Fixed #35521 -- Allowed overriding BoundField class on fields, forms and renderers.

Thank you Sarah Boyce, Carlton Gibson, Tim Schilling and Adam Johnson
for reviews.

Co-authored-by: Christophe Henry <contact@c-henry.fr>
Co-authored-by: David Smith <smithdc@gmail.com>
Co-authored-by: Natalia <124304+nessita@users.noreply.github.com>
Co-authored-by: Matthias Kestenholz <mk@feinheit.ch>

django/forms/fields.py

@@ -95,6 +95,7 @@ class Field:
         "required": _("This field is required."),
     }
     empty_values = list(validators.EMPTY_VALUES)
+    bound_field_class = None
 
     def __init__(
         self,
@@ -111,6 +112,7 @@ class Field:
         disabled=False,
         label_suffix=None,
         template_name=None,
+        bound_field_class=None,
     ):
         # required -- Boolean that specifies whether the field is required.
         #             True by default.
@@ -135,11 +137,13 @@ class Field:
         #             is its widget is shown in the form but not editable.
         # label_suffix -- Suffix to be added to the label. Overrides
         #                 form's label_suffix.
+        # bound_field_class -- BoundField class to use in Field.get_bound_field.
         self.required, self.label, self.initial = required, label, initial
         self.show_hidden_initial = show_hidden_initial
         self.help_text = help_text
         self.disabled = disabled
         self.label_suffix = label_suffix
+        self.bound_field_class = bound_field_class or self.bound_field_class
         widget = widget or self.widget
         if isinstance(widget, type):
             widget = widget()
@@ -251,7 +255,10 @@ class Field:
         Return a BoundField instance that will be used when accessing the form
         field in a template.
         """
-        return BoundField(form, self, field_name)
+        bound_field_class = (
+            self.bound_field_class or form.bound_field_class or BoundField
+        )
+        return bound_field_class(form, self, field_name)
 
     def __deepcopy__(self, memo):
         result = copy.copy(self)

django/forms/forms.py

@@ -68,6 +68,8 @@ class BaseForm(RenderableFormMixin):
     template_name_ul = "django/forms/ul.html"
     template_name_label = "django/forms/label.html"
 
+    bound_field_class = None
+
     def __init__(
         self,
         data=None,
@@ -81,6 +83,7 @@ class BaseForm(RenderableFormMixin):
         field_order=None,
         use_required_attribute=None,
         renderer=None,
+        bound_field_class=None,
     ):
         self.is_bound = data is not None or files is not None
         self.data = MultiValueDict() if data is None else data
@@ -124,6 +127,12 @@ class BaseForm(RenderableFormMixin):
                     renderer = renderer()
         self.renderer = renderer
 
+        self.bound_field_class = (
+            bound_field_class
+            or self.bound_field_class
+            or getattr(self.renderer, "bound_field_class", None)
+        )
+
     def order_fields(self, field_order):
         """
         Rearrange the fields according to field_order.

django/forms/renderers.py

@@ -21,6 +21,8 @@ class BaseRenderer:
     formset_template_name = "django/forms/formsets/div.html"
     field_template_name = "django/forms/field.html"
 
+    bound_field_class = None
+
     def get_template(self, template_name):
         raise NotImplementedError("subclasses must implement get_template()")
 

docs/ref/forms/api.txt

@@ -822,6 +822,9 @@ classes, as needed. The HTML will look something like:
     >>> f["subject"].legend_tag(attrs={"class": "foo"})
     <legend for="id_subject" class="foo required">Subject:</legend>
 
+You may further modify the rendering of form rows by using a
+:ref:`custom BoundField <custom-boundfield>`.
+
 .. _ref-forms-api-configuring-label:
 
 Configuring form elements' HTML ``id`` attributes and ``<label>`` tags
@@ -1149,6 +1152,12 @@ they're not the only way a form object can be displayed.
 
    The ``__str__()`` method of this object displays the HTML for this field.
 
