Clean up ListValue, rename to List.

Tried very hard to clean up all uses in the compass library; hope I got
them all!

9 files changed, 243 insertions, 255 deletions

diff --git a/scss/__init__.py b/scss/__init__.py
index b9764b1..85ca173 100644
--- a/scss/__init__.py
+++ b/scss/__init__.py
@@ -1001,19 +1001,16 @@ class Scss(object):
         if not name:
             return
 
-        name = ListValue(name)
+        iterable = ListValue(name)
         var = var.strip()
         var = calculator.do_glob_math(var)
         var = normalize_var(var)
 
-        for n, v in name.items():
-            v = to_str(v)
+        for v in iterable:
             inner_rule = rule.copy()
             inner_rule.namespace = inner_rule.namespace.derive()
             inner_rule.unparsed_contents = block.unparsed_contents
             inner_rule.namespace.set_variable(var, v)
-            if not isinstance(n, int):
-                inner_rule.namespace.set_variable(n, v)
             self.manage_children(inner_rule, p_children, scope)
 
     # @print_timing(10)
diff --git a/scss/functions/compass/gradients.py b/scss/functions/compass/gradients.py
index 6698283..73dd46b 100644
--- a/scss/functions/compass/gradients.py
+++ b/scss/functions/compass/gradients.py
@@ -11,7 +11,7 @@ import six
 
 from scss.functions.library import FunctionLibrary
 from scss.functions.compass.helpers import opposite_position, position
-from scss.types import ColorValue, ListValue, NumberValue, StringValue
+from scss.types import ColorValue, List, NumberValue, StringValue
 from scss.util import escape, split_params, to_float, to_str
 
 log = logging.getLogger(__name__)
@@ -23,8 +23,8 @@ register = COMPASS_GRADIENTS_LIBRARY.register
 
 def __color_stops(percentages, *args):
     if len(args) == 1:
-        if isinstance(args[0], (list, tuple, ListValue)):
-            return ListValue(args[0]).values()
+        if isinstance(args[0], (list, tuple, List)):
+            list(args[0])
         elif isinstance(args[0], (StringValue, six.string_types)):
             color_stops = []
             colors = split_params(args[0].value)
@@ -43,12 +43,7 @@ def __color_stops(percentages, *args):
     stops = []
     prev_color = False
     for c in args:
-        if isinstance(c, ListValue):
-            cs = c.values()
-        else:
-            cs = [c]
-
-        for c in cs:
+        for c in List.from_maybe(c):
             if isinstance(c, ColorValue):
                 if prev_color:
                     stops.append(None)
@@ -98,8 +93,7 @@ def __color_stops(percentages, *args):
 
 @register('grad-color-stops')
 def grad_color_stops(*args):
-    if len(args) == 1 and isinstance(args[0], (list, tuple, ListValue)):
-        args = ListValue(args[0]).values()
+    args = List.from_maybe_starargs(args)
     color_stops = __color_stops(True, *args)
     ret = ', '.join(['color-stop(%s, %s)' % (to_str(s), c) for s, c in color_stops])
     return StringValue(ret)
@@ -123,7 +117,7 @@ def grad_point(*p):
         vrt = NumberValue(0, '%')
     elif 'bottom' in pos:
         vrt = NumberValue(1, '%')
-    return ListValue([v for v in (hrz, vrt) if v is not None])
+    return List([v for v in (hrz, vrt) if v is not None])
 
 
 def __grad_position(index, default, radial, color_stops):
@@ -144,8 +138,7 @@ def grad_end_position(*color_stops):
 
 @register('color-stops')
 def color_stops(*args):
-    if len(args) == 1 and isinstance(args[0], (list, tuple, ListValue)):
-        args = ListValue(args[0]).values()
+    args = List.from_maybe_starargs(args)
     color_stops = __color_stops(False, *args)
     ret = ', '.join(['%s %s' % (c, to_str(s)) for s, c in color_stops])
     return StringValue(ret)
@@ -153,8 +146,7 @@ def color_stops(*args):
 
 @register('color-stops-in-percentages')
 def color_stops_in_percentages(*args):
-    if len(args) == 1 and isinstance(args[0], (list, tuple, ListValue)):
-        args = ListValue(args[0]).values()
+    args = List.from_maybe_starargs(args)
     color_stops = __color_stops(True, *args)
     ret = ', '.join(['%s %s' % (c, to_str(s)) for s, c in color_stops])
     return StringValue(ret)
@@ -164,7 +156,7 @@ def _get_gradient_position_and_angle(args):
     for arg in args:
         if isinstance(arg, (StringValue, NumberValue, six.string_types)):
             _arg = [arg]
-        elif isinstance(arg, (list, tuple, ListValue)):
+        elif isinstance(arg, (list, tuple, List)):
             _arg = arg
         else:
             continue
@@ -194,7 +186,7 @@ def _get_gradient_shape_and_size(args):
     for arg in args:
         if isinstance(arg, (StringValue, NumberValue, six.string_types)):
             _arg = [arg]
-        elif isinstance(arg, (list, tuple, ListValue)):
+        elif isinstance(arg, (list, tuple, List)):
             _arg = arg
         else:
             continue
@@ -212,20 +204,16 @@ def _get_gradient_shape_and_size(args):
 def _get_gradient_color_stops(args):
     color_stops = []
     for arg in args:
-        if isinstance(arg, ColorValue):
-            color_stops.append(arg)
-        elif isinstance(arg, (list, tuple, ListValue)):
-            for a in arg:
-                if isinstance(a, ColorValue):
-                    color_stops.append(arg)
-                    break
+        for a in List.from_maybe(arg):
+            if isinstance(a, ColorValue):
+                color_stops.append(arg)
+                break
     return color_stops or None
 
