Add tests for usecols and index col combinations (#44951)

Author: phofl

pandas/tests/io/parser/usecols/test_usecols_basic.py

@@ -174,6 +174,28 @@ def test_usecols_implicit_index_col(all_parsers):
     tm.assert_frame_equal(result, expected)
 
 
+def test_usecols_index_col_middle(all_parsers):
+    # GH#9098
+    parser = all_parsers
+    data = """a,b,c,d
+1,2,3,4
+"""
+    result = parser.read_csv(StringIO(data), usecols=["b", "c", "d"], index_col="c")
+    expected = DataFrame({"b": [2], "d": [4]}, index=Index([3], name="c"))
+    tm.assert_frame_equal(result, expected)
+
+
+def test_usecols_index_col_end(all_parsers):
+    # GH#9098
+    parser = all_parsers
+    data = """a,b,c,d
+1,2,3,4
+"""
+    result = parser.read_csv(StringIO(data), usecols=["b", "c", "d"], index_col="d")
+    expected = DataFrame({"b": [2], "c": [3]}, index=Index([4], name="d"))
+    tm.assert_frame_equal(result, expected)
+
+
 def test_usecols_regex_sep(all_parsers):
     # see gh-2733
     parser = all_parsers