ARROW-8136: [Python] Restore creating a dataset from a relative path

Closes #6643 from jorisvandenbossche/ARROW-8136

Authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>

python/pyarrow/dataset.py

@@ -153,11 +153,17 @@ def partitioning(schema=None, field_names=None, flavor=None):
 
 def _ensure_fs(filesystem, path):
     # Validate or infer the filesystem from the path
-    from pyarrow.fs import FileSystem
+    from pyarrow.fs import FileSystem, LocalFileSystem
 
-    if filesystem is not None:
-        return filesystem, path
-    return FileSystem.from_uri(path)
+    if filesystem is None:
+        try:
+            return FileSystem.from_uri(path)
+        except Exception:
+            # when path is not found (e.g. relative path), we fall back to a
+            # local file system
+            filesystem = LocalFileSystem()
+
+    return filesystem, path
 
 
 def _ensure_fs_and_paths(path, filesystem=None):

python/pyarrow/tests/test_dataset.py

@@ -15,7 +15,9 @@
 # specific language governing permissions and limitations
 # under the License.
 
+import contextlib
 import operator
+import os
 
 import numpy as np
 import pytest
@@ -38,6 +40,16 @@
 pytestmark = pytest.mark.dataset
 
 
+@contextlib.contextmanager
+def change_cwd(path):
+    curdir = os.getcwd()
+    os.chdir(str(path))
+    try:
+        yield
+    finally:
+        os.chdir(curdir)
+
+
 def _generate_data(n):
     import datetime
     import itertools
@@ -601,6 +613,13 @@ def _check_dataset_from_path(path, table, **kwargs):
     result = dataset.to_table(use_threads=False)  # deterministic row order
     assert result.equals(table)
 
+    # relative string path
+    with change_cwd(path.parent):
+        dataset = ds.dataset(ds.factory(path.name, **kwargs))
+        assert dataset.schema.equals(table.schema)
+        result = dataset.to_table(use_threads=False)  # deterministic row order
+        assert result.equals(table)
+
     # passing directly to dataset
     dataset = ds.dataset(str(path), **kwargs)
     assert dataset.schema.equals(table.schema)
@@ -642,28 +661,39 @@ def test_open_dataset_list_of_files(tempdir):
 def test_open_dataset_partitioned_directory(tempdir):
     import pyarrow.parquet as pq
     table = pa.table({'a': range(9), 'b': [0.] * 4 + [1.] * 5})
+
+    path = tempdir / "dataset"
+    path.mkdir()
+
     for part in range(3):
-        path = tempdir / "part={}".format(part)
-        path.mkdir()
-        pq.write_table(table, path / "test.parquet")
+        part = path / "part={}".format(part)
+        part.mkdir()
+        pq.write_table(table, part / "test.parquet")
 
     # no partitioning specified, just read all individual files
     full_table = pa.concat_tables([table] * 3)
-    _check_dataset_from_path(tempdir, full_table)
+    _check_dataset_from_path(path, full_table)
 
     # specify partition scheme with discovery
     dataset = ds.dataset(
-        str(tempdir), partitioning=ds.partitioning(flavor="hive"))
+        str(path), partitioning=ds.partitioning(flavor="hive"))
     expected_schema = table.schema.append(pa.field("part", pa.int32()))
     assert dataset.schema.equals(expected_schema)
 
+    # specify partition scheme with discovery and relative path
+    with change_cwd(tempdir):
+        dataset = ds.dataset(
+            "dataset/", partitioning=ds.partitioning(flavor="hive"))
+        expected_schema = table.schema.append(pa.field("part", pa.int32()))
+        assert dataset.schema.equals(expected_schema)
+
     # specify partition scheme with string short-cut
-    dataset = ds.dataset(str(tempdir), partitioning="hive")
+    dataset = ds.dataset(str(path), partitioning="hive")
     assert dataset.schema.equals(expected_schema)
 
     # specify partition scheme with explicit scheme
     dataset = ds.dataset(
-        str(tempdir),
+        str(path),
         partitioning=ds.partitioning(
             pa.schema([("part", pa.int8())]), flavor="hive"))
     expected_schema = table.schema.append(pa.field("part", pa.int8()))
@@ -688,6 +718,11 @@ def test_open_dataset_filesystem(tempdir):
     dataset2 = ds.dataset(str(path), filesystem=fs.LocalFileSystem())
     assert dataset2.schema.equals(table.schema)
 
+    # local filesystem specified with relative path
+    with change_cwd(tempdir):
+        dataset3 = ds.dataset("test.parquet", filesystem=fs.LocalFileSystem())
+    assert dataset3.schema.equals(table.schema)
+
     # passing different filesystem
     with pytest.raises(FileNotFoundError):
         ds.dataset(str(path), filesystem=fs._MockFileSystem())