+   You can use :attr:`.Form.bound_field_class` and
+   :attr:`.Field.bound_field_class` to specify a different ``BoundField`` class
+   per form or per field, respectively.
+
+   See :ref:`custom-boundfield` for examples of overriding a ``BoundField``.
+
 To retrieve a single ``BoundField``, use dictionary lookup syntax on your form
 using the field's name as the key:
 
@@ -1488,23 +1497,34 @@ Methods of ``BoundField``
         >>> print(bound_form["subject"].value())
         hi
 
+.. _custom-boundfield:
+
 Customizing ``BoundField``
 ==========================
 
-If you need to access some additional information about a form field in a
-template and using a subclass of :class:`~django.forms.Field` isn't
-sufficient, consider also customizing :class:`~django.forms.BoundField`.
+.. attribute:: Form.bound_field_class
 
-A custom form field can override ``get_bound_field()``:
+.. versionadded:: 5.2
 
-.. method:: Field.get_bound_field(form, field_name)
+Define a custom :class:`~django.forms.BoundField` class to use when rendering
+the form. This takes precedence over the project-level
+:attr:`.BaseRenderer.bound_field_class` (along with a custom
+:setting:`FORM_RENDERER`), but can be overridden by the field-level
+:attr:`.Field.bound_field_class`.
 
-    Takes an instance of :class:`~django.forms.Form` and the name of the field.
-    The return value will be used when accessing the field in a template. Most
-    likely it will be an instance of a subclass of
-    :class:`~django.forms.BoundField`.
+If not defined as a class variable, ``bound_field_class`` can be set via the
+``bound_field_class`` argument in the :class:`Form` or :class:`Field`
+constructor.
 
-If you have a ``GPSCoordinatesField``, for example, and want to be able to
+For compatibility reasons, a custom form field can still override
+:meth:`.Field.get_bound_field()` to use a custom class, though any of the
+previous options are preferred.
+
+You may want to use a custom :class:`.BoundField` if you need to access some
+additional information about a form field in a template and using a subclass of
+:class:`~django.forms.Field` isn't sufficient.
+
+For example, if you have a ``GPSCoordinatesField``, and want to be able to
 access additional information about the coordinates in a template, this could
 be implemented as follows::
 
@@ -1523,12 +1543,74 @@ be implemented as follows::
 
 
     class GPSCoordinatesField(Field):
-        def get_bound_field(self, form, field_name):
-            return GPSCoordinatesBoundField(form, self, field_name)
+        bound_field_class = GPSCoordinatesBoundField
 
 Now you can access the country in a template with
 ``{{ form.coordinates.country }}``.
 
+You may also want to customize the default form field template rendering. For
+example, you can override :meth:`.BoundField.label_tag` to add a custom class::
+
+   class StyledLabelBoundField(BoundField):
+       def label_tag(self, contents=None, attrs=None, label_suffix=None, tag=None):
+           attrs = attrs or {}
+           attrs["class"] = "wide"
+           return super().label_tag(contents, attrs, label_suffix, tag)
+
+
+   class UserForm(forms.Form):
+       bound_field_class = StyledLabelBoundField
+       name = CharField()
+
+This would update the default form rendering:
+
+.. code-block:: pycon
+
+   >>> f = UserForm()
+   >>> print(f["name"].label_tag)
+   <label for="id_name" class="wide">Name:</label>
+
+To add a CSS class to the wrapping HTML element of all fields, a ``BoundField``
+can be overridden to return a different collection of CSS classes::
+
+    class WrappedBoundField(BoundField):
+        def css_classes(self, extra_classes=None):
+            parent_css_classes = super().css_classes(extra_classes)
+            return f"field-class {parent_css_classes}".strip()
+
+
+    class UserForm(forms.Form):
+        bound_field_class = WrappedBoundField
+        name = CharField()
+
+This would update the form rendering as follows:
+
+.. code-block:: pycon
+
+   >>> f = UserForm()
+   >>> print(f)
+   <div class="field-class"><label for="id_name">Name:</label><input type="text" name="name" required id="id_name"></div>
+
+Alternatively, to override the ``BoundField`` class at the project level,
+:attr:`.BaseRenderer.bound_field_class` can be defined on a custom
+:setting:`FORM_RENDERER`:
+
+.. code-block:: python
+    :caption: ``mysite/renderers.py``
+
+    from django.forms.renderers import DjangoTemplates
+
+    from .forms import CustomBoundField
+
+
+    class CustomRenderer(DjangoTemplates):
+        bound_field_class = CustomBoundField
+
+.. code-block:: python
+    :caption: ``settings.py``
+
+    FORM_RENDERER = "mysite.renderers.CustomRenderer"
+
 .. _binding-uploaded-files:
 