 
 @register('radial-gradient')
 def radial_gradient(*args):
-    if len(args) == 1 and isinstance(args[0], (list, tuple, ListValue)):
-        args = ListValue(args[0]).values()
+    args = List.from_maybe_starargs(args)
 
     position_and_angle = _get_gradient_position_and_angle(args)
     shape_and_size = _get_gradient_shape_and_size(args)
@@ -282,8 +270,7 @@ def radial_gradient(*args):
 
 @register('linear-gradient')
 def linear_gradient(*args):
-    if len(args) == 1 and isinstance(args[0], (list, tuple, ListValue)):
-        args = ListValue(args[0]).values()
+    args = List.from_maybe_starargs(args)
 
     position_and_angle = _get_gradient_position_and_angle(args)
     color_stops = _get_gradient_color_stops(args)
@@ -343,8 +330,7 @@ def linear_gradient(*args):
 
 @register('radial-svg-gradient')
 def radial_svg_gradient(*args):
-    if len(args) == 1 and isinstance(args[0], (list, tuple, ListValue)):
-        args = ListValue(args[0]).values()
+    args = List.from_maybe_starargs(args)
     color_stops = args
     center = None
     if isinstance(args[-1], (StringValue, NumberValue, six.string_types)):
@@ -361,8 +347,7 @@ def radial_svg_gradient(*args):
 
 @register('linear-svg-gradient')
 def linear_svg_gradient(*args):
-    if len(args) == 1 and isinstance(args[0], (list, tuple, ListValue)):
-        args = ListValue(args[0]).values()
+    args = List.from_maybe_starargs(args)
     color_stops = args
     start = None
     if isinstance(args[-1], (StringValue, NumberValue, six.string_types)):
diff --git a/scss/functions/compass/helpers.py b/scss/functions/compass/helpers.py
index 62bdfbb..a12efb5 100644
--- a/scss/functions/compass/helpers.py
+++ b/scss/functions/compass/helpers.py
@@ -17,7 +17,7 @@ import warnings
 
 from scss import config
 from scss.functions.library import FunctionLibrary
-from scss.types import BooleanValue, ListValue, Null, NumberValue, QuotedStringValue, StringValue
+from scss.types import BooleanValue, List, Null, NumberValue, QuotedStringValue, StringValue
 from scss.util import escape, to_str
 
 log = logging.getLogger(__name__)
@@ -51,7 +51,7 @@ def blank(*objs):
             is_blank = not o
         elif isinstance(o, QuotedStringValue):
             is_blank = not len(o.value.strip())
-        elif isinstance(o, ListValue):
+        elif isinstance(o, List):
             is_blank = all(blank(el) for el in o)
         else:
             is_blank = False
@@ -65,37 +65,35 @@ def blank(*objs):
 @register('compact')
 def compact(*args):
     """Returns a new list after removing any non-true values"""
-    sep = ','
-    if len(args) == 1 and isinstance(args[0], ListValue):
-        sep = args[0].value.get('_', '')
+    use_comma = True
+    if len(args) == 1 and isinstance(args[0], List):
+        use_comma = args[0].use_comma
         args = args[0]
 
-    return ListValue(
+    return List(
         [arg for arg in args if arg],
-        separator=sep,
+        use_comma=use_comma,
     )
 
 
 @register('reject')
 def reject(lst, *values):
     """Removes the given values from the list"""
-    ret = {}
-    if not isinstance(lst, ListValue):
-        lst = ListValue(lst)
-    lst = lst.value
+    if not isinstance(lst, List):
+        lst = List(lst)
+
     if len(values) == 1:
         values = values[0]
-        if isinstance(values, ListValue):
-            values = values.value.values()
-        elif not isinstance(values, (list, tuple)):
-            values = ListValue([values])
-    for i, item in lst.items():
+        if isinstance(values, (list, tuple, List)):
+            values = frozenset(values)
+        else:
+            values = frozenset([values])
+
+    ret = []
+    for item in lst:
         if item not in values:
-            ret[i] = item
-    separator = lst.get('_', None)
-    if separator is not None:
-        ret['_'] = separator
-    return ListValue(ret)
+            ret.append(item)
+    return List(ret, use_comma=lst.use_comma)
 
 
 @register('first-value-of')
@@ -103,7 +101,7 @@ def first_value_of(lst):
     if isinstance(lst, QuotedStringValue):
         first = lst.value.split()[0]
         return type(lst)(first)
-    elif isinstance(lst, ListValue):
+    elif isinstance(lst, List):
         if len(lst):
             return lst[0]
         else:
@@ -114,15 +112,12 @@ def first_value_of(lst):
 
 @register('-compass-list')
 def dash_compass_list(*args):
-    separator = None
-    if len(args) == 1 and isinstance(args[0], (list, tuple, ListValue)):
-        args = ListValue(args[0]).values()
-    else:
-        separator = ','
-    ret = ListValue(args)
-    if separator:
-        ret.value['_'] = separator
-    return ret
+    use_comma = False
+    if len(args) == 1 and isinstance(args[0], (list, tuple, List)):
+        args = args[0]
+    if isinstance(args, List):
+        use_comma = args.use_comma
+    return List(args, use_comma=use_comma)
 
 
 @register('-compass-space-list')
@@ -141,14 +136,12 @@ def dash_compass_slice(lst, start_index, end_index=None):
     start_index = NumberValue(start_index).value
     end_index = NumberValue(end_index).value if end_index is not None else None
     ret = {}
-    lst = ListValue(lst).value
-    for i, item in lst.items():
-        if not isinstance(i, int):
-            if i == '_':
-                ret[i] = item
-        elif i > start_index and end_index is None or i <= end_index:
-            ret[i] = item
-    return ListValue(ret)
+    lst = List(lst)
+    if end_index:
+        # This function has an inclusive end, but Python slicing is exclusive
+        end_index += 1
+    ret = lst.value[start_index:end_index]
+    return List(ret, use_comma=lst.use_comma)
 
 
 # ------------------------------------------------------------------------------
@@ -158,8 +151,8 @@ def dash_compass_slice(lst, start_index, end_index=None):
 def prefixed(prefix, *args):
     to_fnct_str = 'to_' + to_str(prefix).replace('-', '_')
     for arg in args:
-        if isinstance(arg, ListValue):
-            for k, iarg in arg.value.items():
+        if isinstance(arg, List):
+            for iarg in arg:
                 if hasattr(iarg, to_fnct_str):
                     return BooleanValue(True)
         else:
@@ -173,22 +166,21 @@ def prefix(prefix, *args):
     to_fnct_str = 'to_' + to_str(prefix).replace('-', '_')
     args = list(args)
     for i, arg in enumerate(args):
-        if isinstance(arg, ListValue):
-            _value = {}
-            for k, iarg in arg.value.items():
+        if isinstance(arg, List):
+            _value = []
+            for iarg in arg:
                 to_fnct = getattr(iarg, to_fnct_str, None)
                 if to_fnct:
-                    _value[k] = to_fnct()
+                    _value.append(to_fnct())
                 else:
-                    _value[k] = iarg
-            args[i] = ListValue(_value)
+                    _value.append(iarg)
+            args[i] = List(_value)
         else:
             to_fnct = getattr(arg, to_fnct_str, None)
             if to_fnct:
                 args[i] = to_fnct()
-    if len(args) == 1:
-        return args[0]
-    return ListValue(args, ',')
+
+    return List.maybe_new(args, use_comma=True)
 
 
 @register('-moz')
@@ -233,8 +225,8 @@ def dash_o(*args):
 
 @register('append-selector', 2)
 def append_selector(selector, to_append):
-    if isinstance(selector, ListValue):
-        lst = selector.values()
+    if isinstance(selector, List):
+        lst = selector.value
     else:
         lst = StringValue(selector).value.split(',')
     to_append = StringValue(to_append).value.strip()
@@ -257,18 +249,18 @@ _elements_of_type_html5_block = 'article, aside, details, figcaption, figure, fo
 _elements_of_type_html5_inline = 'audio, canvas, command, datalist, embed, keygen, mark, meter, output, progress, rp, rt, ruby, time, video, wbr'
 _elements_of_type_html5 = 'article, aside, audio, canvas, command, datalist, details, embed, figcaption, figure, footer, header, hgroup, keygen, mark, menu, meter, nav, output, progress, rp, rt, ruby, section, summary, time, video, wbr'
 _elements_of_type = {
-    'block': dict(enumerate(sorted(_elements_of_type_block.replace(' ', '').split(',')))),
-    'inline': dict(enumerate(sorted(_elements_of_type_inline.replace(' ', '').split(',')))),
-    'table': dict(enumerate(sorted(_elements_of_type_table.replace(' ', '').split(',')))),
-    'list-item': dict(enumerate(sorted(_elements_of_type_list_item.replace(' ', '').split(',')))),
-    'table-row-group': dict(enumerate(sorted(_elements_of_type_table_row_group.replace(' ', '').split(',')))),
-    'table-header-group': dict(enumerate(sorted(_elements_of_type_table_header_group.replace(' ', '').split(',')))),
-    'table-footer-group': dict(enumerate(sorted(_elements_of_type_table_footer_group.replace(' ', '').split(',')))),
-    'table-row': dict(enumerate(sorted(_elements_of_type_table_footer_group.replace(' ', '').split(',')))),
-    'table-cell': dict(enumerate(sorted(_elements_of_type_table_footer_group.replace(' ', '').split(',')))),
-    'html5-block': dict(enumerate(sorted(_elements_of_type_html5_block.replace(' ', '').split(',')))),
-    'html5-inline': dict(enumerate(sorted(_elements_of_type_html5_inline.replace(' ', '').split(',')))),
-    'html5': dict(enumerate(sorted(_elements_of_type_html5.replace(' ', '').split(',')))),
+    'block': sorted(_elements_of_type_block.replace(' ', '').split(',')),
+    'inline': sorted(_elements_of_type_inline.replace(' ', '').split(',')),
+    'table': sorted(_elements_of_type_table.replace(' ', '').split(',')),
+    'list-item': sorted(_elements_of_type_list_item.replace(' ', '').split(',')),
+    'table-row-group': sorted(_elements_of_type_table_row_group.replace(' ', '').split(',')),
+    'table-header-group': sorted(_elements_of_type_table_header_group.replace(' ', '').split(',')),
+    'table-footer-group': sorted(_elements_of_type_table_footer_group.replace(' ', '').split(',')),
+    'table-row': sorted(_elements_of_type_table_footer_group.replace(' ', '').split(',')),
+    'table-cell': sorted(_elements_of_type_table_footer_group.replace(' ', '').split(',')),
+    'html5-block': sorted(_elements_of_type_html5_block.replace(' ', '').split(',')),
+    'html5-inline': sorted(_elements_of_type_html5_inline.replace(' ', '').split(',')),
+    'html5': sorted(_elements_of_type_html5.replace(' ', '').split(',')),
 }
 
