ENH: linearly spaced date_range (GH 20808)

doc/source/whatsnew/v0.23.0.txt

@@ -523,6 +523,7 @@ Other Enhancements
   library. (:issue:`20564`)
 - Added new writer for exporting Stata dta files in version 117, ``StataWriter117``.  This format supports exporting strings with lengths up to 2,000,000 characters (:issue:`16450`)
 - :func:`to_hdf` and :func:`read_hdf` now accept an ``errors`` keyword argument to control encoding error handling (:issue:`20835`)
+- :func:`date_range` now returns a linearly spaced ``DatetimeIndex`` if ``start``, ``stop``, and ``periods`` are specified, but ``freq`` is not. (:issue:`20808`)
 
 .. _whatsnew_0230.api_breaking:
 

pandas/core/indexes/datetimes.py

@@ -358,7 +358,8 @@ def __new__(cls, data=None,
                 msg = 'periods must be a number, got {periods}'
                 raise TypeError(msg.format(periods=periods))
 
-        if data is None and freq is None:
+        if data is None and freq is None \
+                and com._any_none(periods, start, end):
             raise ValueError("Must provide freq argument if no data is "
                              "supplied")
 
@@ -466,9 +467,9 @@ def __new__(cls, data=None,
     @classmethod
     def _generate(cls, start, end, periods, name, freq,
                   tz=None, normalize=False, ambiguous='raise', closed=None):
-        if com._count_not_none(start, end, periods) != 2:
-            raise ValueError('Of the three parameters: start, end, and '
-                             'periods, exactly two must be specified')
+        if com._count_not_none(start, end, periods, freq) != 3:
+            raise ValueError('Of the four parameters: start, end, periods, '
+                             'and freq, exactly three must be specified')
 
         _normalized = True
 
@@ -566,23 +567,30 @@ def _generate(cls, start, end, periods, name, freq,
                 if end.tz is None and start.tz is not None:
                     start = start.replace(tzinfo=None)
 
-            if _use_cached_range(freq, _normalized, start, end):
-                index = cls._cached_range(start, end, periods=periods,
-                                          freq=freq, name=name)
+            if freq is not None:
+                if _use_cached_range(freq, _normalized, start, end):
+                    index = cls._cached_range(start, end, periods=periods,
+                                              freq=freq, name=name)
+                else:
+                    index = _generate_regular_range(start, end, periods, freq)
+
+                if tz is not None and getattr(index, 'tz', None) is None:
+                    index = conversion.tz_localize_to_utc(_ensure_int64(index),
+                                                          tz,
+                                                          ambiguous=ambiguous)
+                    index = index.view(_NS_DTYPE)
+
+                    # index is localized datetime64 array -> have to convert
+                    # start/end as well to compare
+                    if start is not None:
+                        start = start.tz_localize(tz).asm8
+                    if end is not None:
+                        end = end.tz_localize(tz).asm8
             else:
-                index = _generate_regular_range(start, end, periods, freq)
-
-            if tz is not None and getattr(index, 'tz', None) is None:
-                index = conversion.tz_localize_to_utc(_ensure_int64(index), tz,
-                                                      ambiguous=ambiguous)
-                index = index.view(_NS_DTYPE)
-
-                # index is localized datetime64 array -> have to convert
-                # start/end as well to compare
-                if start is not None:
-                    start = start.tz_localize(tz).asm8
-                if end is not None:
-                    end = end.tz_localize(tz).asm8
+                index = tools.to_datetime(np.linspace(start.value,
+                                                      end.value, periods))
+                if tz is not None:
+                    index = index.tz_localize('UTC').tz_convert(tz)
 
         if not left_closed and len(index) and index[0] == start:
             index = index[1:]
@@ -2565,13 +2573,15 @@ def _generate_regular_range(start, end, periods, freq):
     return data
 
 
-def date_range(start=None, end=None, periods=None, freq='D', tz=None,
+def date_range(start=None, end=None, periods=None, freq=None, tz=None,
                normalize=False, name=None, closed=None, **kwargs):
     """
     Return a fixed frequency DatetimeIndex.
 
