Fix SelectField choice backwards compatibility

src/wtforms/fields/choices.py

@@ -49,7 +49,13 @@ def __iter__(self):
             _form=None,
             _meta=self.meta,
         )
-        for i, (value, label, checked, render_kw) in enumerate(self.iter_choices()):
+        for i, choice in enumerate(self.iter_choices()):
+            if len(choice) == 4:
+                value, label, checked, render_kw = choice
+            else:
+                value, label, checked = choice
+                render_kw = {}
+
             opt = self._Option(
                 label=label, id="%s-%d" % (self.id, i), **opts, **render_kw
             )
@@ -141,7 +147,7 @@ def pre_validate(self, form):
         if self.choices is None:
             raise TypeError(self.gettext("Choices cannot be None."))
 
-        for _, _, match, _ in self.iter_choices():
+        for _, _, match, *_ in self.iter_choices():
             if match:
                 break
         else:

src/wtforms/widgets/core.py

@@ -334,7 +334,7 @@ class Select:
 
     The field must provide an `iter_choices()` method which the widget will
     call on rendering; this method must yield tuples of
-    `(value, label, selected, render_kw)`.
+    `(value, label, selected)` or `(value, label, selected, render_kw)`.
     It also must provide a `has_groups()` method which tells whether choices
     are divided into groups, and if they do, the field must have an
     `iter_groups()` method that yields tuples of `(label, choices)`, where
@@ -358,11 +358,21 @@ def __call__(self, field, **kwargs):
         if field.has_groups():
             for group, choices in field.iter_groups():
                 html.append("<optgroup %s>" % html_params(label=group))
-                for val, label, selected, render_kw in choices:
+                for choice in choices:
+                    if len(choice) == 4:
+                        val, label, selected, render_kw = choice
+                    else:
+                        val, label, selected = choice
+                        render_kw = {}
                     html.append(self.render_option(val, label, selected, **render_kw))
                 html.append("</optgroup>")
         else:
-            for val, label, selected, render_kw in field.iter_choices():
+            for choice in field.iter_choices():
+                if len(choice) == 4:
+                    val, label, selected, render_kw = choice
+                else:
+                    val, label, selected = choice
+                    render_kw = {}
                 html.append(self.render_option(val, label, selected, **render_kw))
         html.append("</select>")
         return Markup("".join(html))

tests/conftest.py

@@ -25,7 +25,7 @@ def basic_widget_dummy_field(dummy_field_class):
 
 @pytest.fixture
 def select_dummy_field(dummy_field_class):
-    return dummy_field_class([("foo", "lfoo", True, {}), ("bar", "lbar", False, {})])
+    return dummy_field_class([("foo", "lfoo", True), ("bar", "lbar", False)])
 
 
 @pytest.fixture