TST: Test unnamed columns with index_col for Excel

Closes pandas-devgh-18792.

Author: gfyoung

doc/source/whatsnew/v0.24.0.rst

@@ -1402,7 +1402,7 @@ Notice how we now instead output ``np.nan`` itself instead of a stringified form
 - Bug in :meth:`read_csv()` in which unnecessary warnings were being raised when the dialect's values conflicted with the default arguments (:issue:`23761`)
 - Bug in :meth:`read_html()` in which the error message was not displaying the valid flavors when an invalid one was provided (:issue:`23549`)
 - Bug in :meth:`read_excel()` in which extraneous header names were extracted, even though none were specified (:issue:`11733`)
-- Bug in :meth:`read_excel()` in which ``index_col=None`` was not being respected and parsing index columns anyway (:issue:`20480`)
+- Bug in :meth:`read_excel()` in which ``index_col=None`` was not being respected and parsing index columns anyway (:issue:`18792`, :issue:`20480`)
 - Bug in :meth:`read_excel()` in which ``usecols`` was not being validated for proper column names when passed in as a string (:issue:`20480`)
 - :func:`DataFrame.to_string()`, :func:`DataFrame.to_html()`, :func:`DataFrame.to_latex()` will correctly format output when a string is passed as the ``float_format`` argument (:issue:`21625`, :issue:`22270`)
 

pandas/tests/io/data/test1.xls

@@ -11,7 +11,7 @@
 from numpy import nan
 import pytest
 
-from pandas.compat import PY36, BytesIO, iteritems, map, range, u
+from pandas.compat import PY2, PY36, BytesIO, iteritems, map, range, u
 import pandas.util._test_decorators as td
 
 import pandas as pd
@@ -264,6 +264,23 @@ def test_index_col_empty(self, ext):
                                               names=["A", "B", "C"]))
         tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize("index_col", [None, 2])
+    def test_index_col_with_unnamed(self, ext, index_col):
+        # see gh-18792
+        result = self.get_exceldf("test1", ext, "Sheet4",
+                                  index_col=index_col)
+        expected = DataFrame([["i1", "a", "x"], ["i2", "b", "y"]],
+                             columns=["Unnamed: 0", "col1", "col2"])
+        if index_col:
+            expected = expected.set_index(expected.columns[index_col])
+
+        # Python 2.x somehow interprets column
+        # type as "mixed" instead of a string-type.
+
+        check_column_type = not PY2
+        tm.assert_frame_equal(result, expected,
+                              check_column_type=check_column_type)
+
     def test_usecols_pass_non_existent_column(self, ext):
         msg = ("Usecols do not match columns, "
                "columns expected but not found: " + r"\['E'\]")