ENH: Implement linspace behavior for timedelta_range and interval_range

doc/source/whatsnew/v0.23.0.txt

@@ -526,7 +526,7 @@ Other Enhancements
 - 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:`cut` has gained the ``duplicates='raise'|'drop'`` option to control whether to raise on duplicated edges (:issue:`20947`)
-- :func:`date_range` now returns a linearly spaced ``DatetimeIndex`` if ``start``, ``stop``, and ``periods`` are specified, but ``freq`` is not. (:issue:`20808`, :issue:`20983`)
+- :func:`date_range`, :func:`timedelta_range`, and :func:`interval_range` now return a linearly spaced index if ``start``, ``stop``, and ``periods`` are specified, but ``freq`` is not. (:issue:`20808`, :issue:`20983`, :issue:`20976`)
 
 .. _whatsnew_0230.api_breaking:
 

pandas/core/indexes/datetimes.py

@@ -358,11 +358,6 @@ 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 \
-                and com._any_none(periods, start, end):
-            raise ValueError("Must provide freq argument if no data is "
-                             "supplied")
-
         # if dtype has an embedded tz, capture it
         if dtype is not None:
             try:
@@ -377,9 +372,13 @@ def __new__(cls, data=None,
                 pass
 
         if data is None:
-            return cls._generate(start, end, periods, name, freq,
-                                 tz=tz, normalize=normalize, closed=closed,
-                                 ambiguous=ambiguous)
+            if freq is None and com._any_none(periods, start, end):
+                msg = 'Must provide freq argument if no data is supplied'
+                raise ValueError(msg)
+            else:
+                return cls._generate(start, end, periods, name, freq, tz=tz,
+                                     normalize=normalize, closed=closed,
+                                     ambiguous=ambiguous)
 
         if not isinstance(data, (np.ndarray, Index, ABCSeries)):
             if is_scalar(data):
@@ -2590,11 +2589,6 @@ def date_range(start=None, end=None, periods=None, freq=None, tz=None,
     """
     Return a fixed frequency DatetimeIndex.
 
-    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
     ----------
     start : str or datetime-like, optional
@@ -2628,9 +2622,20 @@ def date_range(start=None, end=None, periods=None, freq=None, tz=None,
     See Also
     --------
     pandas.DatetimeIndex : An immutable container for datetimes.
+    pandas.timedelta_range : Return a fixed frequency TimedeltaIndex.
     pandas.period_range : Return a fixed frequency PeriodIndex.
     pandas.interval_range : Return a fixed frequency IntervalIndex.
 
+    Notes
+    -----
+    Of the four 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).
+
+    To learn more about the frequency strings, please see `this link
+    <http://pandas.pydata.org/pandas-docs/stable/timeseries.html#offset-aliases>`__.
+
     Examples
     --------
     **Specifying the values**
@@ -2769,8 +2774,10 @@ def bdate_range(start=None, end=None, periods=None, freq='B', tz=None,
 
     Notes
     -----
-    Of the three parameters: ``start``, ``end``, and ``periods``, exactly two
-    must be specified.
+    Of the four parameters: ``start``, ``end``, ``periods``, and ``freq``,
+    exactly three must be specified.  Specifying ``freq`` is a requirement
+    for ``bdate_range``.  Use ``date_range`` if specifying ``freq`` is not
+    desired.
 
     To learn more about the frequency strings, please see `this link
     <http://pandas.pydata.org/pandas-docs/stable/timeseries.html#offset-aliases>`__.
@@ -2779,6 +2786,9 @@ def bdate_range(start=None, end=None, periods=None, freq='B', tz=None,
     -------
     rng : DatetimeIndex
     """
+    if freq is None:
+        msg = 'freq must be specified for bdate_range; use date_range instead'
+        raise TypeError(msg)
 
     if is_string_like(freq) and freq.startswith('C'):
         try:

pandas/core/indexes/interval.py

@@ -6,7 +6,8 @@
 from pandas.core.dtypes.missing import notna, isna
 from pandas.core.dtypes.generic import ABCDatetimeIndex, ABCPeriodIndex
 from pandas.core.dtypes.dtypes import IntervalDtype
-from pandas.core.dtypes.cast import maybe_convert_platform, find_common_type
+from pandas.core.dtypes.cast import (
+    maybe_convert_platform, find_common_type, maybe_downcast_to_dtype)
 from pandas.core.dtypes.common import (
     _ensure_platform_int,
     is_list_like,
@@ -1465,8 +1466,13 @@ def interval_range(start=None, end=None, periods=None, freq=None,
 
     Notes
     -----
-    Of the three parameters: ``start``, ``end``, and ``periods``, exactly two
-    must be specified.
+    Of the four parameters ``start``, ``end``, ``periods``, and ``freq``,
+    exactly three must be specified. If ``freq`` is omitted, the resulting
+    ``IntervalIndex`` will have ``periods`` linearly spaced elements between
+    ``start`` and ``end``, inclusively.
+
+    To learn more about datetime-like frequency strings, please see `this link
+    <http://pandas.pydata.org/pandas-docs/stable/timeseries.html#offset-aliases>`__.
 
