Check in SWIFTMask if we are trying to use partial snap

swiftsimio/masks.py

@@ -10,6 +10,8 @@
 
 from swiftsimio import metadata, SWIFTMetadata, SWIFTUnits
 
+from swiftsimio.objects import InvalidSnapshot
+
 from swiftsimio.accelerated import ranges_from_array
 from typing import Dict
 
@@ -46,6 +48,12 @@ def __init__(self, metadata: SWIFTMetadata, spatial_only=True):
         self.units = metadata.units
         self.spatial_only = spatial_only
 
+        if self.metadata.partial_snapshot:
+            raise InvalidSnapshot(
+                "You cannot use masks on partial snapshots. Please use the virtual "
+                "file generated by SWIFT (use snapshot.hdf5, not snapshot.0.hdf5)."
+            )
+
         self._unpack_cell_metadata()
 
         if not spatial_only: