ARROW-16442: [Python][Dataset] Fix fragments of ORC Dataset to use Fi…

…leFragment class

Closes #13052 from jorisvandenbossche/ARROW-16442

Authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>

python/pyarrow/_dataset.pyx

@@ -840,6 +840,7 @@ cdef class Fragment(_Weakrefable):
             # corresponding subclasses of FileFragment
             'ipc': FileFragment,
             'csv': FileFragment,
+            'orc': FileFragment,
             'parquet': _get_parquet_symbol('ParquetFileFragment'),
         }
 

python/pyarrow/tests/test_dataset.py

@@ -2880,6 +2880,8 @@ def test_orc_format(tempdir, dataset_reader):
     orc.write_table(table, path)
 
     dataset = ds.dataset(path, format=ds.OrcFileFormat())
+    fragments = list(dataset.get_fragments())
+    assert isinstance(fragments[0], ds.FileFragment)
     result = dataset_reader.to_table(dataset)
     result.validate(full=True)
     assert result.equals(table)