normalize render attribute names consistently

CHANGES.rst

@@ -31,6 +31,9 @@ Unreleased
     *extra_filters* parameter. :issue:`128` :pr:`592`
 -   Fields can be passed the ``name`` argument to use a HTML name
     different than their Python name. :issue:`205`, :pr:`601`
+-   Render attribute names like ``for_`` and ``class_`` are normalized
+    consistently so later values override those specified earlier.
+    :issue:`449`, :pr:`596`
 
 
 Version 2.3.1

src/wtforms/meta.py

@@ -1,5 +1,6 @@
 from wtforms import i18n
 from wtforms.utils import WebobInputWrapper
+from wtforms.widgets.core import clean_key
 
 
 class DefaultMeta:
@@ -53,8 +54,12 @@ def render_field(self, field, render_kw):
 
         The default implementation calls ``field.widget(field, **render_kw)``
         """
+
+        render_kw = {clean_key(k): v for k, v in render_kw.items()}
+
         other_kw = getattr(field, "render_kw", None)
         if other_kw is not None:
+            other_kw = {clean_key(k): v for k, v in other_kw.items()}
             render_kw = dict(other_kw, **render_kw)
         return field.widget(field, **render_kw)
 

src/wtforms/widgets/core.py

@@ -17,6 +17,13 @@
 )
 
 
+def clean_key(key):
+    key = key.rstrip("_")
+    if key.startswith("data_") or key.startswith("aria_"):
+        key = key.replace("_", "-")
+    return key
+
+
 def html_params(**kwargs):
     """
     Generate HTML attribute syntax from inputted keyword arguments.
@@ -53,10 +60,7 @@ def html_params(**kwargs):
     """
     params = []
     for k, v in sorted(kwargs.items()):
-        if k in ("class_", "class__", "for_"):
-            k = k[:-1]
-        elif k.startswith("data_") or k.startswith("aria_"):
-            k = k.replace("_", "-")
+        k = clean_key(k)
         if v is True:
             params.append(k)
         elif v is False:

tests/test_fields.py

@@ -266,6 +266,42 @@ def test_render_kw(self):
             'type="text" value="hello">'
         )
 
+    def test_render_special(self):
+        class F(Form):
+            s = StringField(render_kw={"class_": "foo"})
+
+        assert F().s() == '<input class="foo" id="s" name="s" type="text" value="">'
+        assert (
+            F().s(**{"class": "bar"})
+            == '<input class="bar" id="s" name="s" type="text" value="">'
+        )
+        assert (
+            F().s(**{"class_": "bar"})
+            == '<input class="bar" id="s" name="s" type="text" value="">'
+        )
+
+        class G(Form):
+            s = StringField(render_kw={"class__": "foo"})
+
+        assert G().s() == '<input class="foo" id="s" name="s" type="text" value="">'
+        assert (
+            G().s(**{"class__": "bar"})
+            == '<input class="bar" id="s" name="s" type="text" value="">'
+        )
+
+        class H(Form):
+            s = StringField(render_kw={"for_": "foo"})
+
+        assert H().s() == '<input for="foo" id="s" name="s" type="text" value="">'
+        assert (
+            H().s(**{"for": "bar"})
+            == '<input for="bar" id="s" name="s" type="text" value="">'
+        )
+        assert (
+            H().s(**{"for_": "bar"})
+            == '<input for="bar" id="s" name="s" type="text" value="">'
+        )
+
     def test_select_field_copies_choices(self):
         class F(Form):
             items = SelectField(choices=[])

tests/test_widgets.py

@@ -21,7 +21,7 @@ class TestHTMLParams:
     def test_basic(self):
         assert html_params(foo=9, k="wuuu") == 'foo="9" k="wuuu"'
         assert html_params(class_="foo") == 'class="foo"'
-        assert html_params(class__="foo") == 'class_="foo"'
+        assert html_params(class__="foo") == 'class="foo"'
         assert html_params(for_="foo") == 'for="foo"'
         assert html_params(readonly=False, foo=9) == 'foo="9"'
         assert (