BUG: to_clipboard fails to format output for Excel

doc/source/whatsnew/v0.23.1.txt

@@ -96,6 +96,7 @@ I/O
 - Bug in IO methods specifying ``compression='zip'`` which produced uncompressed zip archives (:issue:`17778`, :issue:`21144`)
 - Bug in :meth:`DataFrame.to_stata` which prevented exporting DataFrames to buffers and most file-like objects (:issue:`21041`)
 - Bug in :meth:`DataFrame.to_csv` and :meth:`Series.to_csv` causes encoding error when compression and encoding are specified (:issue:`21241`, :issue:`21118`)
+- 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

@@ -1,6 +1,7 @@
 """ io on the clipboard """
 from pandas import compat, get_option, option_context, DataFrame
 from pandas.compat import StringIO, PY2
+import warnings
 
 
 def read_clipboard(sep=r'\s+', **kwargs):  # pragma: no cover
@@ -55,11 +56,14 @@ 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+'
 
+    if sep == r'\s+' and kwargs.get('engine') is None:
+        kwargs['engine'] = 'python'
+
     return read_table(StringIO(text), sep=sep, **kwargs)
 
 
@@ -99,7 +103,7 @@ 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)
@@ -108,8 +112,9 @@ def to_clipboard(obj, excel=True, sep=None, **kwargs):  # pragma: no cover
                 text = text.decode('utf-8')
             clipboard_set(text)
             return
-        except:
-            pass
+        except TypeError:
+            warnings.warn('to_clipboard in excel mode requires a single \
+            character separator. Set "excel=False" or change the separator')
 
     if isinstance(obj, DataFrame):
         # str(df) has various unhelpful defaults, like truncation