add casefold for string

Author: charlesdong1991

Diff for: pandas/core/strings.py

@@ -2933,6 +2933,7 @@ 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
     --------
@@ -2985,6 +2986,7 @@ def rindex(self, sub, start=0, end=None):
     _shared_docs['capitalize'] = dict(type='be capitalized',
                                       method='capitalize')
     _shared_docs['swapcase'] = dict(type='be swapcased', method='swapcase')
+    _shared_docs['casefold'] = dict(type='be casefolded', method='casefold')
     lower = _noarg_wrapper(lambda x: x.lower(),
                            docstring=_shared_docs['casemethods'] %
                            _shared_docs['lower'])
@@ -3000,6 +3002,9 @@ 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
 
@@ -3424,3 +3424,11 @@ def test_method_on_bytes(self):
             expected = Series(np.array(
                 ['ad', 'be', 'cf'], 'S2').astype(object))
             tm.assert_series_equal(result, expected)
+
+    def test_casefold(self):
+        values = Series(['ss', NA, 'case', 'ssd'])
+        s = Series(['ß', NA, 'case', 'ßd'])
+        exp = s.str.casefold()
+
+        assert isinstance(exp, Series)
+        assert_series_equal(exp, values)