Render lazy labels in shorthand dict choices

CHANGES.rst

@@ -1,5 +1,14 @@
 .. currentmodule:: wtforms
 
+Version 3.3.3
+-------------
+
+Unreleased
+
+- Fix :class:`~fields.SelectField` shorthand ``{value: label}`` choices
+  with lazy labels, such as Flask-Babel's ``lazy_gettext``. Each character
+  of the label was rendered as an option of an optgroup.
+
 Version 3.3.2
 -------------
 

src/wtforms/fields/choices.py

@@ -327,9 +327,12 @@ def _choices_from_input(self, choices):
     def _is_shorthand_dict(choices):
         """``True`` if ``choices`` matches the shorthand dict syntax.
 
-        Shorthand is ``dict[str, str | dict[str, str]]``.
+        Shorthand is ``{value: label}`` and ``{optgroup: {value: label}}``.
+        The legacy syntax is ``{optgroup: [choices]}``, so only list and tuple
+        values mark it. Lazy translation strings are not ``str`` but are
+        iterable, so they cannot be told apart from groups otherwise.
         """
-        return all(isinstance(v, (str, dict)) for v in choices.values())
+        return not any(isinstance(v, (list, tuple)) for v in choices.values())
 
     @staticmethod
     def _warn_legacy_choices(choices):

tests/fields/test_select.py

@@ -605,6 +605,43 @@ def test_dict_mixed_flat_and_optgroup_choices():
     assert '<option value="ml">OCaml</option>' in html
 
 
+class LazyLabel:
+    """Iterable lazy string that is not a ``str``, like Flask-Babel's."""
+
+    def __init__(self, text):
+        self.text = text
+
+    def __str__(self):
+        return self.text
+
+    def __html__(self):
+        return self.text
+
+    def __iter__(self):
+        return iter(self.text)
+
+
+def test_dict_shorthand_choices_with_lazy_labels(recwarn):
+    """Lazy labels in the shorthand dict render as labels, not as optgroups."""
+    F = make_form(
+        a=SelectField(
+            choices={
+                "fr": LazyLabel("French"),
+                "Asia": {"ja": LazyLabel("Japanese")},
+            }
+        )
+    )
+    form = F(a="fr")
+    assert form.a() == (
+        '<select id="a" name="a">'
+        '<option selected value="fr">French</option>'
+        '<optgroup label="Asia"><option value="ja">Japanese</option></optgroup>'
+        "</select>"
+    )
+    assert form.validate()
+    assert not [w for w in recwarn if issubclass(w.category, DeprecationWarning)]
+
+
 def test_tuple_choices_deprecation():
     F = make_form(a=SelectField(choices=[("a", "Foo")]))
     with pytest.warns(DeprecationWarning):