* Speed up selecting row-groups (#856)

* xfail warning test

fastparquet/api.py

@@ -311,11 +311,14 @@ def __getitem__(self, item):
         new_rgs = self.row_groups[item]
         if not isinstance(new_rgs, list):
             new_rgs = [new_rgs]
-        new_pf = copy.deepcopy(self)
-        new_pf.fmd.row_groups = new_rgs
-        new_pf._set_attrs()
-        # would otherwise be "simple" when selecting one rg
-        new_pf.file_scheme = self.file_scheme
+        new_pf = object.__new__(ParquetFile)
+        fmd = copy.copy(self.fmd)
+        fmd.row_groups = new_rgs
+        new_pf.__setstate__(
+            {"fn": self.fn, "open": self.open, "fmd": fmd,
+             "pandas_nulls": self.pandas_nulls, "_base_dtype": self._base_dtype,
+             "tz": self.tz}
+        )
         return new_pf
 
     def __len__(self):

fastparquet/test/test_dataframe.py

@@ -2,6 +2,7 @@
 from unittest import mock
 
 import pandas as pd
+import pytest
 from numpy import empty as np_empty
 from pandas.testing import assert_frame_equal
 
@@ -33,6 +34,7 @@ def test_empty():
     assert len(views) == 5
 
 
+@pytest.mark.xfail(reason="df._data is going away")
 def test_empty_tz_utc():
     with warnings.catch_warnings():
         warnings.simplefilter("error")