-    Exactly two of the three parameters `start`, `end` and `periods`
-    must be specified.
+    Of the three parameters `start`, `end`, `periods`, and `freq` exactly
+    three must be specified. If `freq` is omitted, the resulting DatetimeIndex
+    will have `periods` linearly spaced elements between `start` and `end`
+    (closed on both sides).
 
     Parameters
     ----------
@@ -2613,7 +2623,7 @@ def date_range(start=None, end=None, periods=None, freq='D', tz=None,
     --------
     **Specifying the values**
 
-    The next three examples generate the same `DatetimeIndex`, but vary
+    The next four examples generate the same `DatetimeIndex`, but vary
     the combination of `start`, `end` and `periods`.
 
     Specify `start` and `end`, with the default daily frequency.
@@ -2637,6 +2647,13 @@ def date_range(start=None, end=None, periods=None, freq='D', tz=None,
                    '2017-12-29', '2017-12-30', '2017-12-31', '2018-01-01'],
                   dtype='datetime64[ns]', freq='D')
 
+    Specify `start`, `end`, and `periods`; the frequency is generated
+    automatically (linearly spaced).
+
+    >>> pd.date_range(start='2018-04-24', end='2018-04-27', periods=3)
+    DatetimeIndex(['2018-04-24 00:00:00', '2018-04-25 12:00:00',
+                   '2018-04-27 00:00:00'], freq=None)
+
     **Other Parameters**
 
     Changed the `freq` (frequency) to ``'M'`` (month end frequency).
@@ -2687,6 +2704,10 @@ def date_range(start=None, end=None, periods=None, freq='D', tz=None,
     DatetimeIndex(['2017-01-02', '2017-01-03', '2017-01-04'],
                   dtype='datetime64[ns]', freq='D')
     """
+
+    if freq is None and com._any_none(periods, start, end):
+        freq = 'D'
+
     return DatetimeIndex(start=start, end=end, periods=periods,
                          freq=freq, tz=tz, normalize=normalize, name=name,
                          closed=closed, **kwargs)

pandas/tests/indexes/datetimes/test_date_range.py

@@ -157,11 +157,28 @@ def test_date_range_ambiguous_arguments(self):
         start = datetime(2011, 1, 1, 5, 3, 40)
         end = datetime(2011, 1, 1, 8, 9, 40)
 
-        msg = ('Of the three parameters: start, end, and periods, '
-               'exactly two must be specified')
+        msg = ('Of the four parameters: start, end, periods, and '
+               'freq, exactly three must be specified')
         with tm.assert_raises_regex(ValueError, msg):
             date_range(start, end, periods=10, freq='s')
 
+    def test_date_range_convenience_periods(self):
+        # GH 20808
+        rng = date_range('2018-04-24', '2018-04-27', periods=3)
+        exp = DatetimeIndex(['2018-04-24 00:00:00', '2018-04-25 12:00:00',
+                             '2018-04-27 00:00:00'], freq=None)
+
+        tm.assert_index_equal(rng, exp)
+
+        # Test if spacing remains linear if tz changes to dst in range
+        rng = date_range('2018-04-01 01:00:00', '2018-04-01 04:00:00',
+                         tz='Australia/Sydney', periods=3)
+        exp = DatetimeIndex(['2018-04-01 01:00:00+11:00',
+                             '2018-04-01 02:00:00+11:00',
+                             '2018-04-01 02:00:00+10:00',
+                             '2018-04-01 03:00:00+10:00',
+                             '2018-04-01 04:00:00+10:00'], freq=None)
+
     def test_date_range_businesshour(self):
         idx = DatetimeIndex(['2014-07-04 09:00', '2014-07-04 10:00',
                              '2014-07-04 11:00',
@@ -198,8 +215,8 @@ def test_date_range_businesshour(self):
 
     def test_range_misspecified(self):
         # GH #1095
-        msg = ('Of the three parameters: start, end, and periods, '
-               'exactly two must be specified')
+        msg = ('Of the four parameters: start, end, periods, and '
+               'freq, exactly three must be specified')
 
         with tm.assert_raises_regex(ValueError, msg):
             date_range(start='1/1/2000')