 @register('elements-of-type', 1)
@@ -277,8 +269,7 @@ def elements_of_type(display):
     ret = _elements_of_type.get(d.value, None)
     if ret is None:
         raise Exception("Elements of type '%s' not found!" % d.value)
-    ret['_'] = ','
-    return ListValue(ret)
+    return List(ret, use_comma=True)
 
 
 @register('enumerate', 3)
@@ -306,7 +297,7 @@ def enumerate_(prefix, frm, through, separator='-'):
         else:
             ret.append(NumberValue(i))
 
-    return ListValue(ret, separator=',')
+    return List(ret, use_comma=True)
 
 
 @register('headers', 0)
@@ -343,14 +334,14 @@ def headers(frm=None, to=None):
 
 @register('nest')
 def nest(*arguments):
-    if isinstance(arguments[0], ListValue):
-        lst = arguments[0].values()
+    if isinstance(arguments[0], List):
+        lst = arguments[0].value
     else:
         lst = StringValue(arguments[0]).value.split(',')
     ret = [unicode(s).strip() for s in lst if unicode(s).strip()]
     for arg in arguments[1:]:
-        if isinstance(arg, ListValue):
-            lst = arg.values()
+        if isinstance(arg, List):
+            lst = arg.value
         else:
             lst = StringValue(arg).value.split(',')
         new_ret = []
@@ -366,10 +357,8 @@ def nest(*arguments):
                         else:
                             new_ret.append(r + ' ' + s)
         ret = new_ret
-    ret = sorted(set(ret))
-    ret = dict(enumerate(ret))
-    ret['_'] = ','
-    return ret
+
+    return List(sorted(set(ret)), use_comma=True)
 
 
 # This isn't actually from Compass, but it's just a shortcut for enumerate().
@@ -393,8 +382,8 @@ OPPOSITE_POSITIONS = dict(
 )
 
 def _position(opposite, positions):
-    if isinstance(positions, ListValue):
-        positions = positions.values()
+    if isinstance(positions, List):
+        positions = positions.value
     else:
         positions = [positions]
 
@@ -431,7 +420,7 @@ def _position(opposite, positions):
     if len(ret) == 1:
         return ret[0]
     else:
-        return ListValue(ret, separator='')
+        return List(ret, use_comma=False)
 
 
 @register('position')
@@ -495,12 +484,12 @@ def _font_url(path, only_path=False, cache_buster=True, inline=False):
 
 
 def _font_files(args, inline):
-    if len(args) == 1 and isinstance(args[0], (list, tuple, ListValue)):
-        args = ListValue(args[0]).values()
+    if len(args) == 1 and isinstance(args[0], (list, tuple, List)):
+        args = list(args[0])
     n = 0
     params = [[], []]
     for arg in args:
-        if isinstance(arg, ListValue):
+        if isinstance(arg, List):
             if len(arg) == 2:
                 if n % 2 == 1:
                     params[1].append(None)
@@ -527,7 +516,7 @@ def _font_files(args, inline):
             fonts.append('%s format("%s")' % (_font_url(font, inline=inline), StringValue(format).value))
         else:
             fonts.append(_font_url(font, inline=inline))
-    return ListValue(fonts)
+    return List(fonts)
 
 
 @register('font-url', 1)
diff --git a/scss/functions/compass/images.py b/scss/functions/compass/images.py
index d332684..f8ebb6f 100644
--- a/scss/functions/compass/images.py
+++ b/scss/functions/compass/images.py
@@ -17,7 +17,7 @@ from scss import config
 from scss.functions.compass import _image_size_cache
 from scss.functions.compass.helpers import add_cache_buster
 from scss.functions.library import FunctionLibrary
-from scss.types import ColorValue, ListValue, NumberValue, StringValue
+from scss.types import ColorValue, List, NumberValue, StringValue
 from scss.util import escape
 
