Fixed #26533 -- Renamed Widget._format_value() to format_value().

docs/internals/deprecation.txt

@@ -163,6 +163,8 @@ details on these changes.
 * In multi-table inheritance, implicit promotion of a ``OneToOneField`` to a
   ``parent_link`` will be removed.
 
+* Support for ``Widget._format_value()`` will be removed.
+
 .. _deprecation-removed-in-1.10:
 
 1.10

docs/ref/forms/widgets.txt

@@ -228,6 +228,17 @@ foundation for custom widgets.
            In older versions, this attribute was only defined on the date
            and time widgets (as ``False``).
 
+    .. method:: format_value(value)
+
+        Cleans and returns a value for use in the widget template. ``value``
+        isn't guaranteed to be valid input, therefore subclass implementations
+        should program defensively.
+
+        .. versionchanged:: 1.10
+
+            In older versions, this method is a private API named
+            ``_format_value()``. The old name will work until Django 2.0.
+
     .. method:: id_for_label(self, id_)
 
         Returns the HTML ID attribute of this widget for use by a ``<label>``,

docs/releases/1.10.txt

@@ -983,6 +983,10 @@ Miscellaneous
 * In multi-table inheritance, implicit promotion of a ``OneToOneField`` to a
   ``parent_link`` is deprecated. Add ``parent_link=True`` to such fields.
 
+* The private API ``Widget._format_value()`` is made public and renamed to
+  :meth:`~django.forms.Widget.format_value`. The old name will work
+  through a deprecation period.
+
 .. _removed-features-1.10:
 
 Features removed in 1.10