BUG: add 'import warnings' statement in panel.py (GH8152)

Author: Hyungtae Kim

doc/source/v0.15.0.txt

@@ -648,3 +648,4 @@ Bug Fixes
   was a tuple (:issue:`8121`).
 
 
+- Bug in ``panel.py`` where Panel uses ``warnings`` module without importing (:issue:`8152`).

pandas/core/panel.py

@@ -7,6 +7,7 @@
                            OrderedDefaultdict)
 from pandas import compat
 import sys
+import warnings
 import numpy as np
 from pandas.core.common import (PandasError, _try_sort, _default_index,
                                 _infer_dtype_from_scalar, notnull)

pandas/tests/test_panel.py

@@ -18,6 +18,7 @@
                                  assert_frame_equal,
                                  assert_series_equal,
                                  assert_almost_equal,
+                                 assert_produces_warning,
                                  ensure_clean,
                                  assertRaisesRegexp,
                                  makeCustomDataframe as mkdf,
@@ -2324,6 +2325,13 @@ def test_panel_index():
                                        np.repeat([1, 2, 3], 4)])
     assert(index.equals(expected))
 
+
+def test_import_warnings():
+    # GH8152
+    panel = Panel(np.random.rand(3, 3, 3))
+    with assert_produces_warning():
+        panel.major_xs(1, copy=False)
+
 if __name__ == '__main__':
     import nose
     nose.runmodule(argv=[__file__, '-vvs', '-x', '--pdb', '--pdb-failure'],