1 files changed, 56 insertions, 22 deletions

diff --git a/urwid/widget.py b/urwid/widget.py
index 0cb54d1..d2b7a8b 100644
--- a/urwid/widget.py
+++ b/urwid/widget.py
@@ -200,7 +200,7 @@ class Widget(object):
     """
     __metaclass__ = WidgetMeta
     _selectable = False
-    _sizing = set([])
+    _sizing = frozenset([FLOW, BOX, FIXED])
     _command_map = command_map # default to the single shared CommandMap
 
     def _invalidate(self):
@@ -212,29 +212,40 @@ class Widget(object):
         argument.
         """
         signals.emit_signal(self, name, self, *args)
-    
+
     def selectable(self):
         """
         Return True if this widget should take focus.  Default
         implementation returns the value of self._selectable.
         """
         return self._selectable
-    
+
     def sizing(self):
         """
         Return a set including one or more of 'box', 'flow' and
         'fixed'.  Default implementation returns the value of
-        self._sizing.
+        self._sizing, which for the Widget base class includes all
+        three.
+
+        The sizing modes returned indicate the modes that may be
+        supported by this widget, but is not sufficient to know
+        that using that sizing mode will work.  Subclasses should
+        make an effort to remove sizing modes they know will not
+        work given the state of the widget, but many do not yet
+        do this.
+
+        If a sizing mode is missing from the set then the widget
+        should fail when used in that mode.
         """
         return self._sizing
 
     def pack(self, size, focus=False):
         """
-        Return a 'packed' (maxcol, maxrow) for this widget.  Default 
+        Return a 'packed' (maxcol, maxrow) for this widget.  Default
         implementation (no packing defined) returns size, and
         calculates maxrow if not given.
         """
-        if size == ():
+        if not size:
             if FIXED in self.sizing():
                 raise NotImplementedError('Fixed widgets must override'
                     ' Widget.pack()')
@@ -269,7 +280,7 @@ class Widget(object):
         words = []
         if self.selectable():
             words = ["selectable"] + words
-        if self.sizing():
+        if self.sizing() and self.sizing() != frozenset([FLOW, BOX, FIXED]):
             sizing_modes = list(self.sizing())
             sizing_modes.sort()
             words.append("/".join(sizing_modes))
@@ -277,15 +288,21 @@ class Widget(object):
 
     def _repr_attrs(self):
         return {}
-    
+
 
 class FlowWidget(Widget):
     """
-    base class of widgets that determine their rows from the number of
+    Deprecated.  Inherit from Widget and add:
+
+        _sizing = frozenset(['flow'])
+
+    at the top of your class definition instead.
+
+    Base class of widgets that determine their rows from the number of
     columns available.
     """
-    _sizing = set([FLOW])
-    
+    _sizing = frozenset([FLOW])
+
     def rows(self, size, focus=False):
         """
         All flow widgets must implement this function.
@@ -301,23 +318,30 @@ class FlowWidget(Widget):
 
 class BoxWidget(Widget):
     """
-    base class of width and height constrained widgets such as
+    Deprecated.  Inherit from Widget and add:
+
+        _sizing = frozenset(['box'])
+        _selectable = True
+
+    at the top of your class definition instead.
+
+    Base class of width and height constrained widgets such as
     the top level widget attached to the display object
     """
     _selectable = True
-    _sizing = set([BOX])
-    
+    _sizing = frozenset([BOX])
+
     def render(self, size, focus=False):
         """
         All widgets must implement this function.
         """
         raise NotImplementedError()
-    
+
 
 def fixed_size(size):
     """
     raise ValueError if size != ().
-    
+
     Used by FixedWidgets to test size parameter.
     """
     if size != ():
@@ -326,17 +350,23 @@ def fixed_size(size):
 
 class FixedWidget(Widget):
     """
-    base class of widgets that know their width and height and
+    Deprecated.  Inherit from Widget and add:
+
+        _sizing = frozenset(['fixed'])
+
+    at the top of your class definition instead.
+
+    Base class of widgets that know their width and height and
     cannot be resized
     """
-    _sizing = set([FIXED])
-    
+    _sizing = frozenset([FIXED])
+
     def render(self, size, focus=False):
         """
         All widgets must implement this function.
         """
         raise NotImplementedError()
-    
+
     def pack(self, size=None, focus=False):
         """
         All fixed widgets must implement this function.
@@ -344,10 +374,12 @@ class FixedWidget(Widget):
         raise NotImplementedError()
 
 
-class Divider(FlowWidget):
+class Divider(Widget):
     """
     Horizontal divider widget
     """
+    _sizing = frozenset([FLOW])
+
     ignore_focus = True
 
     def __init__(self,div_char=u" ",top=0,bottom=0):
@@ -446,10 +478,12 @@ class SolidFill(BoxWidget):
 class TextError(Exception):
     pass
 
-class Text(FlowWidget):
+class Text(Widget):
     """
     a horizontally resizeable text widget
     """
+    _sizing = frozenset([FLOW])
+
     ignore_focus = True
     _repr_content_length_max = 140