ENH: Add Series.str.casefold (#25419)

Author: charlesdong1991

Diff for: doc/source/reference/series.rst

@@ -409,6 +409,7 @@ strings and apply several methods to it. These can be accessed like
    :template: autosummary/accessor_method.rst
 
    Series.str.capitalize
+   Series.str.casefold
    Series.str.cat
    Series.str.center
    Series.str.contains

Diff for: doc/source/user_guide/text.rst

@@ -600,6 +600,7 @@ Method Summary
     :meth:`~Series.str.partition`;Equivalent to ``str.partition``
     :meth:`~Series.str.rpartition`;Equivalent to ``str.rpartition``
     :meth:`~Series.str.lower`;Equivalent to ``str.lower``
+    :meth:`~Series.str.casefold`;Equivalent to ``str.casefold``
     :meth:`~Series.str.upper`;Equivalent to ``str.upper``
     :meth:`~Series.str.find`;Equivalent to ``str.find``
     :meth:`~Series.str.rfind`;Equivalent to ``str.rfind``

Diff for: doc/source/whatsnew/v0.25.0.rst

@@ -22,6 +22,7 @@ Other Enhancements
 - Indexing of ``DataFrame`` and ``Series`` now accepts zerodim ``np.ndarray`` (:issue:`24919`)
 - :meth:`Timestamp.replace` now supports the ``fold`` argument to disambiguate DST transition times (:issue:`25017`)
 - :meth:`DataFrame.at_time` and :meth:`Series.at_time` now support :meth:`datetime.time` objects with timezones (:issue:`24043`)
+- ``Series.str`` has gained :meth:`Series.str.casefold` method to removes all case distinctions present in a string (:issue:`25405`)
 - :meth:`DataFrame.set_index` now works for instances of ``abc.Iterator``, provided their output is of the same length as the calling frame (:issue:`22484`, :issue:`24984`)
 - :meth:`DatetimeIndex.union` now supports the ``sort`` argument. The behaviour of the sort parameter matches that of :meth:`Index.union` (:issue:`24994`)
 -

Diff for: pandas/core/strings.py

@@ -2926,7 +2926,7 @@ def rindex(self, sub, start=0, end=None):
 
     _shared_docs['casemethods'] = ("""
     Convert strings in the Series/Index to %(type)s.
-
+    %(version)s
     Equivalent to :meth:`str.%(method)s`.
 
     Returns
@@ -2943,6 +2943,7 @@ def rindex(self, sub, start=0, end=None):
         remaining to lowercase.
     Series.str.swapcase : Converts uppercase to lowercase and lowercase to
         uppercase.
+    Series.str.casefold: Removes all case distinctions in the string.
 
     Examples
     --------
@@ -2989,12 +2990,15 @@ def rindex(self, sub, start=0, end=None):
     3              sWaPcAsE
     dtype: object
     """)
-    _shared_docs['lower'] = dict(type='lowercase', method='lower')
-    _shared_docs['upper'] = dict(type='uppercase', method='upper')
-    _shared_docs['title'] = dict(type='titlecase', method='title')
+    _shared_docs['lower'] = dict(type='lowercase', method='lower', version='')
+    _shared_docs['upper'] = dict(type='uppercase', method='upper', version='')
+    _shared_docs['title'] = dict(type='titlecase', method='title', version='')
     _shared_docs['capitalize'] = dict(type='be capitalized',
-                                      method='capitalize')
-    _shared_docs['swapcase'] = dict(type='be swapcased', method='swapcase')
+                                      method='capitalize', version='')
+    _shared_docs['swapcase'] = dict(type='be swapcased', method='swapcase',
+                                    version='')
+    _shared_docs['casefold'] = dict(type='be casefolded', method='casefold',
+                                    version='\n    .. versionadded:: 0.25.0\n')
     lower = _noarg_wrapper(lambda x: x.lower(),
                            docstring=_shared_docs['casemethods'] %
                            _shared_docs['lower'])
@@ -3010,6 +3014,9 @@ def rindex(self, sub, start=0, end=None):
     swapcase = _noarg_wrapper(lambda x: x.swapcase(),
                               docstring=_shared_docs['casemethods'] %
                               _shared_docs['swapcase'])
+    casefold = _noarg_wrapper(lambda x: x.casefold(),
+                              docstring=_shared_docs['casemethods'] %
+                              _shared_docs['casefold'])
 
     _shared_docs['ismethods'] = ("""
     Check whether all characters in each string are %(type)s.

Diff for: pandas/tests/test_strings.py

@@ -76,7 +76,7 @@ def assert_series_or_index_equal(left, right):
     'len', 'lower', 'lstrip', 'partition',
     'rpartition', 'rsplit', 'rstrip',
     'slice', 'slice_replace', 'split',
-    'strip', 'swapcase', 'title', 'upper'
+    'strip', 'swapcase', 'title', 'upper', 'casefold'
 ], [()] * 100, [{}] * 100))
 ids, _, _ = zip(*_any_string_method)  # use method name as fixture-id
 
@@ -3440,3 +3440,12 @@ def test_method_on_bytes(self):
             expected = Series(np.array(
                 ['ad', 'be', 'cf'], 'S2').astype(object))
             tm.assert_series_equal(result, expected)
+
+    @pytest.mark.skipif(compat.PY2, reason='not in python2')
+    def test_casefold(self):
+        # GH25405
+        expected = Series(['ss', NA, 'case', 'ssd'])
+        s = Series(['ß', NA, 'case', 'ßd'])
+        result = s.str.casefold()
+
+        tm.assert_series_equal(result, expected)