[python] improve restore functionality (#451)

Previously the temporary restore feature required copying data. The new
feature in pylance does not.

---------

Co-authored-by: Chang She <chang@lancedb.com>
Co-authored-by: Weston Pace <weston.pace@gmail.com>

python/lancedb/table.py

@@ -311,7 +311,7 @@ class LanceTable(Table):
 
         This allows viewing previous versions of the table. If you wish to
         keep writing to the dataset starting from an old version, then use
-        the `restore` function instead.
+        the `restore` function.
 
         Parameters
         ----------
@@ -341,16 +341,18 @@ class LanceTable(Table):
             raise ValueError(f"Invalid version {version}")
         self._reset_dataset(version=version)
 
-    def restore(self, version: int):
+    def restore(self, version: int = None):
         """Restore a version of the table. This is an in-place operation.
 
         This creates a new version where the data is equivalent to the
-        specified previous version. Note that this creates a new snapshot.
+        specified previous version. Data is not copied (as of python-v0.2.1).
 
         Parameters
         ----------
-        version : int
-            The version to restore.
+        version : int, default None
+            The version to restore. If unspecified then restores the currently
+            checked out version. If the currently checked out version is the
+            latest version then this is a no-op.
 
         Examples
         --------
@@ -373,15 +375,18 @@ class LanceTable(Table):
         3
         """
         max_ver = max([v["version"] for v in self._dataset.versions()])
-        if version < 1 or version >= max_ver:
+        if version is None:
+            version = self.version
+        elif version < 1 or version > max_ver:
             raise ValueError(f"Invalid version {version}")
+        else:
+            self.checkout(version)
+
         if version == max_ver:
-            self._reset_dataset()
+            # no-op if restoring the latest version
             return
-        self.checkout(version)
-        data = self.to_arrow()
-        self.checkout(max_ver)
-        self.add(data, mode="overwrite")
+
+        self._dataset.restore()
         self._reset_dataset()
 
     def __len__(self):

python/tests/test_table.py

@@ -280,3 +280,18 @@ def test_restore(db):
     table.restore(1)
     assert len(table.list_versions()) == 3
     assert len(table) == 1
+
+    expected = table.to_arrow()
+    table.checkout(1)
+    table.restore()
+    assert len(table.list_versions()) == 4
+    assert table.to_arrow() == expected
+
+    table.restore(4)  # latest version should be no-op
+    assert len(table.list_versions()) == 4
+
+    with pytest.raises(ValueError):
+        table.restore(5)
+
+    with pytest.raises(ValueError):
+        table.restore(0)