BUG: Fix to_latex with longtable (#17959)

pandas.DataFrame.to_latex(longtable=True) always contained
\multicolumn{3}{r}{{Continued on next page}}
regardless of the number of columns in the output.

Author: syxolk

pandas/io/formats/format.py

@@ -942,8 +942,8 @@ def get_col_type(dtype):
                 if self.longtable:
                     buf.write('\\endhead\n')
                     buf.write('\\midrule\n')
-                    buf.write('\\multicolumn{3}{r}{{Continued on next '
-                              'page}} \\\\\n')
+                    buf.write('\\multicolumn{{{n}}}{{r}}{{{{Continued on next '
+                              'page}}}} \\\\\n'.format(n=len(row)))
                     buf.write('\\midrule\n')
                     buf.write('\\endfoot\n\n')
                     buf.write('\\bottomrule\n')

pandas/tests/io/formats/test_to_latex.py

@@ -365,7 +365,7 @@ def test_to_latex_longtable(self, frame):
 \midrule
 \endhead
 \midrule
-\multicolumn{3}{r}{{Continued on next page}} \\
+\multicolumn{2}{r}{{Continued on next page}} \\
 \midrule
 \endfoot