BUG: Fix MI repr with long names (#21655)

Author: TomAugspurger

doc/source/whatsnew/v0.23.2.txt

@@ -55,6 +55,7 @@ Fixed Regressions
 - Fixed regression in :meth:`to_csv` when handling file-like object incorrectly (:issue:`21471`)
 - Re-allowed duplicate level names of a ``MultiIndex``. Accessing a level that has a duplicate name by name still raises an error (:issue:`19029`).
 - Bug in both :meth:`DataFrame.first_valid_index` and :meth:`Series.first_valid_index` raised for a row index having duplicate values (:issue:`21441`)
+- Fixed printing of DataFrames with hierarchical columns with long names (:issue:`21180`)
 - Fixed regression in :meth:`~DataFrame.reindex` and :meth:`~DataFrame.groupby`
   with a MultiIndex or multiple keys that contains categorical datetime-like values (:issue:`21390`).
 - Fixed regression in unary negative operations with object dtype (:issue:`21380`)

pandas/io/formats/format.py

@@ -636,10 +636,14 @@ def to_string(self):
                     mid = int(round(n_cols / 2.))
                     mid_ix = col_lens.index[mid]
                     col_len = col_lens[mid_ix]
-                    adj_dif -= (col_len + 1)  # adjoin adds one
+                    # adjoin adds one
+                    adj_dif -= (col_len + 1)
                     col_lens = col_lens.drop(mid_ix)
                     n_cols = len(col_lens)
-                max_cols_adj = n_cols - self.index  # subtract index column
+                # subtract index column
+                max_cols_adj = n_cols - self.index
+                # GH-21180. Ensure that we print at least two.
+                max_cols_adj = max(max_cols_adj, 2)
                 self.max_cols_adj = max_cols_adj
 
                 # Call again _chk_truncate to cut frame appropriately
@@ -778,7 +782,7 @@ def space_format(x, y):
 
             str_columns = list(zip(*[[space_format(x, y) for y in x]
                                      for x in fmt_columns]))
-            if self.sparsify:
+            if self.sparsify and len(str_columns):
                 str_columns = _sparsify(str_columns)
 
             str_columns = [list(x) for x in zip(*str_columns)]

pandas/tests/io/formats/test_format.py

@@ -305,6 +305,44 @@ def test_repr_non_interactive(self):
             assert not has_truncated_repr(df)
             assert not has_expanded_repr(df)
 
+    def test_repr_truncates_terminal_size(self):
+        # https://github.com/pandas-dev/pandas/issues/21180
+        # TODO: use mock fixutre.
+        # This is being backported, so doing it directly here.
+        try:
+            from unittest import mock
+        except ImportError:
+            mock = pytest.importorskip("mock")
+
+        terminal_size = (118, 96)
+        p1 = mock.patch('pandas.io.formats.console.get_terminal_size',
+                        return_value=terminal_size)
+        p2 = mock.patch('pandas.io.formats.format.get_terminal_size',
+                        return_value=terminal_size)
+
+        index = range(5)
+        columns = pd.MultiIndex.from_tuples([
+            ('This is a long title with > 37 chars.', 'cat'),
+            ('This is a loooooonger title with > 43 chars.', 'dog'),
+        ])
+        df = pd.DataFrame(1, index=index, columns=columns)
+
+        with p1, p2:
+            result = repr(df)
+
+        h1, h2 = result.split('\n')[:2]
+        assert 'long' in h1
+        assert 'loooooonger' in h1
+        assert 'cat' in h2
+        assert 'dog' in h2
+
+        # regular columns
+        df2 = pd.DataFrame({"A" * 41: [1, 2], 'B' * 41: [1, 2]})
+        with p1, p2:
+            result = repr(df2)
+
+        assert df2.columns[0] in result.split('\n')[0]
+
     def test_repr_max_columns_max_rows(self):
         term_width, term_height = get_terminal_size()
         if term_width < 10 or term_height < 10: