Fixed issue GH7777, HDFStore.read_column did not preserve UTC tzinfo

Author: alorenzo175

doc/source/v0.15.0.txt

@@ -196,6 +196,7 @@ Bug Fixes
 - Bug in Series 0-division with a float and integer operand dtypes  (:issue:`7785`)
 - Bug in ``Series.astype("unicode")`` not calling ``unicode`` on the values correctly (:issue:`7758`)
 - Bug in ``DataFrame.as_matrix()`` with mixed ``datetime64[ns]`` and ``timedelta64[ns]`` dtypes (:issue:`7778`)
+- Bug in ``HDFStore.select_column()`` not preserving UTC timezone info when selecting a DatetimeIndex (:issue:`7777`)
 
 
 - Bug in pickles contains ``DateOffset`` may raise ``AttributeError`` when ``normalize`` attribute is reffered internally (:issue:`7748`)

pandas/io/pytables.py

@@ -62,6 +62,18 @@ def _ensure_encoding(encoding):
             encoding = _default_encoding
     return encoding
 
+def _set_tz(values, tz, preserve_UTC=False):
+    """ set the timezone if values are an Index """
+    if tz is not None and isinstance(values, Index):
+        tz = _ensure_decoded(tz)
+        if values.tz is None:
+            values = values.tz_localize('UTC').tz_convert(tz)
+        if preserve_UTC:
+            if tslib.get_timezone(tz) == 'UTC':
+                values = list(values)
+
+    return values
+
 
 Term = Expr
 
@@ -1464,11 +1476,7 @@ def convert(self, values, nan_rep, encoding):
                 kwargs['freq'] = None
             self.values = Index(values, **kwargs)
 
-        # set the timezone if indicated
-        # we stored in utc, so reverse to local timezone
-        if self.tz is not None:
-            self.values = self.values.tz_localize(
-                'UTC').tz_convert(_ensure_decoded(self.tz))
+        self.values = _set_tz(self.values, self.tz)
 
         return self
 
@@ -3443,8 +3451,11 @@ def read_column(self, column, where=None, start=None, stop=None, **kwargs):
                 # column must be an indexable or a data column
                 c = getattr(self.table.cols, column)
                 a.set_info(self.info)
-                return Series(a.convert(c[start:stop], nan_rep=self.nan_rep,
-                                        encoding=self.encoding).take_data())
+                return Series(_set_tz(a.convert(c[start:stop], 
+                                                nan_rep=self.nan_rep,
+                                                encoding=self.encoding
+                                                ).take_data(), 
+                                      a.tz, True))
 
         raise KeyError("column [%s] not found in the table" % column)
 

pandas/io/tests/test_pytables.py

@@ -4299,6 +4299,38 @@ def test_tseries_indices_frame(self):
             self.assertEqual(type(result.index), type(df.index))
             self.assertEqual(result.index.freq, df.index.freq)
 
+    def test_tseries_select_index_column(self):
+        # GH7777
+        # selecting a UTC datetimeindex column did
+        # not preserve UTC tzinfo set before storing
+
+        # check that no tz still works
+        rng = date_range('1/1/2000', '1/30/2000')
+        frame = DataFrame(np.random.randn(len(rng), 4), index=rng)
+        
+        with ensure_clean_store(self.path) as store:
+            store.append('frame', frame)
+            result = store.select_column('frame', 'index')
+            self.assertEqual(rng.tz, DatetimeIndex(result.values).tz)
+
+        # check utc
+        rng = date_range('1/1/2000', '1/30/2000', tz='UTC')
+        frame = DataFrame(np.random.randn(len(rng), 4), index=rng)
+        
+        with ensure_clean_store(self.path) as store:
+            store.append('frame', frame)
+            result = store.select_column('frame', 'index')
+            self.assertEqual(rng.tz, DatetimeIndex(result.values).tz)
+
+        # double check non-utc
+        rng = date_range('1/1/2000', '1/30/2000', tz='US/Eastern')
+        frame = DataFrame(np.random.randn(len(rng), 4), index=rng)
+
+        with ensure_clean_store(self.path) as store:
+            store.append('frame', frame)
+            result = store.select_column('frame', 'index')
+            self.assertEqual(rng.tz, DatetimeIndex(result.values).tz)
+
     def test_unicode_index(self):
 
         unicode_values = [u('\u03c3'), u('\u03c3\u03c3')]