BUG: fix HDFStore.append with all empty strings error (GH12242) (#23435)

Author: josham

doc/source/whatsnew/v0.24.0.txt

@@ -1272,6 +1272,7 @@ Notice how we now instead output ``np.nan`` itself instead of a stringified form
 - Bug in :meth:`detect_client_encoding` where potential ``IOError`` goes unhandled when importing in a mod_wsgi process due to restricted access to stdout. (:issue:`21552`)
 - Bug in :func:`to_string()` that broke column alignment when ``index=False`` and width of first column's values is greater than the width of first column's header (:issue:`16839`, :issue:`13032`)
 - Bug in :func:`DataFrame.to_csv` where a single level MultiIndex incorrectly wrote a tuple. Now just the value of the index is written (:issue:`19589`).
+- Bug in :meth:`HDFStore.append` when appending a :class:`DataFrame` with an empty string column and ``min_itemsize`` < 8 (:issue:`12242`)
 
 Plotting
 ^^^^^^^^

pandas/io/pytables.py

@@ -4637,7 +4637,7 @@ def _convert_string_array(data, encoding, errors, itemsize=None):
     # create the sized dtype
     if itemsize is None:
         ensured = ensure_object(data.ravel())
-        itemsize = libwriters.max_len_string_array(ensured)
+        itemsize = max(1, libwriters.max_len_string_array(ensured))
 
     data = np.asarray(data, dtype="S%d" % itemsize)
     return data

pandas/tests/io/test_pytables.py

@@ -1482,6 +1482,16 @@ def check_col(key, name, size):
             pytest.raises(ValueError, store.append, 'df',
                           df, min_itemsize={'foo': 20, 'foobar': 20})
 
+    def test_append_with_empty_string(self):
+
+        with ensure_clean_store(self.path) as store:
+
+            # with all empty strings (GH 12242)
+            df = DataFrame({'x': ['a', 'b', 'c', 'd', 'e', 'f', '']})
+            store.append('df', df[:-1], min_itemsize={'x': 1})
+            store.append('df', df[-1:], min_itemsize={'x': 1})
+            tm.assert_frame_equal(store.select('df'), df)
+
     def test_to_hdf_with_min_itemsize(self):
 
         with ensure_clean_path(self.path) as path: