Fix #445 - add `__len__` to listbox with validation if body `Sized` (#534)

* If body is not `Sized` - `__len__` call should raise `AttributeError`
* Downside: since body can be replaced, `__len__` is always set ant `ListBox` by itself will pass `Sized` instance check
* Relax `SimpleListWalker` constructor rules: any Iterable is allowed as source due to `list` is used internally

Co-authored-by: Aleksei Stepanov <alekseis@nvidia.com>

2 files changed, 51 insertions, 7 deletions

diff --git a/urwid/listbox.py b/urwid/listbox.py
index a8cd91c..47b8376 100644
--- a/urwid/listbox.py
+++ b/urwid/listbox.py
@@ -24,7 +24,7 @@ from __future__ import annotations
 
 import typing
 import warnings
-from collections.abc import MutableSequence
+from collections.abc import Iterable, Sized
 
 from urwid import signals
 from urwid.canvas import CanvasCombine, SolidCanvas
@@ -100,7 +100,7 @@ class ListWalker(metaclass=signals.MetaSignals):
 
 
 class SimpleListWalker(MonitoredList, ListWalker):
-    def __init__(self, contents, wrap_around: bool = False):
+    def __init__(self, contents: Iterable[typing.Any], wrap_around: bool = False):
         """
         contents -- list to copy into this object
 
@@ -113,7 +113,7 @@ class SimpleListWalker(MonitoredList, ListWalker):
         detected automatically and will cause ListBox objects using
         this list walker to be updated.
         """
-        if not isinstance(contents, MutableSequence):
+        if not isinstance(contents, Iterable):
             raise ListWalkerError(f"SimpleListWalker expecting list like object, got: {contents!r}")
         MonitoredList.__init__(self, contents)
         self.focus = 0
@@ -191,7 +191,7 @@ class SimpleListWalker(MonitoredList, ListWalker):
 
 
 class SimpleFocusListWalker(ListWalker, MonitoredFocusList):
-    def __init__(self, contents, wrap_around=False):
+    def __init__(self, contents: Iterable[typing.Any], wrap_around: bool = False):
         """
         contents -- list to copy into this object
 
@@ -208,8 +208,8 @@ class SimpleFocusListWalker(ListWalker, MonitoredFocusList):
         normal list methods will cause the focus to be updated
         intelligently.
         """
-        if not isinstance(contents, MutableSequence):
-            raise ListWalkerError(f"SimpleFocusListWalker expecting list like object, got: {contents!r}")
+        if not isinstance(contents, Iterable):
+            raise ListWalkerError(f"SimpleFocusListWalker expecting iterable object, got: {contents!r}")
         MonitoredFocusList.__init__(self, contents)
         self.wrap_around = wrap_around
 
@@ -274,7 +274,12 @@ class ListBox(Widget, WidgetContainerMixin):
             widgets to be displayed inside the list box
         :type body: ListWalker
         """
-        self.body = body
+        if getattr(body, 'get_focus', None):
+            self._body: ListWalker = body
+        else:
+            self._body = SimpleListWalker(body)
+
+        self.body = self._body  # Initialization hack
 
         # offset_rows is the number of rows between the top of the view
         # and the top of the focused item
@@ -340,6 +345,11 @@ class ListBox(Widget, WidgetContainerMixin):
         )
         self.body = body
 
+    def __len__(self) -> int:
+        if isinstance(self._body, Sized):
+            return len(self._body)
+        raise AttributeError(f"{self._body.__class__.__name__} is not Sized")
+
     def calculate_visible(self, size: tuple[int, int], focus: bool = False):
         """
         Returns the widgets that would be displayed in
diff --git a/urwid/tests/test_listbox.py b/urwid/tests/test_listbox.py
index 846b17b..044a0df 100644
--- a/urwid/tests/test_listbox.py
+++ b/urwid/tests/test_listbox.py
@@ -94,6 +94,30 @@ class ListBoxCalculateVisibleTest(unittest.TestCase):
         self.cvtest( "cursor way off bottom",
             l1, 3, 100, (0,1), 2, (1, 4) )
 
+    def test_sized(self):
+        lbox = urwid.ListBox(urwid.SimpleListWalker([urwid.Text(str(num)) for num in range(5)]))
+        self.assertEqual(5, len(lbox))
+
+    def test_not_sized(self):
+        class TestWalker(urwid.ListWalker):
+            @property
+            def contents(self):
+                return self
+
+            @staticmethod
+            def next_position(position: int) -> tuple[urwid.Text, int]:
+                return urwid.Text(str(position)), position
+
+            @staticmethod
+            def prev_position(position: int) -> tuple[urwid.Text, int]:
+                return urwid.Text(str(position)), position
+
+        lbox = urwid.ListBox(TestWalker())
+        with self.assertRaises(AttributeError) as exc:
+            len(lbox)
+
+        self.assertEqual(f"{TestWalker.__name__} is not Sized", str(exc.exception))
+
 
 class ListBoxChangeFocusTest(unittest.TestCase):
     def cftest(self, desc, body, pos, offset_inset,
@@ -814,3 +838,13 @@ class ListBoxSetBodyTest(unittest.TestCase):
                 "outdated canvas cache reuse "
                 "after ListWalker's contents modified"
                 )
+
+
+class TestListWalkerFromIterable(unittest.TestCase):
+    def test_01_simple_list_walker(self):
+        walker = urwid.SimpleListWalker(str(num) for num in range(5))
+        self.assertEqual(5, len(walker))
+
+    def test_02_simple_focus_list_walker(self):
+        walker = urwid.SimpleFocusListWalker(str(num) for num in range(5))
+        self.assertEqual(5, len(walker))