BUG: DTI.freqstr raises AttributeError when freq is None

doc/source/v0.14.1.txt

@@ -258,7 +258,7 @@ Bug Fixes
 - Bug in ``Index.astype(float)`` where it would return an ``object`` dtype
   ``Index`` (:issue:`7464`).
 - Bug in ``DataFrame.reset_index`` loses ``tz`` (:issue:`3950`)
-
+- Bug in ``DatetimeIndex.freqstr`` raises ``AttributeError`` when ``freq`` is ``None`` (:issue:`7606`)
 
 - Bug in non-monotonic ``Index.union`` may preserve ``name`` incorrectly (:issue:`7458`)
 - Bug in ``DatetimeIndex.intersection`` doesn't preserve timezone (:issue:`4690`)

pandas/core/frame.py

@@ -4311,12 +4311,8 @@ def to_period(self, freq=None, axis=0, copy=True):
 
         axis = self._get_axis_number(axis)
         if axis == 0:
-            if freq is None:
-                freq = self.index.freqstr or self.index.inferred_freq
             new_data.set_axis(1, self.index.to_period(freq=freq))
         elif axis == 1:
-            if freq is None:
-                freq = self.columns.freqstr or self.columns.inferred_freq
             new_data.set_axis(0, self.columns.to_period(freq=freq))
         else:  # pragma: no cover
             raise AssertionError('Axis must be 0 or 1. Got %s' % str(axis))

pandas/core/series.py

@@ -2374,8 +2374,6 @@ def to_period(self, freq=None, copy=True):
         if copy:
             new_values = new_values.copy()
 
-        if freq is None:
-            freq = self.index.freqstr or self.index.inferred_freq
         new_index = self.index.to_period(freq=freq)
         return self._constructor(new_values,
                                  index=new_index).__finalize__(self)

pandas/tseries/index.py

@@ -48,8 +48,7 @@ def f(self):
                     'is_quarter_start', 'is_quarter_end',
                     'is_year_start', 'is_year_end']:
             month_kw = self.freq.kwds.get('startingMonth', self.freq.kwds.get('month', 12)) if self.freq else 12
-            freqstr = self.freqstr if self.freq else None
-            return tslib.get_start_end_field(values, field, freqstr, month_kw)
+            return tslib.get_start_end_field(values, field, self.freqstr, month_kw)
         else:
             return tslib.get_date_field(values, field)
     f.__name__ = name
@@ -573,10 +572,7 @@ def __unicode__(self):
 
         values = self.values
 
-        freq = None
-        if self.offset is not None:
-            freq = self.offset.freqstr
-
+        freq = self.freqstr
         summary = str(self.__class__)
         if len(self) == 1:
             first = formatter(values[0], tz=self.tz)
@@ -794,12 +790,14 @@ def to_period(self, freq=None):
         """
         from pandas.tseries.period import PeriodIndex
 
-        if self.freq is None and freq is None:
-            msg = "You must pass a freq argument as current index has none."
-            raise ValueError(msg)
-
         if freq is None:
-            freq = get_period_alias(self.freqstr)
+            freq = self.freqstr or self.inferred_freq
+
+            if freq is None:
+                msg = "You must pass a freq argument as current index has none."
+                raise ValueError(msg)
+
+            freq = get_period_alias(freq)
 
         return PeriodIndex(self.values, name=self.name, freq=freq, tz=self.tz)
 
@@ -1440,6 +1438,8 @@ def inferred_freq(self):
     @property
     def freqstr(self):
         """ return the frequency object as a string if its set, otherwise None """
+        if self.freq is None:
+            return None
         return self.offset.freqstr
 
     _year = _field_accessor('year', 'Y')

pandas/tseries/tests/test_timeseries.py

@@ -1477,7 +1477,28 @@ def test_to_period(self):
         assert_series_equal(pts, exp)
 
         pts = ts.to_period('M')
+        exp.index = exp.index.asfreq('M')
         self.assertTrue(pts.index.equals(exp.index.asfreq('M')))
+        assert_series_equal(pts, exp)
+
+        # GH 7606 without freq
+        idx = DatetimeIndex(['2011-01-01', '2011-01-02', '2011-01-03', '2011-01-04'])
+        exp_idx = pd.PeriodIndex(['2011-01-01', '2011-01-02', '2011-01-03',
+                                  '2011-01-04'], freq='D')
+
+        s = Series(np.random.randn(4), index=idx)
+        expected = s.copy()
+        expected.index = exp_idx
+        assert_series_equal(s.to_period(), expected)
+
+        df = DataFrame(np.random.randn(4, 4), index=idx, columns=idx)
+        expected = df.copy()
+        expected.index = exp_idx
+        assert_frame_equal(df.to_period(), expected)
+
+        expected = df.copy()
+        expected.columns = exp_idx
+        assert_frame_equal(df.to_period(axis=1), expected)
 
     def create_dt64_based_index(self):
         data = [Timestamp('2007-01-01 10:11:12.123456Z'),
@@ -2102,7 +2123,14 @@ def test_to_period_nofreq(self):
 
         idx = DatetimeIndex(['2000-01-01', '2000-01-02', '2000-01-03'],
                             freq='infer')
-        idx.to_period()
+        self.assertEqual(idx.freqstr, 'D')
+        expected = pd.PeriodIndex(['2000-01-01', '2000-01-02', '2000-01-03'], freq='D')
+        self.assertTrue(idx.to_period().equals(expected))
+
+        # GH 7606
+        idx = DatetimeIndex(['2000-01-01', '2000-01-02', '2000-01-03'])
+        self.assertEqual(idx.freqstr, None)
+        self.assertTrue(idx.to_period().equals(expected))
 
     def test_000constructor_resolution(self):
         # 2252