 try:
@@ -68,26 +68,16 @@ def _image_url(path, only_path=False, cache_buster=True, dst_color=None, src_col
             filetime = 'NA'
     BASE_URL = config.STATIC_URL
     if path:
-        dst_colors = dst_color
-        if isinstance(dst_colors, ListValue):
-            dst_colors = [list(ColorValue(v).value[:3]) for n, v in dst_colors.items() if v]
-        else:
-            dst_colors = [list(ColorValue(dst_colors).value[:3])] if dst_colors else []
+        dst_colors = [list(ColorValue(v).value[:3]) for v in List.from_maybe(dst_color) if v]
 
         src_colors = src_color
-        if isinstance(src_colors, ListValue):
-            src_colors = [tuple(ColorValue(v).value[:3]) if v else (0, 0, 0) for n, v in src_colors.items()]
-        else:
-            src_colors = [tuple(ColorValue(src_colors).value[:3]) if src_colors else (0, 0, 0)]
+        src_colors = [tuple(ColorValue(v).value[:3]) if v else (0, 0, 0) for v in List.from_maybe(src_color)]
 
         len_colors = max(len(dst_colors), len(src_colors))
         dst_colors = (dst_colors * len_colors)[:len_colors]
         src_colors = (src_colors * len_colors)[:len_colors]
 
-        if isinstance(spacing, ListValue):
-            spacing = [int(NumberValue(v).value) for n, v in spacing.items()]
-        else:
-            spacing = [int(NumberValue(spacing).value)]
+        spacing = [int(NumberValue(v).value) for v in List.from_maybe(spacing)]
         spacing = (spacing * 4)[:4]
 
         file_name, file_ext = os.path.splitext(os.path.normpath(filepath).replace('\\', '_').replace('/', '_'))
diff --git a/scss/functions/compass/sprites.py b/scss/functions/compass/sprites.py
index a441e20..5dd7fcf 100644
--- a/scss/functions/compass/sprites.py
+++ b/scss/functions/compass/sprites.py
@@ -33,7 +33,7 @@ from scss import config
 from scss.functions.compass import _image_size_cache
 from scss.functions.compass.layouts import PackedSpritesLayout, HorizontalSpritesLayout, VerticalSpritesLayout, DiagonalSpritesLayout
 from scss.functions.library import FunctionLibrary
-from scss.types import ColorValue, ListValue, NumberValue, QuotedStringValue, StringValue
+from scss.types import ColorValue, List, NumberValue, QuotedStringValue, StringValue
 from scss.util import escape
 
 log = logging.getLogger(__name__)
@@ -172,7 +172,7 @@ def sprite_map(g, **kwargs):
             direction = kwargs.get('direction', config.SPRTE_MAP_DIRECTION)
             repeat = StringValue(kwargs.get('repeat', 'no-repeat'))
             collapse = kwargs.get('collapse') or 0
-            if isinstance(collapse, ListValue):
+            if isinstance(collapse, List):
                 collapse_x = int(NumberValue(collapse[0]).value)
                 collapse_y = int(NumberValue(collapse[-1]).value)
             else:
@@ -194,22 +194,13 @@ def sprite_map(g, **kwargs):
                 position = 1.0
 
             padding = kwargs.get('padding', kwargs.get('spacing', 0))
-            if isinstance(padding, ListValue):
-                padding = [int(NumberValue(v).value) for n, v in padding.items()]
-            else:
-                padding = [int(NumberValue(padding).value)]
+            padding = [int(NumberValue(v).value) for v in List.from_maybe(padding)]
             padding = (padding * 4)[:4]
 
             dst_colors = kwargs.get('dst_color')
-            if isinstance(dst_colors, ListValue):
-                dst_colors = [list(ColorValue(v).value[:3]) for n, v in dst_colors.items() if v]
-            else:
-                dst_colors = [list(ColorValue(dst_colors).value[:3])] if dst_colors else []
+            dst_colors = [list(ColorValue(v).value[:3]) for v in List.from_maybe(dst_colors) if v]
             src_colors = kwargs.get('src_color')
-            if isinstance(src_colors, ListValue):
-                src_colors = [tuple(ColorValue(v).value[:3]) if v else (0, 0, 0) for n, v in src_colors.items()]
-            else:
-                src_colors = [tuple(ColorValue(src_colors).value[:3]) if src_colors else (0, 0, 0)]
+            src_colors = [tuple(ColorValue(v).value[:3]) if v else (0, 0, 0) for v in List.from_maybe(src_colors)]
             len_colors = max(len(dst_colors), len(src_colors))
             dst_colors = (dst_colors * len_colors)[:len_colors]
             src_colors = (src_colors * len_colors)[:len_colors]
@@ -253,10 +244,7 @@ def sprite_map(g, **kwargs):
                 if _padding is None:
                     _padding = padding
                 else:
-                    if isinstance(_padding, ListValue):
-                        _padding = [int(NumberValue(v).value) for n, v in _padding.items()]
-                    else:
-                        _padding = [int(NumberValue(_padding).value)]
+                    _padding = [int(NumberValue(v).value) for v in List.from_maybe(_padding)]
                     _padding = (_padding * 4)[:4]
                 all_paddings.append(_padding)
 
