fix: make sure netcdf exporters can handle list of timesteps

pysteps/io/exporters.py

@@ -392,9 +392,10 @@ def initialize_forecast_exporter_netcdf(
         Start date of the forecast.
     timestep: int
         Time step of the forecast (minutes).
-    n_timesteps: int
-        Number of time steps in the forecast this argument is ignored if
-        incremental is set to 'timestep'.
+    n_timesteps: int or list of integers
+      Number of time steps to forecast or a list of time steps for which the
+      forecasts are computed (relative to the input time step). The elements of
+      the list are required to be in ascending order.
     shape: tuple of int
         Two-element tuple defining the shape (height,width) of the forecast
         grids.
@@ -460,8 +461,14 @@ def initialize_forecast_exporter_netcdf(
             + "'timestep' or 'member'"
         )
 
+    n_timesteps_is_list = isinstance(n_timesteps, list)
+    if n_timesteps_is_list:
+        num_timesteps = len(n_timesteps)
+    else:
+        num_timesteps = n_timesteps
+
     if incremental == "timestep":
-        n_timesteps = None
+        num_timesteps = None
     elif incremental == "member":
         n_ens_members = None
     elif incremental is not None:
@@ -498,7 +505,7 @@ def initialize_forecast_exporter_netcdf(
     h, w = shape
 
     ncf.createDimension("ens_number", size=n_ens_members)
-    ncf.createDimension("time", size=n_timesteps)
+    ncf.createDimension("time", size=num_timesteps)
     ncf.createDimension("y", size=h)
     ncf.createDimension("x", size=w)
 
@@ -585,7 +592,10 @@ def initialize_forecast_exporter_netcdf(
 
     var_time = ncf.createVariable("time", int, dimensions=("time",))
     if incremental != "timestep":
-        var_time[:] = [i * timestep * 60 for i in range(1, n_timesteps + 1)]
+        if n_timesteps_is_list:
+            var_time[:] = np.array(n_timesteps) * timestep * 60
+        else:
+            var_time[:] = [i * timestep * 60 for i in range(1, n_timesteps + 1)]
     var_time.long_name = "forecast time"
     startdate_str = datetime.strftime(startdate, "%Y-%m-%d %H:%M:%S")
     var_time.units = "seconds since %s" % startdate_str
@@ -635,7 +645,8 @@ def initialize_forecast_exporter_netcdf(
     exporter["timestep"] = timestep
     exporter["metadata"] = metadata
     exporter["incremental"] = incremental
-    exporter["num_timesteps"] = n_timesteps
+    exporter["num_timesteps"] = num_timesteps
+    exporter["timesteps"] = n_timesteps
     exporter["num_ens_members"] = n_ens_members
     exporter["shape"] = shape
 
@@ -853,7 +864,12 @@ def _export_netcdf(field, exporter):
         else:
             var_f[var_f.shape[0], :, :] = field
         var_time = exporter["var_time"]
-        var_time[len(var_time) - 1] = len(var_time) * exporter["timestep"] * 60
+        if isinstance(exporter["timesteps"], list):
+            var_time[len(var_time) - 1] = (
+                exporter["timesteps"][len(var_time) - 1] * exporter["timestep"] * 60
+            )
+        else:
+            var_time[len(var_time) - 1] = len(var_time) * exporter["timestep"] * 60
     else:
         var_f[var_f.shape[0], :, :, :] = field
         var_ens_num = exporter["var_ens_num"]

pysteps/tests/test_exporters.py

@@ -23,14 +23,17 @@
     "fill_value",
     "scale_factor",
     "offset",
+    "n_timesteps",
 )
 
 exporter_arg_values = [
-    (1, None, np.float32, None, None, None),
-    (1, "timestep", np.float32, 65535, None, None),
-    (2, None, np.float32, 65535, None, None),
-    (2, "timestep", np.float32, None, None, None),
-    (2, "member", np.float64, None, 0.01, 1.0),
+    (1, None, np.float32, None, None, None, 3),
+    (1, "timestep", np.float32, 65535, None, None, 3),
+    (2, None, np.float32, 65535, None, None, 3),
+    (2, None, np.float32, 65535, None, None, [1, 2, 4]),
+    (2, "timestep", np.float32, None, None, None, 3),
+    (2, "timestep", np.float32, None, None, None, [1, 2, 4]),
+    (2, "member", np.float64, None, 0.01, 1.0, 3),
 ]
 
 
@@ -54,7 +57,7 @@ def test_get_geotiff_filename():
 
 @pytest.mark.parametrize(exporter_arg_names, exporter_arg_values)
 def test_io_export_netcdf_one_member_one_time_step(
-    n_ens_members, incremental, datatype, fill_value, scale_factor, offset
+    n_ens_members, incremental, datatype, fill_value, scale_factor, offset, n_timesteps
 ):
     """
     Test the export netcdf.
@@ -75,7 +78,6 @@ def test_io_export_netcdf_one_member_one_time_step(
         file_path = os.path.join(outpath, outfnprefix + ".nc")
         startdate = metadata["timestamps"][0]
         timestep = metadata["accutime"]
-        n_timesteps = 3
         shape = precip.shape[1:]
 
         exporter = initialize_forecast_exporter_netcdf(
@@ -100,7 +102,11 @@ def test_io_export_netcdf_one_member_one_time_step(
         if incremental == None:
             export_forecast_dataset(precip, exporter)
         if incremental == "timestep":
-            for t in range(n_timesteps):
+            if isinstance(n_timesteps, list):
+                timesteps = len(n_timesteps)
+            else:
+                timesteps = n_timesteps
+            for t in range(timesteps):
                 if n_ens_members > 1:
                     export_forecast_dataset(precip[:, t, :, :], exporter)
                 else: