Update docstrings and add support for returning None from a CLI.

Author: gavinevans

improver/calibration/__init__.py

@@ -679,6 +679,7 @@ def forecast_and_truth_dataframes_to_cubes(
             Training length in days as an integer.
         percentiles:
             The set of percentiles to be used for estimating EMOS coefficients.
+            These should be a set of equally spaced quantiles.
 
     Returns:
         Forecasts and truths for the training period in Cube format.

improver/cli/__init__.py

@@ -328,7 +328,8 @@ def with_output(
     from improver.utilities.save import save_netcdf
 
     result = wrapped(*args, **kwargs)
-    if output:
+
+    if output and result:
         save_netcdf(result, output, compression_level, least_significant_digit)
         return
     return result

improver/cli/estimate_emos_coefficients_from_table.py

@@ -130,6 +130,7 @@ def process(
             coefficients to solve.
         percentiles (List[float]):
             The set of percentiles to be used for estimating EMOS coefficients.
+            These should be a set of equally spaced quantiles.
 
     Returns:
         iris.cube.CubeList:
@@ -172,6 +173,9 @@ def process(
         percentiles=percentiles,
     )
 
+    if not forecast_cube or not truth_cube:
+        return
+
     # Extract WMO IDs from the additional predictors.
     if additional_predictors:
         constr = iris.Constraint(wmo_id=truth_cube.coord("wmo_id").points)

improver_tests/acceptance/test_estimate_emos_coefficients_from_table.py

@@ -204,3 +204,33 @@ def test_invalid_truth_filter(tmp_path,):
         IOError, match="The requested filepath.*temperature_at_screen_level.*"
     ):
         run_cli(args)
+
+
+@pytest.mark.slow
+def test_return_none(tmp_path,):
+    """
+    Test that None is returned if a non-existent forecast period is requested.
+    """
+    kgo_dir = acc.kgo_root() / "estimate-emos-coefficients-from-table/"
+    history_path = kgo_dir / "forecast_table_quantiles"
+    truth_path = kgo_dir / "truth_table"
+    output_path = tmp_path / "output.nc"
+    args = [
+        history_path,
+        truth_path,
+        "--diagnostic",
+        "temperature_at_screen_level",
+        "--cycletime",
+        "20210805T2100Z",
+        "--forecast-period",
+        "1000",
+        "--training-length",
+        "5",
+        "--distribution",
+        "norm",
+        "--tolerance",
+        EST_EMOS_TOL,
+        "--output",
+        output_path,
+    ]
+    assert run_cli(args) is None