@@ -267,18 +255,12 @@ def sprite_map(g, **kwargs):
                         has_dst_colors = True
                 else:
                     has_dst_colors = True
-                    if isinstance(_dst_colors, ListValue):
-                        _dst_colors = [list(ColorValue(v).value[:3]) for n, v in _dst_colors.items() if v]
-                    else:
-                        _dst_colors = [list(ColorValue(_dst_colors).value[:3])] if _dst_colors else []
+                    _dst_colors = [list(ColorValue(v).value[:3]) for v in List.from_maybe(_dst_colors) if v]
                 _src_colors = kwargs.get(name + '_src_color')
                 if _src_colors is None:
                     _src_colors = src_colors
                 else:
-                    if isinstance(_src_colors, ListValue):
-                        _src_colors = [tuple(ColorValue(v).value[:3]) if v else (0, 0, 0) for n, v in _src_colors.items()]
-                    else:
-                        _src_colors = [tuple(ColorValue(_src_colors).value[:3]) if _src_colors else (0, 0, 0)]
+                    _src_colors = [tuple(ColorValue(v).value[:3]) if v else (0, 0, 0) for v in List.from_maybe(_src_colors)]
                 _len_colors = max(len(_dst_colors), len(_src_colors))
                 _dst_colors = (_dst_colors * _len_colors)[:_len_colors]
                 _src_colors = (_src_colors * _len_colors)[:_len_colors]
@@ -419,7 +401,7 @@ def sprite_file(map, sprite):
 def sprites(map):
     map = StringValue(map).value
     sprite_map = sprite_maps.get(map, {})
-    return ListValue(sorted(s for s in sprite_map if not s.startswith('*')))
+    return List(list(sorted(s for s in sprite_map if not s.startswith('*'))))
 
 
 @register('sprite', 2)
diff --git a/scss/functions/core.py b/scss/functions/core.py
index dd4b00e..2d49a0d 100644
--- a/scss/functions/core.py
+++ b/scss/functions/core.py
@@ -14,7 +14,7 @@ from six.moves import xrange
 
 from scss.cssdefs import _variable_re
 from scss.functions.library import FunctionLibrary
-from scss.types import BooleanValue, ColorValue, ListValue, NumberValue, QuotedStringValue, StringValue, String
+from scss.types import BooleanValue, ColorValue, List, NumberValue, QuotedStringValue, StringValue, String
 
 log = logging.getLogger(__name__)
 
@@ -428,16 +428,21 @@ CORE_LIBRARY.add(NumberValue.wrap_python_function(math.floor), 'floor', 1)
 
 # ------------------------------------------------------------------------------
 # List functions
-def __parse_separator(separator):
+def __parse_separator(separator, default_from=None):
     if separator is None:
         return None
     separator = StringValue(separator).value
     if separator == 'comma':
-        return ','
+        return True
     elif separator == 'space':
-        return ' '
+        return False
     elif separator == 'auto':
-        return None
+        if not default_from:
+            return True
+        elif len(default_from) < 2:
+            return True
+        else:
+            return default_from.use_comma
     else:
         raise ValueError('Separator must be auto, comma, or space')
 
@@ -446,9 +451,8 @@ def __parse_separator(separator):
 @register('-compass-list-size')
 @register('length')
 def _length(*lst):
-    if len(lst) == 1 and isinstance(lst[0], (list, tuple, ListValue)):
-        lst = ListValue(lst[0]).values()
-    lst = ListValue(lst)
+    if len(lst) == 1 and isinstance(lst[0], (list, tuple, List)):
+        lst = lst[0]
     return NumberValue(len(lst))
 
 
@@ -460,7 +464,7 @@ def nth(lst, n):
     Return the Nth item in the string
     """
     n = NumberValue(n).value
-    lst = ListValue(lst).value
+    lst = List(lst).value
     try:
         n = int(float(n)) - 1
         n = n % len(lst)
@@ -483,39 +487,37 @@ def nth(lst, n):
 @register('join', 2)
 @register('join', 3)
 def join(lst1, lst2, separator=None):
-    ret = ListValue(lst1)
-    lst2 = ListValue(lst2).value
-    lst_len = len(ret.value)
-    ret.value.update((k + lst_len if isinstance(k, int) else k, v) for k, v in lst2.items())
-    separator = __parse_separator(separator)
-    if separator is not None:
-        ret.value['_'] = separator
-    return ret
+    ret = []
+    ret.extend(List.from_maybe(lst1))
+    ret.extend(List.from_maybe(lst2))
+
+    use_comma = __parse_separator(separator, default_from=lst1)
+    return List(ret, use_comma=use_comma)
 
 
 @register('min')
 def min_(*lst):
-    if len(lst) == 1 and isinstance(lst[0], (list, tuple, ListValue)):
-        lst = ListValue(lst[0]).values()
-    lst = ListValue(lst).value
-    return min(lst.values())
+    if len(lst) == 1 and isinstance(lst[0], (list, tuple, List)):
+        lst = lst[0]
+    return min(lst)
 
 
 @register('max')
 def max_(*lst):
-    if len(lst) == 1 and isinstance(lst[0], (list, tuple, ListValue)):
-        lst = ListValue(lst[0]).values()
-    lst = ListValue(lst).value
-    return max(lst.values())
+    if len(lst) == 1 and isinstance(lst[0], (list, tuple, List)):
+        lst = lst[0]
+    return max(lst)
 
 
 @register('append', 2)
 @register('append', 3)
 def append(lst, val, separator=None):
-    separator = __parse_separator(separator)
-    ret = ListValue(lst, separator)
-    ret.value[len(ret)] = val
-    return ret
+    ret = []
+    ret.extend(List.from_maybe(lst))
+    ret.append(val)
+
+    use_comma = __parse_separator(separator, default_from=lst)
+    return List(ret, use_comma=use_comma)
 
 
 @register('index', 2)
@@ -535,9 +537,15 @@ def _type_of(obj):  # -> bool, number, string, color, list
 
 
 @register('unit', 1)
-def _unit(number):  # -> px, em, cm, etc.
-    unit = NumberValue(number).unit
-    return StringValue(unit)
+def unit(number):  # -> px, em, cm, etc.
+    numer = '*'.join(number.unit_numer)
+    denom = '*'.join(number.unit_denom)
+
+    if denom:
+        ret = numer + '/' + denom
+    else:
+        ret = numer
+    return StringValue(ret)
 
 
 @register('unitless', 1)
diff --git a/scss/functions/extra.py b/scss/functions/extra.py
index 66f05e6..70ad480 100644
--- a/scss/functions/extra.py
+++ b/scss/functions/extra.py
@@ -13,7 +13,7 @@ from six.moves import xrange
 
 from scss import config
 from scss.functions.library import FunctionLibrary
-from scss.types import ColorValue, NumberValue, StringValue, ListValue
+from scss.types import ColorValue, NumberValue, StringValue, List
 from scss.util import escape
 
 try:
@@ -273,25 +273,10 @@ def background_noise(density=None, opacity=None, size=None, monochrome=False, in
     if not Image:
         raise Exception("Images manipulation require PIL")
 
-    if isinstance(density, ListValue):
-        density = [NumberValue(v).value for n, v in density.items()]
-    else:
-        density = [NumberValue(density).value]
-
-    if isinstance(intensity, ListValue):
-        intensity = [NumberValue(v).value for n, v in intensity.items()]
-    else:
-        intensity = [NumberValue(intensity).value]
-
-    if isinstance(color, ListValue):
-        color = [ColorValue(v).value for n, v in color.items() if v]
-    else:
-        color = [ColorValue(color).value] if color else []
-
-    if isinstance(opacity, ListValue):
-        opacity = [NumberValue(v).value for n, v in opacity.items()]
-    else:
-        opacity = [NumberValue(opacity).value]
+    density = [NumberValue(v).value for v in List.from_maybe(density)]
+    intensity = [NumberValue(v).value for v in List.from_maybe(intensity)]
+    color = [ColorValue(v).value for v in List.from_maybe(color) if v]
+    opacity = [NumberValue(v).value for v in List.from_maybe(opacity)]
 
     size = int(NumberValue(size).value) if size else 0
     if size < 1 or size > 512:
@@ -347,25 +332,10 @@ def background_brushed(density=None, intensity=None, color=None, opacity=None, s
     if not Image:
         raise Exception("Images manipulation require PIL")
 
-    if isinstance(density, ListValue):
-        density = [NumberValue(v).value for n, v in density.items()]
-    else:
-        density = [NumberValue(density).value]
-
-    if isinstance(intensity, ListValue):
-        intensity = [NumberValue(v).value for n, v in intensity.items()]
-    else:
-        intensity = [NumberValue(intensity).value]
-
-    if isinstance(color, ListValue):
-        color = [ColorValue(v).value for n, v in color.items() if v]
-    else:
-        color = [ColorValue(color).value] if color else []
-
-    if isinstance(opacity, ListValue):
-        opacity = [NumberValue(v).value for n, v in opacity.items()]
-    else:
-        opacity = [NumberValue(opacity).value]
+    density = [NumberValue(v).value for v in List.from_maybe(density)]
+    intensity = [NumberValue(v).value for v in List.from_maybe(intensity)]
+    color = [ColorValue(v).value for v in List.from_maybe(color) if v]
+    opacity = [NumberValue(v).value for v in List.from_maybe(opacity)]
 
     size = int(NumberValue(size).value) if size else -1
     if size < 0 or size > 512:
@@ -373,15 +343,8 @@ def background_brushed(density=None, intensity=None, color=None, opacity=None, s
 
     monochrome = bool(monochrome)
 
-    if isinstance(direction, ListValue):
-        direction = [NumberValue(v).value for n, v in direction.items()]
-    else:
-        direction = [NumberValue(direction).value]
-
-    if isinstance(spread, ListValue):
-        spread = [NumberValue(v).value for n, v in spread.items()]
-    else:
-        spread = [NumberValue(spread).value]
+    direction = [NumberValue(v).value for v in List.from_maybe(direction)]
+    spread = [NumberValue(v).value for v in List.from_maybe(spread)]
 
     background = ColorValue(background).value if background else None
 
diff --git a/scss/tests/functions/test_core.py b/scss/tests/functions/test_core.py
index 53b03b0..d02a479 100644
--- a/scss/tests/functions/test_core.py
+++ b/scss/tests/functions/test_core.py
@@ -260,7 +260,6 @@ def test_nth(calc):
     assert calc('nth(10px 20px 30px, 1)') == calc('10px')
     assert calc('nth((Helvetica, Arial, sans-serif), 3)') == calc('sans-serif')
 
