Hide UNSET in the callback

Refs: #3030 (comment)

Author: kdeldycke

CHANGES.rst

@@ -13,8 +13,6 @@ Unreleased
     This allow ``default`` to be of any type, including ``bool`` or ``None``, fixing
     inconsistencies reported in: :issue:`1992` :issue:`2012` :issue:`2514`
     :issue:`2610` :issue:`3024` :pr:`3030`
--   Custom ``callback`` function are now allowed to receive the ``UNSET`` sentinel
-    value.  :pr:`3030`
 -   Allow ``default`` to be set on ``Argument`` for ``nargs = -1``. :issue:`2164`
     :pr:`3030`
 -   Show correct auto complete value for ``nargs`` option in combination with flag

src/click/core.py

@@ -2363,12 +2363,9 @@ def process_value(self, ctx: Context, value: t.Any) -> t.Any:
         2. Check if the value is missing (see: :meth:`value_is_missing`), and raise
            :exc:`MissingParameter` if it is required.
         3. If a :attr:`callback` is set, call it to have the value replaced by the
-           result of the callback. The callback is receiving the value as-is, which let
-           the developer decide how to handle the different cases.
-
-        .. versionchanged:: 8.3
-            The :attr:`callback` gets an internal sentinel if the parameter was not set
-            by the user and no default was specified.
+           result of the callback. If the value was not set, the callback receive
+           ``None``. This keep the legacy behavior as it was before the introduction of
+           the :attr:`UNSET` sentinel.
 
         :meta private:
         """
@@ -2378,6 +2375,10 @@ def process_value(self, ctx: Context, value: t.Any) -> t.Any:
             raise MissingParameter(ctx=ctx, param=self)
 
         if self.callback is not None:
+            # Legacy case: UNSET is not exposed directly to the callback, but converted
+            # to None.
+            if value is UNSET:
+                value = None
             value = self.callback(ctx, self, value)
 
         return value

tests/test_options.py

@@ -1478,7 +1478,8 @@ def cmd(foo):
         ("xMl", [], "xMl"),
         (" ᕕ( ᐛ )ᕗ ", [], " ᕕ( ᐛ )ᕗ "),
         (None, [], None),
-        (UNSET, [], UNSET),
+        # Legacy case: UNSET is not exposed directly to the callback, but converted to None.
+        (UNSET, [], None),
         # Legacy case: if default=True and flag_value is set, The value returned is the
         # flag_value, not default itself.
         (True, [], "js"),