BUG: read_excel return empty dataframe when using usecols

- [x] closes #18273
- [x] tests added / passed
- [x] passes git diff master --name-only -- "*.py" | grep "pandas/" | xargs -r flake8
- [x] whatsnew entry

As mentioned read_excel returns an empty DataFrame when usecols argument is a list of strings.
Now lists of strings are correctly interpreted by read_excel function.

Author: jacksonjos

doc/source/whatsnew/v0.23.0.txt

@@ -979,6 +979,7 @@ I/O
 - Bug in :func:`DataFrame.to_latex()` where pairs of braces meant to serve as invisible placeholders were escaped (:issue:`18667`)
 - Bug in :func:`read_json` where large numeric values were causing an ``OverflowError`` (:issue:`18842`)
 - Bug in :func:`DataFrame.to_parquet` where an exception was raised if the write destination is S3 (:issue:`19134`)
+- Bug in :func:`read_excel` where `usecols` named argument as a list of strings were returning a empty DataFrame (:issue:`18273`)
 - :class:`Interval` now supported in :func:`DataFrame.to_excel` for all Excel file types (:issue:`19242`)
 - :class:`Timedelta` now supported in :func:`DataFrame.to_excel` for all Excel file types (:issue:`19242`, :issue:`9155`, :issue:`19900`)
 - Bug in :meth:`pandas.io.stata.StataReader.value_labels` raising an ``AttributeError`` when called on very old files. Now returns an empty dict (:issue:`19417`)

pandas/io/excel.py

@@ -96,7 +96,10 @@
     * If int then indicates last column to be parsed
     * If list of ints then indicates list of column numbers to be parsed
     * If string then indicates comma separated list of Excel column letters and
-      column ranges (e.g. "A:E" or "A,C,E:F").  Ranges are inclusive of
+      column ranges (e.g. "A:E" or "A,C,E:F") to be parsed. Ranges are
+      inclusive of both sides.
+    * If list of strings each string shall be a Excel column letter or column
+      range (e.g. "A:E" or "A,C,E:F") to be parsed. Ranges are inclusive of
       both sides.
 squeeze : boolean, default False
     If the parsed data only contains one column then return a Series
@@ -477,8 +480,15 @@ def _excel2num(x):
 
         if isinstance(usecols, int):
             return i <= usecols
+        # check if usecols is a string that indicates a comma separated list of
+        # Excel column letters and column ranges
         elif isinstance(usecols, compat.string_types):
             return i in _range2cols(usecols)
+        # check if usecols is a list of strings, each one indicating a Excel
+        # column letter or a column range
+        elif all(isinstance(x, compat.string_types) for x in usecols):
+            usecols_str = ",".join(usecols)
+            return i in _range2cols(usecols_str)
         else:
             return i in usecols
 

pandas/tests/io/test_excel.py

@@ -179,6 +179,32 @@ def test_usecols_str(self, ext):
         tm.assert_frame_equal(df2, df1, check_names=False)
         tm.assert_frame_equal(df3, df1, check_names=False)
 
+    @pytest.mark.parametrize("columns,usecols,parse_cols", [
+        (['A', 'B', 'C'], ['A:D'], ['A:D']),
+        (['B', 'C'], ['A', 'C', 'D'], ['A', 'C', 'D']),
+        (['B', 'C'], ['A', 'C:D'], ['A', 'C:D'])
+    ])
+    # GH18273 - read_excel return empty dataframe when using usecols as a list
+    # of strings
+    def test_usecols_str_list(self, ext, columns, usecols, parse_cols):
+
+        dfref = self.get_csv_refdf('test1')
+
+        df1 = dfref.reindex(columns=columns)
+        df2 = self.get_exceldf('test1', ext, 'Sheet1', index_col=0,
+                               usecols=usecols)
+        df3 = self.get_exceldf('test1', ext, 'Sheet2', skiprows=[1],
+                               index_col=0, usecols=usecols)
+
+        with tm.assert_produces_warning(FutureWarning):
+            df4 = self.get_exceldf('test1', ext, 'Sheet2', skiprows=[1],
+                                   index_col=0, parse_cols=parse_cols)
+
+        # TODO add index to xls, read xls ignores index name ?
+        tm.assert_frame_equal(df2, df1, check_names=False)
+        tm.assert_frame_equal(df3, df1, check_names=False)
+        tm.assert_frame_equal(df4, df1, check_names=False)
+
     def test_excel_stop_iterator(self, ext):
 
         parsed = self.get_exceldf('test2', ext, 'Sheet1')