BUG: LatexFormatter.write_result multi-index

Fixed GH issue 14484:
`LatexFormatter.write_result`` now does not print blanks if a
higher-order index differs from the previous row.
Also added testcase for this.

Author: Maximilian Köstler

doc/source/whatsnew/v0.21.0.txt

@@ -415,6 +415,7 @@ I/O
 - Bug in :func:`read_stata` where value labels could not be read when using an iterator (:issue:`16923`)
 - Bug in :func:`read_html` where import check fails when run in multiple threads (:issue:`16928`)
 - Bug in :func:`read_csv` where automatic delimiter detection caused a ``TypeError`` to be thrown when a bad line was encountered rather than the correct error message (:issue:`13374`)
+- 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`)
 
 Plotting
 ^^^^^^^^

pandas/io/formats/format.py

@@ -45,7 +45,6 @@
 import pandas as pd
 import numpy as np
 
-import itertools
 import csv
 from functools import partial
 
@@ -897,10 +896,11 @@ def get_col_type(dtype):
                     lev3 = [blank] * clevels
                 if name:
                     lev3.append(lev.name)
-                for level_idx, group in itertools.groupby(
-                        self.frame.index.labels[i]):
-                    count = len(list(group))
-                    lev3.extend([lev2[level_idx]] + [blank] * (count - 1))
+                for level_idx, group in self.frame.groupby(
+                        level=list(range(0, i + 1))):
+                    level_idx_val = str(level_idx[-1] if i > 0 else level_idx)
+                    count = group.shape[0]
+                    lev3.extend([level_idx_val] + [blank] * (count - 1))
                 strcols.insert(i, lev3)
 
         column_format = self.column_format

pandas/tests/io/formats/test_to_latex.py

@@ -217,6 +217,28 @@ def test_to_latex_multiindex(self):
 1 &   2.0 &  3.5 &  0.707107 &  3.0 &  3.25 &  3.5 &  3.75 &  4.0 \\
 \bottomrule
 \end{tabular}
+"""
+
+        assert result == expected
+
+        # GH 14484
+        # If an index is repeated in subsequent rows, it should be
+        # replaced with a blank in the created table.
+        # This should ONLY happen if all higher order indicies
+        # (to the left) are equal, too.
+        # In this test, 'c' has to be printed both times, because
+        # the higher order index 'A' != 'B'
+        df = pd.DataFrame(index=pd.MultiIndex.from_tuples(
+            [('A', 'c'), ('B', 'c')]), columns=['col'])
+        result = df.to_latex()
+        expected = r"""\begin{tabular}{lll}
+\toprule
+  &   &  col \\
+\midrule
+A & c &  NaN \\
+B & c &  NaN \\
+\bottomrule
+\end{tabular}
 """
 
         assert result == expected