pct2rgb.py: emit explicit exception when source file has no color tab…

…le (fixes #8793)

autotest/pyscripts/test_pct.py

@@ -170,6 +170,29 @@ def test_pct2rgb_1(script_path, tmp_path, rgb2pct1_tif):
     ori_ds = None
 
 
+###############################################################################
+# Test pct2rgb when invoked on a dataset without color table
+
+
+def test_pct2rgb_no_color_table(script_path, tmp_path, rgb2pct1_tif):
+    gdal_array = pytest.importorskip("osgeo.gdal_array")
+    try:
+        gdal_array.BandRasterIONumPy
+    except AttributeError:
+        pytest.skip("osgeo.gdal_array.BandRasterIONumPy is unavailable")
+
+    output_tif = str(tmp_path / "test_pct2rgb_no_color_table.tif")
+
+    from osgeo_utils import pct2rgb
+
+    with pytest.raises(Exception, match="has no color table"):
+        pct2rgb.pct2rgb(
+            src_filename="../gcore/data/byte.tif",
+            pct_filename=None,
+            dst_filename=output_tif,
+        )
+
+
 ###############################################################################
 # Test rgb2pct -n option
 

swig/python/gdal-utils/osgeo_utils/pct2rgb.py

@@ -84,6 +84,8 @@ def pct2rgb(
         ct = get_color_table(pal)
     else:
         ct = src_band.GetRasterColorTable()
+        if ct is None:
+            raise Exception(f"{src_filename} has no color table")
 
     ct_size = ct.GetCount()
     lookup = [