 Binding uploaded files to a form

docs/ref/forms/fields.txt

@@ -397,6 +397,16 @@ default this value is set to ``"django/forms/field.html"``. Can be changed per
 field by overriding this attribute or more generally by overriding the default
 template, see also :ref:`overriding-built-in-field-templates`.
 
+``bound_field_class``
+---------------------
+
+.. attribute:: Field.bound_field_class
+
+.. versionadded:: 5.2
+
+The ``bound_field_class`` attribute allows a per-field override of
+:attr:`.Form.bound_field_class`.
+
 Checking if the field data has changed
 ======================================
 
@@ -1635,4 +1645,14 @@ only requirements are that it implement a ``clean()`` method and that its
 ``label``, ``initial``, ``widget``, ``help_text``).
 
 You can also customize how a field will be accessed by overriding
-:meth:`~django.forms.Field.get_bound_field()`.
+:attr:`~django.forms.Field.bound_field_class` or override
+:meth:`.Field.get_bound_field()` if you need more flexibility when creating
+the ``BoundField``:
+
+.. method:: Field.get_bound_field(form, field_name)
+
+    Takes an instance of :class:`~django.forms.Form` and the name of the field.
+    The returned :class:`.BoundField` instance will be used when accessing the
+    field in a template.
+
+See :ref:`custom-boundfield` for examples of overriding a ``BoundField``.

docs/ref/forms/renderers.txt

@@ -65,6 +65,18 @@ should return a rendered templates (as a string) or raise
 
         Defaults to ``"django/forms/field.html"``
 
+    .. attribute:: bound_field_class
+
+        .. versionadded:: 5.2
+
+        The default class used to represent form fields across the project.
+
+        Defaults to :class:`.BoundField` class.
+
+        This can be customized further using :attr:`.Form.bound_field_class`
+        for per-form overrides, or :attr:`.Field.bound_field_class` for
+        per-field overrides.
+
     .. method:: get_template(template_name)
 
         Subclasses must implement this method with the appropriate template

docs/releases/5.2.txt

@@ -70,6 +70,62 @@ to be a ``CompositePrimaryKey``::
 
 See :doc:`/topics/composite-primary-key` for more details.
 
