ENH: Improve error message for empty object array (pandas-dev#23718)

* ENH: Improve error message for empty object array

Improve the error message shown when an object array is empty

closes pandas-dev#23572

* TST: Add tests for all None

Test exception is hit when all values in an object column are None
Extend the test for strl conversion to ensure this case passes (as expected)

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]