feat: add static target handling in resampling validation (#494)

Author: rbeucher

src/access_moppy/utilities.py

@@ -687,6 +687,8 @@ def parse_cmip6_table_frequency(compound_name: str) -> pd.Timedelta:
         "Eday": pd.Timedelta(days=1),
         "Eyr": pd.Timedelta(days=365),
         "Efx": pd.Timedelta(days=0),
+        "fx": pd.Timedelta(days=0),
+        "Ofx": pd.Timedelta(days=0),
         # Sea ice tables
         "SImon": pd.Timedelta(days=30),
         "SIday": pd.Timedelta(days=1),
@@ -2164,6 +2166,36 @@ def validate_and_resample_if_needed(
     Returns:
         tuple of (dataset, was_resampled)
     """
+    # Get target frequency
+    target_freq = parse_cmip6_table_frequency(compound_name)
+    target_seconds = target_freq.total_seconds()
+
+    # Time-invariant (fx/*fx) targets do not require temporal validation/resampling.
+    # Accept datasets with no time axis or a singleton time axis; reject true
+    # time-varying inputs for fixed-output tables.
+    if target_seconds == 0:
+        if time_coord not in ds.coords:
+            logger.debug(
+                "Static target table for %s with no '%s' coordinate - no resampling needed",
+                compound_name,
+                time_coord,
+            )
+            return ds, False
+
+        time_size = ds[time_coord].size
+        if time_size <= 1:
+            logger.debug(
+                "Static target table for %s with singleton '%s' coordinate - no resampling needed",
+                compound_name,
+                time_coord,
+            )
+            return ds, False
+
+        raise IncompatibleFrequencyError(
+            f"Cannot process '{compound_name}' as a fixed (fx) variable: "
+            f"dataset has {time_size} time steps on '{time_coord}'."
+        )
+
     # Detect current frequency
     detected_freq = detect_time_frequency_lazy(ds, time_coord)
     if detected_freq is None:
@@ -2172,13 +2204,8 @@ def validate_and_resample_if_needed(
             f"Check that the '{time_coord}' coordinate has valid units and sufficient time points."
         )
 
-    # Get target frequency
-    target_freq = parse_cmip6_table_frequency(compound_name)
-
     # Check if resampling is needed
     input_seconds = detected_freq.total_seconds()
-    target_seconds = target_freq.total_seconds()
-
     # Check compatibility first
     is_compatible, reason = is_frequency_compatible(detected_freq, target_freq)
     if not is_compatible:

tests/unit/test_temporal_resampling.py

@@ -355,6 +355,46 @@ def test_invalid_compound_name_raises_error(self):
         with pytest.raises(ValueError):
             validate_and_resample_if_needed(ds, "Unknown.tas", "tas")
 
+    def test_static_target_without_time_coord_no_resampling(self):
+        """Static targets (fx) should skip time validation when no time coord exists."""
+        ds = xr.Dataset(
+            {"mrsofc": (["lat", "lon"], np.ones((2, 3), dtype="f4"))},
+            coords={"lat": [0.0, 1.0], "lon": [10.0, 20.0, 30.0]},
+        )
+
+        ds_result, was_resampled = validate_and_resample_if_needed(
+            ds, "Efx.mrsofc", "mrsofc"
+        )
+
+        assert not was_resampled
+        assert ds_result is ds
+
+    def test_static_target_singleton_time_coord_no_resampling(self):
+        """Static targets tolerate a singleton source time axis (common UM fx inputs)."""
+        time = pd.date_range("2000-01-01", periods=1, freq="D")
+        ds = xr.Dataset(
+            {"mrsofc": (["time", "lat", "lon"], np.ones((1, 2, 3), dtype="f4"))},
+            coords={"time": time, "lat": [0.0, 1.0], "lon": [10.0, 20.0, 30.0]},
+        )
+
+        ds_result, was_resampled = validate_and_resample_if_needed(
+            ds, "Efx.mrsofc", "mrsofc"
+        )
+
+        assert not was_resampled
+        assert ds_result is ds
+
+    def test_static_target_rejects_time_varying_input(self):
+        """Static targets should reject inputs with multiple time steps."""
+        time = pd.date_range("2000-01-01", periods=2, freq="D")
+        ds = xr.Dataset(
+            {"mrsofc": (["time", "lat", "lon"], np.ones((2, 2, 3), dtype="f4"))},
+            coords={"time": time, "lat": [0.0, 1.0], "lon": [10.0, 20.0, 30.0]},
+        )
+
+        with pytest.raises(IncompatibleFrequencyError, match=r"fixed \(fx\)"):
+            validate_and_resample_if_needed(ds, "Efx.mrsofc", "mrsofc")
+
 
 class TestErrorHandling:
     """Tests for error handling in resampling operations."""