BUG: resampling with a how could trigger asfreq instead

closes #12332

Author: jreback

doc/source/whatsnew/v0.18.0.txt

@@ -581,7 +581,7 @@ 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`, :issue:`12202`).
+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`, :issue:`12332`).
 
 .. ipython:: python
 

pandas/tseries/resample.py

@@ -500,7 +500,7 @@ def _downsample(self, how, **kwargs):
         # do we have a regular frequency
         if ax.freq is not None or ax.inferred_freq is not None:
 
-            if len(self.grouper.binlabels) > len(ax):
+            if len(self.grouper.binlabels) > len(ax) and how is None:
 
                 # let's do an asfreq
                 return self.asfreq()

pandas/tseries/tests/test_resample.py

@@ -925,6 +925,19 @@ def test_resample_ohlc(self):
         self.assertEqual(xs['low'], s[:5].min())
         self.assertEqual(xs['close'], s[4])
 
+    def test_resample_ohlc_result(self):
+
+        # GH 12332
+        index = pd.date_range('1-1-2000', '2-15-2000', freq='h')
+        index = index.union(pd.date_range('4-15-2000', '5-15-2000', freq='h'))
+        s = Series(range(len(index)), index=index)
+
+        a = s.loc[:'4-15-2000'].resample('30T').ohlc()
+        self.assertIsInstance(a, DataFrame)
+
+        b = s.loc[:'4-14-2000'].resample('30T').ohlc()
+        self.assertIsInstance(b, DataFrame)
+
     def test_resample_ohlc_dataframe(self):
         df = (
             pd.DataFrame({