BUG: Fix to_latex with longtable (#17959)

doc/source/whatsnew/v0.21.1.txt

@@ -127,6 +127,7 @@ I/O
 - Bug in :func:`pandas.io.json.json_normalize` to avoid modification of ``meta`` (:issue:`18610`)
 - Bug in :func:`to_latex` where repeated multi-index values were not printed even though a higher level index differed from the previous row (:issue:`14484`)
 - Bug when reading NaN-only categorical columns in :class:`HDFStore` (:issue:`18413`)
+- Bug in :meth:`DataFrame.to_latex` with ``longtable=True`` where a latex multicolumn always spanned over three columns (:issue:`17959`)
 
 Plotting
 ^^^^^^^^

pandas/io/formats/format.py

@@ -962,8 +962,8 @@ 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

@@ -91,6 +91,29 @@ def test_to_latex_format(self, frame):
 
         assert withindex_result == withindex_expected
 
+    def test_to_latex_empty(self):
+        df = DataFrame()
+        result = df.to_latex()
+        expected = r"""\begin{tabular}{l}
+\toprule
+Empty DataFrame
+Columns: Index([], dtype='object')
+Index: Index([], dtype='object') \\
+\bottomrule
+\end{tabular}
+"""
+        assert result == expected
+
+        result = df.to_latex(longtable=True)
+        expected = r"""\begin{longtable}{l}
+\toprule
+Empty DataFrame
+Columns: Index([], dtype='object')
+Index: Index([], dtype='object') \\
+\end{longtable}
+"""
+        assert result == expected
+
     def test_to_latex_with_formatters(self):
         df = DataFrame({'int': [1, 2, 3],
                         'float': [1.0, 2.0, 3.0],
@@ -377,7 +400,7 @@ def test_to_latex_longtable(self, frame):
 1 &  2 &  b2 \\
 \end{longtable}
 """
-
+        open("expected.txt", "w").write(withindex_result)
         assert withindex_result == withindex_expected
 
         withoutindex_result = df.to_latex(index=False, longtable=True)
@@ -387,7 +410,7 @@ 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
 
@@ -400,6 +423,14 @@ def test_to_latex_longtable(self, frame):
 
         assert withoutindex_result == withoutindex_expected
 
+        df = DataFrame({'a': [1, 2]})
+        with1column_result = df.to_latex(index=False, longtable=True)
+        assert "\multicolumn{1}" in with1column_result
+
+        df = DataFrame({'a': [1, 2], 'b': [3, 4], 'c': [5, 6]})
+        with3columns_result = df.to_latex(index=False, longtable=True)
+        assert "\multicolumn{3}" in with3columns_result
+
     def test_to_latex_escape_special_chars(self):
         special_characters = ['&', '%', '$', '#', '_', '{', '}', '~', '^',
                               '\\']