Fix reading Series with read_hdf

doc/source/whatsnew/v0.20.3.txt

@@ -56,6 +56,7 @@ I/O
 ^^^
 
 -- Bug in ``pd.read_csv()`` in which files weren't opened as binary files by the C engine on Windows, causing EOF characters mid-field, which would fail (:issue:`16039`, :issue:`16559`, :issue`16675`)
+-- Bug in ``pd.read_hdf()`` in which reading a ``Series`` saved to an HDF file in 'fixed' format fails when an explicit ``mode='r'`` argument is supplied (:issue:`16583`)
 
 Plotting
 ^^^^^^^^

pandas/io/pytables.py

@@ -282,21 +282,24 @@ def to_hdf(path_or_buf, key, value, mode=None, complevel=None, complib=None,
         f(path_or_buf)
 
 
-def read_hdf(path_or_buf, key=None, **kwargs):
+def read_hdf(path_or_buf, key=None, mode='r', **kwargs):
     """ read from the store, close it if we opened it
 
         Retrieve pandas object stored in file, optionally based on where
         criteria
 
         Parameters
         ----------
-        path_or_buf : path (string), buffer, or path object (pathlib.Path or
-            py._path.local.LocalPath) to read from
+        path_or_buf : path (string), buffer or path object (pathlib.Path or
+            py._path.local.LocalPath) designating the file to open, or an
+            already opened pd.HDFStore object
 
             .. versionadded:: 0.19.0 support for pathlib, py.path.
 
         key : group identifier in the store. Can be omitted if the HDF file
             contains a single pandas object.
+        mode : string, {'r', 'r+', 'a'}, default 'r'. Mode to use when opening
+            the file. Ignored if path_or_buf is a pd.HDFStore.
         where : list of Term (or convertable) objects, optional
         start : optional, integer (defaults to None), row number to start
             selection
@@ -313,10 +316,9 @@ def read_hdf(path_or_buf, key=None, **kwargs):
 
         """
 
-    if kwargs.get('mode', 'a') not in ['r', 'r+', 'a']:
+    if mode not in ['r', 'r+', 'a']:
         raise ValueError('mode {0} is not allowed while performing a read. '
-                         'Allowed modes are r, r+ and a.'
-                         .format(kwargs.get('mode')))
+                         'Allowed modes are r, r+ and a.'.format(mode))
     # grab the scope
     if 'where' in kwargs:
         kwargs['where'] = _ensure_term(kwargs['where'], scope_level=1)
@@ -343,7 +345,7 @@ def read_hdf(path_or_buf, key=None, **kwargs):
             raise compat.FileNotFoundError(
                 'File %s does not exist' % path_or_buf)
 
-        store = HDFStore(path_or_buf, **kwargs)
+        store = HDFStore(path_or_buf, mode=mode, **kwargs)
         # can't auto open/close if we are using an iterator
         # so delegate to the iterator
         auto_close = True

pandas/tests/io/test_pytables.py

@@ -5247,6 +5247,17 @@ def test_query_compare_column_type(self):
                         expected = df.loc[[], :]
                     tm.assert_frame_equal(expected, result)
 
+    @pytest.mark.parametrize('format', ['fixed', 'table'])
+    def test_read_hdf_series_mode_r(self, format):
+        # GH 16583
+        # Tests that reading a Series saved to an HDF file
+        # still works if a mode='r' argument is supplied
+        series = tm.makeFloatSeries()
+        with ensure_clean_path(self.path) as path:
+            series.to_hdf(path, key='data', format=format)
+            result = pd.read_hdf(path, key='data', mode='r')
+        tm.assert_series_equal(result, series)
+
     @pytest.mark.skipif(sys.version_info < (3, 6), reason="Need python 3.6")
     def test_fspath(self):
         with tm.ensure_clean('foo.h5') as path: