config: support comma in config ListAttribute item

This is a first step towards completing sopel-irc#1455, since, as @dgw mentioned,
to do so would require accepting commas in a single entry of
`ListAttribute`.

The following tests are now added/updated for "`ListAttribute` items...":

- ... containing comma(s)
- ... with a non-escape backslash
- ... with escape sequences
- ... ending in special characters (escape character and delimiter)
- ... containing adjacent special characters

sopel/config/types.py

@@ -212,13 +212,34 @@ class ListAttribute(BaseValidated):
     currently support commas within items in the list. By default, the spaces
     before and after each item are stripped; you can override this by passing
     ``strip=False``."""
+
+    ESCAPE_CHARACTER = '\\'
+    DELIMITER = ','
+
     def __init__(self, name, strip=True, default=None):
         default = default or []
         super(ListAttribute, self).__init__(name, default=default)
         self.strip = strip
 
     def parse(self, value):
-        value = list(filter(None, value.split(',')))
+        items = []
+        is_escape_on = False
+        current_token = []
+        for char in value:
+            if not is_escape_on:
+                if char == ListAttribute.ESCAPE_CHARACTER:
+                    is_escape_on = True
+                elif char == ListAttribute.DELIMITER:
+                    items.append(''.join(current_token))
+                    current_token = []
+                else:
+                    current_token.append(char)
+            else:
+                current_token.append(char)
+                is_escape_on = False
+        items.append(''.join(current_token))
+
+        value = list(filter(None, items))
         if self.strip:
             return [v.strip() for v in value]
         else:
@@ -227,7 +248,11 @@ def parse(self, value):
     def serialize(self, value):
         if not isinstance(value, (list, set)):
             raise ValueError('ListAttribute value must be a list.')
-        return ','.join(value)
+
+        items = value
+        for special_char in [ListAttribute.ESCAPE_CHARACTER, ListAttribute.DELIMITER]:
+            items = map(lambda item: item.replace(special_char, '{}{}'.format(ListAttribute.ESCAPE_CHARACTER, special_char)), items)
+        return ','.join(items)
 
     def configure(self, prompt, default, parent, section_name):
         each_prompt = '?'

test/test_config.py

@@ -65,7 +65,50 @@ def test_listattribute_with_multiple_values(self):
 
     def test_listattribute_with_value_containing_comma(self):
         self.config.fake.listattr = ['spam, egg, sausage', 'bacon']
-        self.assertEqual(self.config.fake.listattr, ['spam', 'egg', 'sausage', 'bacon'])
+        self.assertEqual(self.config.fake.listattr, ['spam, egg, sausage', 'bacon'])
+
+    def test_listattribute_with_value_containing_nonescape_backslash(self):
+        self.config.fake.listattr = ['spam', r'egg\sausage', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', r'egg\sausage', 'bacon'])
+
+        self.config.fake.listattr = ['spam', r'egg\tacos', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', r'egg\tacos', 'bacon'])
+
+    def test_listattribute_with_value_containing_standard_escape_sequence(self):
+        self.config.fake.listattr = ['spam', 'egg\tsausage', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', 'egg\tsausage', 'bacon'])
+
+        self.config.fake.listattr = ['spam', 'egg\nsausage', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', 'egg\nsausage', 'bacon'])
+
+        self.config.fake.listattr = ['spam', 'egg\\sausage', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', 'egg\\sausage', 'bacon'])
+
+    def test_listattribute_with_value_ending_in_special_chars(self):
+        self.config.fake.listattr = ['spam', 'egg', 'sausage\\', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', 'egg', 'sausage\\', 'bacon'])
+
+        self.config.fake.listattr = ['spam', 'egg', 'sausage,', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', 'egg', 'sausage,', 'bacon'])
+
+        self.config.fake.listattr = ['spam', 'egg', 'sausage,,', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', 'egg', 'sausage,,', 'bacon'])
+
+    def test_listattribute_with_value_containing_adjacent_special_chars(self):
+        self.config.fake.listattr = ['spam', r'egg\,sausage', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', r'egg\,sausage', 'bacon'])
+
+        self.config.fake.listattr = ['spam', r'egg\,\sausage', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', r'egg\,\sausage', 'bacon'])
+
+        self.config.fake.listattr = ['spam', r'egg,\,sausage', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', r'egg,\,sausage', 'bacon'])
+
+        self.config.fake.listattr = ['spam', 'egg,,sausage', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', 'egg,,sausage', 'bacon'])
+
+        self.config.fake.listattr = ['spam', r'egg\\sausage', 'bacon']
+        self.assertEqual(self.config.fake.listattr, ['spam', r'egg\\sausage', 'bacon'])
 
     def test_choiceattribute_when_none(self):
         self.config.fake.choiceattr = None