deprecate 'c',fix examples in v0.22.0, add tests and a note to v3.0.0

Author: natmokval

doc/source/whatsnew/v0.22.0.rst

@@ -157,16 +157,27 @@ sum and ``1`` for product.
 
 *pandas 0.22.0*
 
-.. ipython:: python
+.. code-block:: ipython
+
+   In [11]: s = pd.Series([1, 1, np.nan, np.nan],
+      ....:               index=pd.date_range("2017", periods=4))
 
-   s = pd.Series([1, 1, np.nan, np.nan], index=pd.date_range("2017", periods=4))
-   s.resample("2d").sum()
+   In [12]: s.resample("2d").sum()
+   Out[12]:
+   2017-01-01    2.0
+   2017-01-03    0.0
+   Freq: 2D, Length: 2, dtype: float64
 
 To restore the 0.21 behavior of returning ``NaN``, use ``min_count>=1``.
 
-.. ipython:: python
+.. code-block:: ipython
+
+   In [13]: s.resample("2d").sum(min_count=1)
+   Out[13]:
+   2017-01-01    2.0
+   2017-01-03    NaN
+   Freq: 2D, Length: 2, dtype: float64
 
-   s.resample("2d").sum(min_count=1)
 
 In particular, upsampling and taking the sum or product is affected, as
 upsampling introduces missing values even if the original series was

doc/source/whatsnew/v3.0.0.rst

@@ -272,6 +272,8 @@ Other Deprecations
 - Deprecated allowing non-keyword arguments in :meth:`Series.to_markdown` except ``buf``. (:issue:`57280`)
 - Deprecated allowing non-keyword arguments in :meth:`Series.to_string` except ``buf``. (:issue:`57280`)
 - Deprecated behavior of :meth:`Series.dt.to_pytimedelta`, in a future version this will return a :class:`Series` containing python ``datetime.timedelta`` objects instead of an ``ndarray`` of timedelta; this matches the behavior of other :meth:`Series.dt` properties. (:issue:`57463`)
+- Deprecated lowercase strings ``d``, ``b`` and ``c`` denoting frequencies in :class:`Day`, :class:`BusinessDay` and :class:`CustomBusinessDay` in favour of ``D``, ``B`` and ``C`` (:issue:`58998`)
+- Deprecated lowercase strings ``w``, ``w-mon``, ``w-tue``, etc. denoting frequencies in :class:`Week` in favour of ``W``, ``W-MON``, ``W-TUE``, etc. (:issue:`58998`)
 - Deprecated parameter ``method`` in :meth:`DataFrame.reindex_like` / :meth:`Series.reindex_like` (:issue:`58667`)
 - Deprecated using ``epoch`` date format in :meth:`DataFrame.to_json` and :meth:`Series.to_json`, use ``iso`` instead. (:issue:`57063`)
 

pandas/_libs/tslibs/dtypes.pyx

@@ -357,6 +357,7 @@ cdef dict c_PERIOD_AND_OFFSET_DEPR_FREQSTR = {
     "w-sun": "W-SUN",
     "d": "D",
     "b": "B",
+    "c": "C",
     "MIN": "min",
 }
 

pandas/tests/indexes/datetimes/test_date_range.py

@@ -795,6 +795,7 @@ def test_frequency_A_raises(self, freq):
             ("2W-WED", "2w-wed"),
             ("2B", "2b"),
             ("2D", "2d"),
+            ("2C", "2c"),
         ],
     )
     def test_date_range_depr_lowercase_frequency(self, freq, freq_depr):

pandas/tests/resample/test_datetime_index.py

@@ -2044,6 +2044,7 @@ def test_resample_BM_BQ_raises(freq):
         ("1W-SUN", "1w-sun", ["2013-01-06"]),
         ("1D", "1d", ["2013-01-01"]),
         ("1B", "1b", ["2013-01-01"]),
+        ("1C", "1c", ["2013-01-01"]),
     ],
 )
 def test_resample_depr_lowercase_frequency(freq, freq_depr, data):