BUG: Fix Index.__repr__ when display.max_seq_items = 1 (pandas-dev#38443)

Author: skvrahul

doc/source/whatsnew/v1.3.0.rst

@@ -250,6 +250,7 @@ MultiIndex
 I/O
 ^^^
 
+- Bug in :meth:`Index.__repr__` when ``display.max_seq_items=1`` (:issue:`38415`)
 - Bug in :func:`read_csv` interpreting ``NA`` value as comment, when ``NA`` does contain the comment string fixed for ``engine="python"`` (:issue:`34002`)
 - Bug in :func:`read_csv` raising ``IndexError`` with multiple header columns and ``index_col`` specified when file has no data rows (:issue:`38292`)
 - Bug in :func:`read_csv` not accepting ``usecols`` with different length than ``names`` for ``engine="python"`` (:issue:`16469`)

pandas/io/formats/printing.py

@@ -382,7 +382,11 @@ def best_len(values: List[str]) -> int:
         summary = f"[{first}, {last}]{close}"
     else:
 
-        if n > max_seq_items:
+        if max_seq_items == 1:
+            # If max_seq_items=1 show only last element
+            head = []
+            tail = [formatter(x) for x in obj[-1:]]
+        elif n > max_seq_items:
             n = min(max_seq_items // 2, 10)
             head = [formatter(x) for x in obj[:n]]
             tail = [formatter(x) for x in obj[-n:]]

pandas/tests/groupby/test_groupby.py

@@ -2058,6 +2058,7 @@ def test_groupby_list_level():
     [
         (5, "{0: [0], 1: [1], 2: [2], 3: [3], 4: [4]}"),
         (4, "{0: [0], 1: [1], 2: [2], 3: [3], ...}"),
+        (1, "{0: [0], ...}"),
     ],
 )
 def test_groups_repr_truncates(max_seq_items, expected):

pandas/tests/indexes/multi/test_formats.py

@@ -89,6 +89,20 @@ def test_unicode_repr_issues(self):
         # NumPy bug
         # repr(index.get_level_values(1))
 
+    def test_repr_max_seq_items_equal_to_n(self, idx):
+        # display.max_seq_items == n
+        with pd.option_context("display.max_seq_items", 6):
+            result = idx.__repr__()
+            expected = """\
+MultiIndex([('foo', 'one'),
+            ('foo', 'two'),
+            ('bar', 'one'),
+            ('baz', 'two'),
+            ('qux', 'one'),
+            ('qux', 'two')],
+           names=['first', 'second'])"""
+            assert result == expected
+
     def test_repr(self, idx):
         result = idx[:1].__repr__()
         expected = """\
@@ -118,6 +132,15 @@ def test_repr(self, idx):
            names=['first', 'second'], length=6)"""
             assert result == expected
 
+        # display.max_seq_items == 1
+        with pd.option_context("display.max_seq_items", 1):
+            result = idx.__repr__()
+            expected = """\
+MultiIndex([...
+            ('qux', 'two')],
+           names=['first', ...], length=6)"""
+            assert result == expected
+
     def test_rjust(self, narrow_multi_index):
         mi = narrow_multi_index
         result = mi[:1].__repr__()

pandas/tests/io/formats/test_format.py

@@ -297,6 +297,9 @@ def test_repr_obeys_max_seq_limit(self):
         with option_context("display.max_seq_items", 5):
             assert len(printing.pprint_thing(list(range(1000)))) < 100
 
+        with option_context("display.max_seq_items", 1):
+            assert len(printing.pprint_thing(list(range(1000)))) < 9
+
     def test_repr_set(self):
         assert printing.pprint_thing({1}) == "{1}"