     Returns
     -------
@@ -1505,6 +1511,14 @@ def interval_range(start=None, end=None, periods=None, freq=None,
                    (2017-03-01, 2017-04-01]]
                   closed='right', dtype='interval[datetime64[ns]]')
 
+    Specify ``start``, ``end``, and ``periods``; the frequency is generated
+    automatically (linearly spaced).
+
+    >>> pd.interval_range(start=0, end=6, periods=4)
+    IntervalIndex([(0.0, 1.5], (1.5, 3.0], (3.0, 4.5], (4.5, 6.0]]
+              closed='right',
+              dtype='interval[float64]')
+
     The ``closed`` parameter specifies which endpoints of the individual
     intervals within the ``IntervalIndex`` are closed.
 
@@ -1516,19 +1530,21 @@ def interval_range(start=None, end=None, periods=None, freq=None,
     --------
     IntervalIndex : an Index of intervals that are all closed on the same side.
     """
-    if com._count_not_none(start, end, periods) != 2:
-        raise ValueError('Of the three parameters: start, end, and periods, '
-                         'exactly two must be specified')
-
     start = com._maybe_box_datetimelike(start)
     end = com._maybe_box_datetimelike(end)
-    endpoint = next(com._not_none(start, end))
+    endpoint = start if start is not None else end
+
+    if freq is None and com._any_none(periods, start, end):
+        freq = 1 if is_number(endpoint) else 'D'
+
+    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')
 
     if not _is_valid_endpoint(start):
         msg = 'start must be numeric or datetime-like, got {start}'
         raise ValueError(msg.format(start=start))
-
-    if not _is_valid_endpoint(end):
+    elif not _is_valid_endpoint(end):
         msg = 'end must be numeric or datetime-like, got {end}'
         raise ValueError(msg.format(end=end))
 
@@ -1538,8 +1554,7 @@ def interval_range(start=None, end=None, periods=None, freq=None,
         msg = 'periods must be a number, got {periods}'
         raise TypeError(msg.format(periods=periods))
 
-    freq = freq or (1 if is_number(endpoint) else 'D')
-    if not is_number(freq):
+    if freq is not None and not is_number(freq):
         try:
             freq = to_offset(freq)
         except ValueError:
@@ -1552,28 +1567,34 @@ def interval_range(start=None, end=None, periods=None, freq=None,
                 _is_type_compatible(end, freq)]):
         raise TypeError("start, end, freq need to be type compatible")
 
+    # +1 to convert interval count to breaks count (n breaks = n-1 intervals)
+    if periods is not None:
+        periods += 1
+
     if is_number(endpoint):
+        # compute the period/start/end if unspecified (at most one)
         if periods is None:
-            periods = int((end - start) // freq)
-
-        if start is None:
-            start = end - periods * freq
-
-        # force end to be consistent with freq (lower if freq skips over end)
-        end = start + periods * freq
-
-        # end + freq for inclusive endpoint
-        breaks = np.arange(start, end + freq, freq)
-    elif isinstance(endpoint, Timestamp):
-        # add one to account for interval endpoints (n breaks = n-1 intervals)
-        if periods is not None:
-            periods += 1
-        breaks = date_range(start=start, end=end, periods=periods, freq=freq)
+            periods = int((end - start) // freq) + 1
+        elif start is None:
+            start = end - (periods - 1) * freq
+        elif end is None:
+            end = start + (periods - 1) * freq
+
+        # force end to be consistent with freq (lower if freq skips end)
+        if freq is not None:
+            end -= end % freq
+
+        breaks = np.linspace(start, end, periods)
+        if all(is_integer(x) for x in com._not_none(start, end, freq)):
+            # np.linspace always produces float output
+            breaks = maybe_downcast_to_dtype(breaks, 'int64')
     else:
-        # add one to account for interval endpoints (n breaks = n-1 intervals)
-        if periods is not None:
-            periods += 1
-        breaks = timedelta_range(start=start, end=end, periods=periods,
-                                 freq=freq)
+        # delegate to the appropriate range function
+        if isinstance(endpoint, Timestamp):
+            range_func = date_range
+        else:
+            range_func = timedelta_range
+
+        breaks = range_func(start=start, end=end, periods=periods, freq=freq)
 
     return IntervalIndex.from_breaks(breaks, name=name, closed=closed)

pandas/core/indexes/timedeltas.py

@@ -225,13 +225,13 @@ def __new__(cls, data=None, unit=None, freq=None, start=None, end=None,
                 msg = 'periods must be a number, got {periods}'
                 raise TypeError(msg.format(periods=periods))
 
