[python] Temporary restore feature (#428)

This adds LanceTable.restore as a temporary feature. It reads data from
a previous version and creates
a new snapshot version using that data. This makes the version writeable
unlike checkout. This should be replaced once the feature is implemented
in pylance.

Co-authored-by: Chang She <chang@lancedb.com>

python/lancedb/table.py

@@ -268,10 +268,11 @@ class LanceTable(Table):
         self.name = name
         self._version = version
 
-    def _reset_dataset(self):
+    def _reset_dataset(self, version=None):
         try:
             if "_dataset" in self.__dict__:
                 del self.__dict__["_dataset"]
+            self._version = version
         except AttributeError:
             pass
 
@@ -297,7 +298,9 @@ class LanceTable(Table):
     def checkout(self, version: int):
         """Checkout a version of the table. This is an in-place operation.
 
-        This allows viewing previous versions of the 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.
 
         Parameters
         ----------
@@ -325,7 +328,49 @@ class LanceTable(Table):
         max_ver = max([v["version"] for v in self._dataset.versions()])
         if version < 1 or version > max_ver:
             raise ValueError(f"Invalid version {version}")
-        self._version = version
+        self._reset_dataset(version=version)
+
+    def restore(self, version: int):
+        """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.
+
+        Parameters
+        ----------
+        version : int
+            The version to restore.
+
+        Examples
+        --------
+        >>> import lancedb
+        >>> db = lancedb.connect("./.lancedb")
+        >>> table = db.create_table("my_table", [{"vector": [1.1, 0.9], "type": "vector"}])
+        >>> table.version
+        1
+        >>> table.to_pandas()
+               vector    type
+        0  [1.1, 0.9]  vector
+        >>> table.add([{"vector": [0.5, 0.2], "type": "vector"}])
+        >>> table.version
+        2
+        >>> table.restore(1)
+        >>> table.to_pandas()
+               vector    type
+        0  [1.1, 0.9]  vector
+        >>> len(table.list_versions())
+        3
+        """
+        max_ver = max([v["version"] for v in self._dataset.versions()])
+        if version < 1 or version >= max_ver:
+            raise ValueError(f"Invalid version {version}")
+        if version == max_ver:
+            self._reset_dataset()
+            return
+        self.checkout(version)
+        data = self.to_arrow()
+        self.checkout(max_ver)
+        self.add(data, mode="overwrite")
         self._reset_dataset()
 
     def __len__(self):