fix zero length RangeIndex + add tests

Author: topper-123

pandas/core/indexes/range.py

@@ -188,6 +188,8 @@ def _format_data(self, name=None):
         return None
 
     def _format_with_header(self, header: List[str], na_rep: str = "NaN") -> List[str]:
+        if len(self._range) == 0:
+            return []
         first_val_str = str(self._range[0])
         last_val_str = str(self._range[-1])
         max_length = max(len(first_val_str), len(last_val_str))

pandas/tests/indexes/common.py

@@ -1,5 +1,5 @@
 import gc
-from typing import Optional, Type
+from typing import Type
 
 import numpy as np
 import pytest
@@ -33,7 +33,7 @@
 class Base:
     """ base class for index sub-class tests """
 
-    _holder: Optional[Type[Index]] = None
+    _holder: Type[Index]
     _compat_props = ["shape", "ndim", "size", "nbytes"]
 
     def create_index(self) -> Index:
@@ -681,6 +681,10 @@ def test_format(self):
         expected = [str(x) for x in idx]
         assert idx.format() == expected
 
+    def test_format_empty(self):
+        # GH35712
+        assert self._holder([]).format() == []
+
     def test_hasnans_isnans(self, index):
         # GH 11343, added tests for hasnans / isnans
         if isinstance(index, MultiIndex):

pandas/tests/indexes/period/test_period.py

@@ -536,6 +536,10 @@ def test_contains_raise_error_if_period_index_is_in_multi_index(self, msg, key):
         with pytest.raises(KeyError, match=msg):
             df.loc[key]
 
+    def test_format_empty(self):
+        # GH35712
+        assert self._holder([], freq="A").format() == []
+
 
 def test_maybe_convert_timedelta():
     pi = PeriodIndex(["2000", "2001"], freq="D")

pandas/tests/indexes/ranges/test_range.py

@@ -521,3 +521,7 @@ def test_engineless_lookup(self):
             idx.get_loc("a")
 
         assert "_engine" not in idx._cache
+
+    def test_format_empty(self):
+        # GH35712
+        assert self._holder(0).format() == []