MNT: Add useful error message when packet to datasets fails (#763)

This occurs when there is a nested packet file definition with
different variables for packets with the same apid. For example,
with data spread across multiple packets and that definition being
written into the XTCE logic.

imap_processing/tests/test_utils.py

@@ -97,6 +97,16 @@ def test_packet_file_to_datasets(use_derived_value, expected_mode):
     np.testing.assert_array_equal(data["mode"], [expected_mode] * len(data["mode"]))
 
 
+def test_packet_file_to_datasets_flat_definition():
+    test_file = "tests/idex/imap_idex_l0_raw_20230725_v001.pkts"
+    packet_files = imap_module_directory / test_file
+    packet_definition = (
+        imap_module_directory / "idex/packet_definitions/idex_packet_definition.xml"
+    )
+    with pytest.raises(ValueError, match="Packet fields do not match"):
+        utils.packet_file_to_datasets(packet_files, packet_definition)
+
+
 def test__create_minimum_dtype_array():
     """Test expected return types for minimum data types."""
     result = utils._create_minimum_dtype_array([1, 2, 3], "uint8")

imap_processing/utils.py

@@ -348,6 +348,8 @@ def packet_file_to_datasets(
     data_dict: dict[int, dict] = dict()
     # Also keep track of the datatype mapping for each field
     datatype_mapping: dict[int, dict] = dict()
+    # Keep track of which variables (keys) are in the dataset
+    variable_mapping: dict[int, set] = dict()
 
     # Set up the parser from the input packet definition
     packet_definition = xtcedef.XtcePacketDefinition(xtce_packet_definition)
@@ -361,6 +363,14 @@ def packet_file_to_datasets(
                 # This is the first packet for this APID
                 data_dict[apid] = collections.defaultdict(list)
                 datatype_mapping[apid] = dict()
+                variable_mapping[apid] = packet.data.keys()
+            if variable_mapping[apid] != packet.data.keys():
+                raise ValueError(
+                    f"Packet fields do not match for APID {apid}. This could be "
+                    f"due to a conditional packet definition in the XTCE, while this "
+                    f"function currently only supports flat packet definitions."
+                    f"\nExpected: {variable_mapping[apid]},\ngot: {packet.data.keys()}"
+                )
 
             # TODO: Do we want to give an option to remove the header content?
             packet_content = packet.data | packet.header