Quote list_values=False values that would not survive a reload

CHANGES.rst

@@ -6,6 +6,9 @@ Unreleased
 
 * raise ``InterpolationError`` instead of leaking a raw ``TypeError`` when a
   value interpolates a reference to an option whose value is a list
+* quote values on writing with ``list_values=False`` when they contain ``#``
+  or start with a quote character, and unquote them again on reading, so
+  written output can be loaded back unchanged (issue #270)
 
 Release 5.0.9
 """""""""""""

src/configobj/__init__.py

@@ -1709,7 +1709,8 @@ def _quote(self, value, multiline=True):
         * Obey list syntax for empty and single member lists.
 
         If ``list_values=False`` then the value is only quoted if it contains
-        a ``\\n`` (is multiline) or '#'.
+        a ``\\n`` (is multiline), starts with a quote character, or contains
+        '#', so that written output can be read back unchanged.
 
         If ``write_empty_values`` is set, and the value is an empty string, it
         won't be quoted.
@@ -1737,15 +1738,23 @@ def _quote(self, value, multiline=True):
         if not value:
             return '""'
 
-        no_lists_no_quotes = not self.list_values and '\n' not in value and '#' not in value
+        no_lists_no_quotes = (not self.list_values and '\n' not in value
+            and '#' not in value and value[0] not in ('"', "'"))
         need_triple = multiline and ((("'" in value) and ('"' in value)) or ('\n' in value ))
         hash_triple_quote = multiline and not need_triple and ("'" in value) and ('"' in value) and ('#' in value)
         check_for_single = (no_lists_no_quotes or not need_triple) and not hash_triple_quote
 
         if check_for_single:
             if not self.list_values:
-                # we don't quote if ``list_values=False``
-                quot = noquot
+                if no_lists_no_quotes:
+                    # we don't quote if ``list_values=False``
+                    # and the bare form cannot be misparsed when read back
+                    quot = noquot
+                else:
+                    # a ``#`` would truncate the value at reading time, and
+                    # a leading quote character makes the line unparseable,
+                    # so quote to keep the output re-parseable
+                    quot = self._get_single_quote(value)
             # for normal values either single or double quotes will do
             elif '\n' in value:
                 # will only happen if multiline is off - e.g. '\n' in key
@@ -1799,8 +1808,16 @@ def _handle_value(self, value):
             mat = self._nolistvalue.match(value)
             if mat is None:
                 raise SyntaxError()
-            # NOTE: we don't unquote here
-            return mat.groups()
+            entry = mat.group(1)
+            comment = mat.group(2)
+            if (len(entry) >= 2 and entry[0] == entry[-1]
+                    and entry[0] in ('"', "'")
+                    and entry[0] not in entry[1:-1]):
+                # a fully quoted value is unquoted here, symmetrically to
+                # how ``_quote`` writes it with ``list_values=False``;
+                # anything else keeps its verbatim text
+                entry = self._unquote(entry)
+            return (entry, comment)
         #
         mat = self._valueexp.match(value)
         if mat is None:

src/tests/test_configobj.py

@@ -394,8 +394,8 @@ def test_behavior_when_list_values_is_false():
     cfg = ConfigObj(cfg_lines(c), list_values=False)
     assert cfg == {
         'key1': 'no quotes',
-        'key2': "'single quotes'",
-        'key3': '"double quotes"',
+        'key2': 'single quotes',
+        'key3': 'double quotes',
         'key4': '"list", \'with\', several, "quotes"'
     }
 
@@ -404,7 +404,7 @@ def test_behavior_when_list_values_is_false():
     cfg2['key2'] = '''"Value" with 'quotes' !'''
     assert cfg2.write() == [
         "key1 = '''Multiline\nValue'''",
-        'key2 = "Value" with \'quotes\' !'
+        'key2 = \'\'\'"Value" with \'quotes\' !\'\'\''
     ]
 
     cfg2.list_values = True
@@ -414,6 +414,27 @@ def test_behavior_when_list_values_is_false():
     ]
 
 
+def test_list_values_false_written_output_can_be_read_back():
+    # A ``#`` would truncate the value at reading time and a leading quote
+    # character would make the line unparseable, so such values are quoted
+    # on writing and unquoted on reading again.
+    values = [
+        'hello # world',
+        '"hello" world',
+        "'quoted' # with hash",
+        '',
+        'no special characters',
+    ]
+    for value in values:
+        cfg = ConfigObj(list_values=False)
+        cfg['key'] = value
+        assert ConfigObj(cfg.write(), list_values=False)['key'] == value
+
+    cfg = ConfigObj(list_values=False)
+    cfg['key'] = 'hello # world'
+    assert cfg.write() == ['key = "hello # world"']
+
+
 def test_flatten_errors(val, cfg_contents):
     config = cfg_contents("""
        test1=40