+Simplified override of :class:`~django.forms.BoundField`
+--------------------------------------------------------
+
+Prior to version 5.2, overriding :meth:`.Field.get_bound_field()` was the only
+option to use a custom :class:`~django.forms.BoundField`. Django now supports
+specifying the following attributes to customize form rendering:
+
+* :attr:`.BaseRenderer.bound_field_class` at the project level,
+* :attr:`.Form.bound_field_class` at the form level, and
+* :attr:`.Field.bound_field_class` at the field level.
+
+For example, to customize the ``BoundField`` of a ``Form`` class::
+
+    from django.forms import Form
+
+
+    class CustomBoundField(forms.BoundField):
+
+        custom_class = "custom"
+
+        def css_classes(self, extra_classes=None):
+            result = super().css_classes(extra_classes)
+            if self.custom_class not in result:
+                result += f" {self.custom_class}"
+            return result.strip()
+
+
+    class CustomForm(forms.Form):
+        bound_field_class = CustomBoundField
+
+        name = forms.CharField(
+            label="Your Name",
+            max_length=100,
+            required=False,
+            widget=forms.TextInput(attrs={"class": "name-input-class"}),
+        )
+        email = forms.EmailField(label="Your Email")
+
+
+When rendering a ``CustomForm`` instance, the following HTML is included:
+
+.. code:: html
+
+    <div class="custom">
+      <label for="id_name">Your Name:</label>
+      <input type="text" name="name" class="name-input-class" maxlength="100" id="id_name">
+    </div>
+
+    <div class="custom">
+      <label for="id_email">Your Email:</label>
+      <input type="email" name="email" maxlength="320" required="" id="id_email">
+    </div>
+
+
+See :ref:`custom-boundfield` for more details about this feature.
+
 Minor features
 --------------
 

tests/forms_tests/tests/test_forms.py

@@ -8,6 +8,7 @@ from django.core.files.uploadedfile import SimpleUploadedFile
 from django.core.validators import MaxValueValidator, RegexValidator
 from django.forms import (
     BooleanField,
+    BoundField,
     CharField,
     CheckboxSelectMultiple,
     ChoiceField,
@@ -4971,6 +4972,22 @@ class RendererTests(SimpleTestCase):
         context = form.get_context()
         self.assertEqual(context["errors"].renderer, custom)
 
+    def test_boundfield_fallback(self):
+        class RendererWithoutBoundFieldClassAttribute:
+            form_template_name = "django/forms/div.html"
+            formset_template_name = "django/forms/formsets/div.html"
+            field_template_name = "django/forms/field.html"
+
+            def render(self, template_name, context, request=None):
+                return "Nice"
+
+        class UserForm(Form):
+            name = CharField()
+
+        form = UserForm(renderer=RendererWithoutBoundFieldClassAttribute())
+        self.assertIsInstance(form["name"], BoundField)
+        self.assertEqual(form["name"].as_field_group(), "Nice")
+
 
