BUG: Reject in-memory streams for SEGY export (#1538)

Author: GibranAlfa

src/xtgeo/cube/_cube_export.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import io
 import json
 import math
 import struct
@@ -26,7 +27,16 @@ def export_segy(cube: Cube, sfile: str) -> None:
     Args:
         cube (:class:`xtgeo.cube.Cube`): The instance
         sfile (str): File name to export to.
+
+    Raises:
+        TypeError: If ``sfile`` is an in-memory stream (e.g. ``BytesIO``).
     """
+    if isinstance(sfile, (io.StringIO, io.BytesIO)):
+        raise TypeError(
+            "SEGY export requires a filesystem path; in-memory streams are not "
+            "supported."
+        )
+
     logger.debug("Exporting segy format using segyio")
     cvalues = cube.values
 

tests/test_cube/test_cube.py

@@ -1,3 +1,4 @@
+import io
 import logging
 import pathlib
 
@@ -231,6 +232,16 @@ def test_segyio_import_export(tmp_path, pristine, smallcube):
     assert input_cube.values.flatten().tolist() == read_cube.values.flatten().tolist()
 
 
+def test_segy_export_to_bytesio_raises():
+    """SEGY cube export to BytesIO is not supported."""
+    cube = xtgeo.Cube(ncol=3, nrow=2, nlay=5, xinc=10, yinc=10, zinc=1)
+    cube.values = list(range(30))
+
+    stream = io.BytesIO()
+    with pytest.raises(TypeError, match="filesystem path"):
+        cube.to_file(stream, fformat="segy")
+
+
 def test_cube_resampling(loadsfile1):
     """Import a cube, then make a smaller and resample, then export the new"""