0) == func_shepard(0, 0)
+    assert func_fallback(1, 1) == func_shepard(1, 1)
+
+    # Test interpolation at an intermediate point
+    result_fallback = func_fallback(0.5, 0.5)
+    result_shepard = func_shepard(0.5, 0.5)
+    assert np.isclose(result_fallback, result_shepard, atol=1e-6)
+
+
+def test_shepard_fallback_3d_interpolation():
+    """Test that shepard_fallback produces correct interpolation for 3D data.
+
+    This test verifies the fallback interpolation works correctly for
+    3-dimensional input data.
+    """
+    # Create a 3D function: w = x + y + z
+    source = [
+        (0, 0, 0, 0),  # f(0, 0, 0) = 0
+        (1, 0, 0, 1),  # f(1, 0, 0) = 1
+        (0, 1, 0, 1),  # f(0, 1, 0) = 1
+        (0, 0, 1, 1),  # f(0, 0, 1) = 1
+        (1, 1, 1, 3),  # f(1, 1, 1) = 3
+    ]
+
+    # Create with shepard to get baseline results
+    func_shepard = Function(
+        source=source,
+        inputs=["x", "y", "z"],
+        outputs="w",
+        interpolation="shepard",
+    )
+
+    # Create another function and trigger the fallback
+    func_fallback = Function(
+        source=source,
+        inputs=["x", "y", "z"],
+        outputs="w",
+        interpolation="shepard",
+    )
+
+    # Trigger fallback
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore")
+        func_fallback.set_interpolation("linear_grid")
+
+    # Test that both produce the same results at exact points
+    assert func_fallback(0, 0, 0) == func_shepard(0, 0, 0)
+    assert func_fallback(1, 1, 1) == func_shepard(1, 1, 1)
+
+    # Test interpolation at an intermediate point
+    result_fallback = func_fallback(0.5, 0.5, 0.5)
+    result_shepard = func_shepard(0.5, 0.5, 0.5)
+    assert np.isclose(result_fallback, result_shepard, atol=1e-6)
+
+
+def test_shepard_fallback_at_exact_data_points():
+    """Test that shepard_fallback returns exact values at data points.
+
+    When querying at exact data points, the fallback should return the
+    exact value stored at that point.
+    """
+    # Create a 2D function
+    source = [
+        (0, 0, 10),
+        (1, 0, 20),
+        (0, 1, 30),
+        (1, 1, 40),
+    ]
+
+    func = Function(
+        source=source, inputs=["x", "y"], outputs="z", interpolation="shepard"
+    )
+
+    # Trigger fallback
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore")
+        func.set_interpolation("linear_grid")
+
+    # Test exact data points - should return exact values
+    assert func(0, 0) == 10
+    assert func(1, 0) == 20
+    assert func(0, 1) == 30
+    assert func(1, 1) == 40