BUG: Allow freq conversion from dt64 to period (pandas-dev#23460)

Closes pandas-dev#23438

pandas/core/arrays/period.py

@@ -967,24 +967,15 @@ def dt64arr_to_periodarr(data, freq, tz=None):
     if data.dtype != np.dtype('M8[ns]'):
         raise ValueError('Wrong dtype: %s' % data.dtype)
 
-    if freq is not None:
-        freq = Period._maybe_convert_freq(freq)
+    if freq is None:
+        if isinstance(data, ABCIndexClass):
+            data, freq = data._values, data.freq
+        elif isinstance(data, ABCSeries):
+            data, freq = data._values, data.dt.freq
 
-    if isinstance(data, ABCIndexClass):
-        if freq is None:
-            freq = data.freq
-        elif freq != data.freq:
-            msg = DIFFERENT_FREQ_INDEX.format(freq.freqstr, data.freq.freqstr)
-            raise IncompatibleFrequency(msg)
-        data = data._values
+    freq = Period._maybe_convert_freq(freq)
 
-    elif isinstance(data, ABCSeries):
-        if freq is None:
-            freq = data.dt.freq
-        elif freq != data.dt.freq:
-            msg = DIFFERENT_FREQ_INDEX.format(freq.freqstr,
-                                              data.dt.freq.freqstr)
-            raise IncompatibleFrequency(msg)
+    if isinstance(data, (ABCIndexClass, ABCSeries)):
         data = data._values
 
     base, mult = frequencies.get_freq_code(freq)

pandas/tests/arrays/test_period.py

@@ -34,10 +34,13 @@ def test_period_array_ok(data, freq, expected):
     tm.assert_numpy_array_equal(result, expected)
 
 
-def test_from_datetime64_raises():
+def test_from_datetime64_freq_changes():
+    # https://github.com/pandas-dev/pandas/issues/23438
     arr = pd.date_range("2017", periods=3, freq="D")
-    with tm.assert_raises_regex(IncompatibleFrequency, "freq"):
-        PeriodArray._from_datetime64(arr, freq="M")
+    result = PeriodArray._from_datetime64(arr, freq="M")
+    expected = period_array(['2017-01-01', '2017-01-01', '2017-01-01'],
+                            freq="M")
+    tm.assert_period_array_equal(result, expected)
 
 
 @pytest.mark.parametrize("data, freq, msg", [

pandas/tests/indexes/period/test_construction.py

@@ -158,6 +158,21 @@ def test_constructor_datetime64arr(self):
 
         pytest.raises(ValueError, PeriodIndex, vals, freq='D')
 
+    @pytest.mark.parametrize('box', [None, 'series', 'index'])
+    def test_constructor_datetime64arr_ok(self, box):
+        # https://github.com/pandas-dev/pandas/issues/23438
+        data = pd.date_range('2017', periods=4, freq="M")
+        if box is None:
+            data = data._values
+        elif box == 'series':
+            data = pd.Series(data)
+
+        result = PeriodIndex(data, freq='D')
+        expected = PeriodIndex([
+            '2017-01-31', '2017-02-28', '2017-03-31', '2017-04-30'
+        ], freq="D")
+        tm.assert_index_equal(result, expected)
+
     def test_constructor_dtype(self):
         # passing a dtype with a tz should localize
         idx = PeriodIndex(['2013-01', '2013-03'], dtype='period[M]')