BUG: to_clipboard fails to format output for Excel

doc/source/whatsnew/v0.23.1.txt

@@ -70,7 +70,7 @@ Indexing
 I/O
 ^^^
 
--
+-Bug in :func:`DataFrame.to_clipboard` where data sent to clipboard was not properly tab-delimited even when ``excel=True`` (:issue:`21104`)
 -
 
 Plotting

pandas/io/clipboards.py

@@ -55,7 +55,7 @@ def read_clipboard(sep=r'\s+', **kwargs):  # pragma: no cover
 
     counts = {x.lstrip().count('\t') for x in lines}
     if len(lines) > 1 and len(counts) == 1 and counts.pop() != 0:
-        sep = r'\t'
+        sep = '\t'
 
     if sep is None and kwargs.get('delim_whitespace') is None:
         sep = r'\s+'
@@ -99,7 +99,7 @@ def to_clipboard(obj, excel=True, sep=None, **kwargs):  # pragma: no cover
     if excel:
         try:
             if sep is None:
-                sep = r'\t'
+                sep = '\t'
             buf = StringIO()
             # clipboard_set (pyperclip) expects unicode
             obj.to_csv(buf, sep=sep, encoding='utf-8', **kwargs)

pandas/tests/io/test_clipboard.py

@@ -12,7 +12,7 @@
 from pandas.util import testing as tm
 from pandas.util.testing import makeCustomDataframe as mkdf
 from pandas.io.clipboard.exceptions import PyperclipException
-from pandas.io.clipboard import clipboard_set
+from pandas.io.clipboard import clipboard_set, clipboard_get
 
 
 try:
@@ -81,6 +81,7 @@ def test_round_trip_frame_sep(self):
             self.check_round_trip_frame(dt, sep=',')
             self.check_round_trip_frame(dt, sep=r'\s+')
             self.check_round_trip_frame(dt, sep='|')
+            self.check_round_trip_frame(dt, sep='\t')
 
     def test_round_trip_frame_string(self):
         for dt in self.data_types:
@@ -123,6 +124,15 @@ def test_read_clipboard_infer_excel(self):
         exp = pd.read_clipboard(**clip_kwargs)
 
         tm.assert_frame_equal(res, exp)
+
+    def test_excel_clipboard_format(self):
+        for dt in self.data_types:
+            for sep in ['\t', None]:
+                data = self.data[dt]
+                data.to_clipboard(excel=True, sep=sep)
+                result = read_clipboard(sep='\t', index_col=0)
+                tm.assert_frame_equal(data, result, check_dtype=False)
+                assert clipboard_get().count('\t') > 0
 
     def test_invalid_encoding(self):
         # test case for testing invalid encoding