-@xfail(reason="second example broken, unclear why")
 def test_join(calc):
     # Examples from the Ruby docs
     assert calc('join(10px 20px, 30px 40px)') == calc('10px 20px 30px 40px')
@@ -269,7 +268,6 @@ def test_join(calc):
     assert calc('join(10px, 20px, comma)') == calc('10px, 20px')
     assert calc('join((blue, red), (#abc, #def), space)') == calc('blue red #abc #def')
 
-@xfail(reason="possibly a problem with list equality?")
 def test_append(calc):
     # Examples from the Ruby docs
     assert calc('append(10px 20px, 30px)') == calc('10px 20px 30px')
@@ -301,7 +299,6 @@ def test_type_of(calc):
     assert calc('type-of(#fff)') == calc('color')
     assert calc('type-of(blue)') == calc('color')
 
-@xfail(reason="numeric type does not yet understand multiple units")
 def test_unit(calc):
     # Examples from the Ruby docs
     assert calc('unit(100)') == calc('""')
diff --git a/scss/types.py b/scss/types.py
index 47d4f56..89aa226 100644
--- a/scss/types.py
+++ b/scss/types.py
@@ -391,10 +391,32 @@ class NumberValue(Value):
         return val + unit
 
 
-class ListValue(Value):
+class List(Value):
+    """A list of other values.  May be delimited by commas or spaces.
+
+    Lists of one item don't make much sense in CSS, but can exist in Sass.  Use ......
+
+    Lists may also contain zero items, but these are forbidden from appearing
+    in CSS output.
+    """
+
     sass_type_name = u'list'
 
-    def __init__(self, tokens, separator=None):
+    def __init__(self, tokens, separator=None, use_comma=True):
+        if isinstance(tokens, ListValue):
+            tokens = tokens.value
+
+        if not isinstance(tokens, (list, tuple)):
+            raise TypeError("Expected list, got %r" % (tokens,))
+
+        self.value = list(tokens)
+        # TODO...
+        self.use_comma = separator == ","
+        return
+
+
+
+
         self.tokens = tokens
         if tokens is None:
             self.value = {}
@@ -423,10 +445,65 @@ class ListValue(Value):
         if separator:
             self.value['_'] = separator
 
+    @classmethod
+    def maybe_new(cls, values, use_comma=True):
+        """If `values` contains only one item, return that item.  Otherwise,
+        return a List as normal.
+        """
+        if len(values) == 1:
+            return values[0]
+        else:
+            return cls(values, use_comma=use_comma)
+
+    def maybe(self):
+        """If this List contains only one item, return it.  Otherwise, return
+        the List.
+        """
+        if len(self.value) == 1:
+            return self.value[0]
+        else:
+            return self
+
+    @classmethod
+    def from_maybe(cls, values, use_comma=True):
+        """If `values` appears to not be a list, return a list containing it.
+        Otherwise, return a List as normal.
+        """
+        if not isinstance(values, (list, tuple, List)):
+            values = [values]
+
+        return cls(values, use_comma=use_comma)
+
+    @classmethod
+    def from_maybe_starargs(cls, args):
+        """If `args` has one element which appears to be a list, return it.
+        Otherwise, return a list as normal.
+
+        Mainly used by Sass function implementations that predate `...`
+        support, so they can accept both a list of arguments and a single list
+        stored in a variable.
+        """
+        if len(args) == 1:
+            if isinstance(args[0], cls):
+                return args[0]
+            elif isinstance(args[0], (list, tuple)):
+                return cls(args[0])
+
+        return cls(args)
+
     @property
     def separator(self):
         return self.value.get('_', '')
 
+    def delimiter(self, compress=False):
+        if self.use_comma:
+            if compress:
+                return ','
+            else:
+                return ', '
+        else:
+            return ' '
+
     @classmethod
     def _do_op(cls, first, second, op):
         if isinstance(first, ListValue) and isinstance(second, ListValue):
@@ -453,34 +530,32 @@ class ListValue(Value):
         return dict((i if isinstance(k, int) else k, v) for i, (k, v) in enumerate(sorted(lst.items())))
 
     def __len__(self):
-        return len(self.value) - (1 if '_' in self.value else 0)
+        return len(self.value)
 
     def __str__(self):
         return self.render()
 
     def __iter__(self):
-        return iter(self.values())
+        return iter(self.value)
 
     def values(self):
-        return list(zip(*self.items()))[1]
+        return self.value
 
     def keys(self):
-        return list(zip(*self.items()))[0]
+        return range(len(self.value))
 
     def items(self):
-        return sorted((k, v) for k, v in self.value.items() if k != '_')
+        return enumerate(self.value)
 
     def __getitem__(self, key):
         return self.value[key]
 
     def render(self, compress=False, short_colors=False):
-        delim = self.separator
-        if not compress or not delim:
-            delim += ' '
+        delim = self.delimiter(compress)
 
         return delim.join(
             item.render(compress=compress, short_colors=short_colors)
-            for item in self.values()
+            for item in self.value
         )
 
 
@@ -759,5 +834,7 @@ class String(Value):
         return self.__str__()
 
 
+# Backwards-compatibility.
+ListValue = List
 QuotedStringValue = String
 StringValue = String