TST: validation tests for resample/groupby preservation

closes #12202

Author: Jeff Reback <jeff@reback.net>

Closes #12314 from jreback/resample and squashes the following commits:

bde178a [Jeff Reback] TST: validation tests for resample/groupby preservation

Author: jreback

doc/source/whatsnew/v0.18.0.txt

@@ -574,7 +574,7 @@ other anchored offsets like ``MonthBegin`` and ``YearBegin``.
 Resample API
 ^^^^^^^^^^^^
 
-Like the change in the window functions API :ref:`above <whatsnew_0180.enhancements.moments>`, ``.resample(...)`` is changing to have a more groupby-like API. (:issue:`11732`, :issue:`12702`).
+Like the change in the window functions API :ref:`above <whatsnew_0180.enhancements.moments>`, ``.resample(...)`` is changing to have a more groupby-like API. (:issue:`11732`, :issue:`12702`, :issue:`12202`).
 
 .. ipython:: python
 

pandas/tseries/tests/test_resample.py

@@ -1220,6 +1220,24 @@ def test_resample_empty(self):
                         # (ex: doing mean with dtype of np.object)
                         pass
 
+    def test_resample_dtype_preservation(self):
+
+        # GH 12202
+        # validation tests for dtype preservation
+
+        df = DataFrame({'date': pd.date_range(start='2016-01-01',
+                                              periods=4, freq='W'),
+                        'group': [1, 1, 2, 2],
+                        'val': Series([5, 6, 7, 8],
+                                      dtype='int32')}
+                       ).set_index('date')
+
+        result = df.resample('1D').ffill()
+        self.assertEqual(result.val.dtype, np.int32)
+
+        result = df.groupby('group').resample('1D').ffill()
+        self.assertEqual(result.val.dtype, np.int32)
+
     def test_weekly_resample_buglet(self):
         # #1327
         rng = date_range('1/1/2000', freq='B', periods=20)