-        if data is None and freq is None:
-            raise ValueError("Must provide freq argument if no data is "
-                             "supplied")
-
         if data is None:
-            return cls._generate(start, end, periods, name, freq,
-                                 closed=closed)
+            if freq is None and com._any_none(periods, start, end):
+                msg = 'Must provide freq argument if no data is supplied'
+                raise ValueError(msg)
+            else:
+                return cls._generate(start, end, periods, name, freq,
+                                     closed=closed)
 
         if unit is not None:
             data = to_timedelta(data, unit=unit, box=False)
@@ -266,10 +266,10 @@ def __new__(cls, data=None, unit=None, freq=None, start=None, end=None,
         return cls._simple_new(data, name=name, freq=freq)
 
     @classmethod
-    def _generate(cls, start, end, periods, name, offset, 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')
+    def _generate(cls, start, end, periods, name, freq, closed=None):
+        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')
 
         if start is not None:
             start = Timedelta(start)
@@ -295,8 +295,11 @@ def _generate(cls, start, end, periods, name, offset, closed=None):
         else:
             raise ValueError("Closed has to be either 'left', 'right' or None")
 
-        index = _generate_regular_range(start, end, periods, offset)
-        index = cls._simple_new(index, name=name, freq=offset)
+        if freq is not None:
+            index = _generate_regular_range(start, end, periods, freq)
+            index = cls._simple_new(index, name=name, freq=freq)
+        else:
+            index = to_timedelta(np.linspace(start.value, end.value, periods))
 
         if not left_closed:
             index = index[1:]
@@ -1046,7 +1049,7 @@ def _generate_regular_range(start, end, periods, offset):
     return data
 
 
-def timedelta_range(start=None, end=None, periods=None, freq='D',
+def timedelta_range(start=None, end=None, periods=None, freq=None,
                     name=None, closed=None):
     """
     Return a fixed frequency TimedeltaIndex, with day as the default
@@ -1074,8 +1077,10 @@ def timedelta_range(start=None, end=None, periods=None, freq='D',
 
     Notes
     -----
-    Of the three parameters: ``start``, ``end``, and ``periods``, exactly two
-    must be specified.
+    Of the four parameters ``start``, ``end``, ``periods``, and ``freq``,
+    exactly three must be specified. If ``freq`` is omitted, the resulting
+    ``TimedeltaIndex`` will have ``periods`` linearly spaced elements between
+    ``start`` and ``end`` (closed on both sides).
 
     To learn more about the frequency strings, please see `this link
     <http://pandas.pydata.org/pandas-docs/stable/timeseries.html#offset-aliases>`__.
@@ -1102,6 +1107,17 @@ def timedelta_range(start=None, end=None, periods=None, freq='D',
     TimedeltaIndex(['1 days 00:00:00', '1 days 06:00:00', '1 days 12:00:00',
                     '1 days 18:00:00', '2 days 00:00:00'],
                    dtype='timedelta64[ns]', freq='6H')
+
+    Specify ``start``, ``end``, and ``periods``; the frequency is generated
+    automatically (linearly spaced).
+
+    >>> pd.timedelta_range(start='1 day', end='5 days', periods=4)
+    TimedeltaIndex(['1 days 00:00:00', '2 days 08:00:00', '3 days 16:00:00',
+                '5 days 00:00:00'],
+               dtype='timedelta64[ns]', freq=None)
     """
+    if freq is None and com._any_none(periods, start, end):
+        freq = 'D'
+
     return TimedeltaIndex(start=start, end=end, periods=periods,
                           freq=freq, name=name, closed=closed)

pandas/tests/indexes/datetimes/test_date_range.py

@@ -361,6 +361,10 @@ def test_constructor(self):
         with tm.assert_raises_regex(TypeError, msg):
             bdate_range('2011-1-1', '2012-1-1', 'B')
 
+        msg = 'freq must be specified for bdate_range; use date_range instead'
+        with tm.assert_raises_regex(TypeError, msg):
+            bdate_range(START, END, periods=10, freq=None)
+
     def test_naive_aware_conflicts(self):
         naive = bdate_range(START, END, freq=BDay(), tz=None)
         aware = bdate_range(START, END, freq=BDay(), tz="Asia/Hong_Kong")