 class TemplateTests(SimpleTestCase):
     def test_iterate_radios(self):
@@ -5473,3 +5490,146 @@ class OverrideTests(SimpleTestCase):
             '<label for="id_name" class="required">Name:</label>'
             '<legend class="required">Language:</legend>',
         )
+
+
+class BoundFieldWithoutColon(BoundField):
+    def label_tag(self, contents=None, attrs=None, label_suffix=None, tag=None):
+        return super().label_tag(
+            contents=contents, attrs=attrs, label_suffix="", tag=None
+        )
+
+
+class BoundFieldWithTwoColons(BoundField):
+    def label_tag(self, contents=None, attrs=None, label_suffix=None, tag=None):
+        return super().label_tag(
+            contents=contents, attrs=attrs, label_suffix="::", tag=None
+        )
+
+
+class BoundFieldWithCustomClass(BoundField):
+    def label_tag(self, contents=None, attrs=None, label_suffix=None, tag=None):
+        attrs = attrs or {}
+        attrs["class"] = "custom-class"
+        return super().label_tag(contents, attrs, label_suffix, tag)
+
+
+class BoundFieldWithWrappingClass(BoundField):
+    def css_classes(self, extra_classes=None):
+        parent_classes = super().css_classes(extra_classes)
+        return f"field-class {parent_classes}"
+
+
+class BoundFieldOverrideRenderer(DjangoTemplates):
+    bound_field_class = BoundFieldWithoutColon
+
+
+@override_settings(
+    FORM_RENDERER="forms_tests.tests.test_forms.BoundFieldOverrideRenderer"
+)
+class CustomBoundFieldTest(SimpleTestCase):
+    def test_renderer_custom_bound_field(self):
+        t = Template("{{ form }}")
+        html = t.render(Context({"form": Person()}))
+        expected = """
+            <div><label for="id_first_name">First name</label>
+            <input type="text" name="first_name" required
+            id="id_first_name"></div>
+            <div><label for="id_last_name">Last name</label>
+            <input type="text" name="last_name" required
+            id="id_last_name"></div><div>
+            <label for="id_birthday">Birthday</label>
+            <input type="text" name="birthday" required
+            id="id_birthday"></div>"""
+        self.assertHTMLEqual(html, expected)
+
+    def test_form_custom_boundfield(self):
+        class CustomBoundFieldPerson(Person):
+            bound_field_class = BoundFieldWithTwoColons
+
+        with self.subTest("form's BoundField takes over renderer's BoundField"):
+            t = Template("{{ form }}")
+            html = t.render(Context({"form": CustomBoundFieldPerson()}))
+            expected = """
+                <div><label for="id_first_name">First name::</label>
+                <input type="text" name="first_name" required
+                id="id_first_name"></div>
+                <div><label for="id_last_name">Last name::</label>
+                <input type="text" name="last_name" required
+                id="id_last_name"></div><div>
+                <label for="id_birthday">Birthday::</label>
+                <input type="text" name="birthday" required
+                id="id_birthday"></div>"""
+            self.assertHTMLEqual(html, expected)
+
+        with self.subTest("Constructor argument takes over class property"):
+            t = Template("{{ form }}")
+            html = t.render(
+                Context(
+                    {
+                        "form": CustomBoundFieldPerson(
+                            bound_field_class=BoundFieldWithCustomClass
+                        )
+                    }
+                )
+            )
+            expected = """
+                <div><label class="custom-class" for="id_first_name">First name:</label>
+                <input type="text" name="first_name" required
+                id="id_first_name"></div>
+                <div><label class="custom-class" for="id_last_name">Last name:</label>
+                <input type="text" name="last_name" required
+                id="id_last_name"></div><div>
+                <label class="custom-class" for="id_birthday">Birthday:</label>
+                <input type="text" name="birthday" required
+                id="id_birthday"></div>"""
+            self.assertHTMLEqual(html, expected)
+
+        with self.subTest("Overriding css_classes works as expected"):
+            t = Template("{{ form }}")
+            html = t.render(
+                Context(
+                    {
+                        "form": CustomBoundFieldPerson(
+                            bound_field_class=BoundFieldWithWrappingClass
+                        )
+                    }
+                )
+            )
+            expected = """
+                <div class="field-class"><label for="id_first_name">First name:</label>
+                <input type="text" name="first_name" required
+                id="id_first_name"></div>
+                <div class="field-class"><label for="id_last_name">Last name:</label>
+                <input type="text" name="last_name" required
+                id="id_last_name"></div><div class="field-class">
+                <label for="id_birthday">Birthday:</label>
+                <input type="text" name="birthday" required
+                id="id_birthday"></div>"""
+            self.assertHTMLEqual(html, expected)
+
+    def test_field_custom_bound_field(self):
+        class BoundFieldWithTwoColonsCharField(CharField):
+            bound_field_class = BoundFieldWithTwoColons
+
+        class CustomFieldBoundFieldPerson(Person):
+            bound_field_class = BoundField
+
+            first_name = BoundFieldWithTwoColonsCharField()
+            last_name = BoundFieldWithTwoColonsCharField(
+                bound_field_class=BoundFieldWithCustomClass
+            )
+
+        html = Template("{{ form }}").render(
+            Context({"form": CustomFieldBoundFieldPerson()})
+        )
+        expected = """
+            <div><label for="id_first_name">First name::</label>
+            <input type="text" name="first_name" required
+            id="id_first_name"></div>
+            <div><label class="custom-class" for="id_last_name">Last name:</label>
+            <input type="text" name="last_name" required
+            id="id_last_name"></div><div>
+            <label for="id_birthday">Birthday:</label>
+            <input type="text" name="birthday" required
+            id="id_birthday"></div>"""
+        self.assertHTMLEqual(html, expected)