ENH: Improve error message for empty object array

pandas/io/stata.py

@@ -1868,7 +1868,14 @@ def _dtype_to_default_stata_fmt(dtype, column, dta_version=114,
         inferred_dtype = infer_dtype(column.dropna())
         if not (inferred_dtype in ('string', 'unicode') or
                 len(column) == 0):
-            raise ValueError('Writing general object arrays is not supported')
+            raise ValueError('Column `{col}` cannot be exported.\n\nOnly '
+                             'string-like object arrays containing all '
+                             'strings or a mix of strings and None can be '
+                             'exported. Object arrays containing only null '
+                             'values are prohibited. Other object types'
+                             'cannot be exported and must first be converted '
+                             'to one of the supported '
+                             'types.'.format(col=column.name))
         itemsize = max_len_string_array(ensure_object(column.values))
         if itemsize > max_str_len:
             if dta_version >= 117:

pandas/tests/io/test_stata.py

@@ -1514,11 +1514,35 @@ def test_mixed_string_strl(self):
             {'mixed': None,
              'number': 1}
         ]
-
         output = pd.DataFrame(output)
+        output.number = output.number.astype('int32')
+
         with tm.ensure_clean() as path:
             output.to_stata(path, write_index=False, version=117)
             reread = read_stata(path)
             expected = output.fillna('')
-            expected.number = expected.number.astype('int32')
             tm.assert_frame_equal(reread, expected)
+
+            # Check strl supports all None (null)
+            output.loc[:, 'mixed'] = None
+            output.to_stata(path, write_index=False, convert_strl=['mixed'],
+                            version=117)
+            reread = read_stata(path)
+            expected = output.fillna('')
+            tm.assert_frame_equal(reread, expected)
+
+    @pytest.mark.parametrize('version', [114, 117])
+    def test_all_none_exception(self, version):
+        output = [
+            {'none': 'none',
+             'number': 0},
+            {'none': None,
+             'number': 1}
+        ]
+        output = pd.DataFrame(output)
+        output.loc[:, 'none'] = None
+        with tm.ensure_clean() as path:
+            with pytest.raises(ValueError) as excinfo:
+                output.to_stata(path, version=version)
+        assert 'Only string-like' in excinfo.value.args[0]
+        assert 'Column `none`' in excinfo.value.args[0]