Drop set_context() in favour of 'requires_context = True'

docs/api-guide/fields.md

@@ -50,7 +50,19 @@ If set, this gives the default value that will be used for the field if no input
 
 The `default` is not applied during partial update operations. In the partial update case only fields that are provided in the incoming data will have a validated value returned.
 
-May be set to a function or other callable, in which case the value will be evaluated each time it is used. When called, it will receive no arguments. If the callable has a `set_context` method, that will be called each time before getting the value with the field instance as only argument.
+May be set to a function or other callable, in which case the value will be evaluated each time it is used. When called, it will receive no arguments. If the callable has a `requires_context = True` attribute, then the serializer field will be passed as an argument.
+
+For example:
+
+    class CurrentUserDefault:
+        """
+        May be applied as a `default=...` value on a serializer field.
+        Returns the current user.
+        """
+        requires_context = True
+
+        def __call__(self, serializer_field):
+            return serializer_field.context['request'].user
 
 When serializing the instance, default will be used if the object attribute or dictionary key is not present in the instance.
 

docs/api-guide/validators.md

@@ -294,15 +294,14 @@ To write a class-based validator, use the `__call__` method. Class-based validat
 #### Accessing the context
 
 In some advanced cases you might want a validator to be passed the serializer
-field it is being used with as additional context. You can do so by using
+field it is being used with as additional context. You can do so by setting
-`rest_framework.validators.ContextBasedValidator` as a base class for the
+a `requires_context = True` attribute on the validator. The `__call__` method
-validator. The `__call__` method will then be called with the `serializer_field`
+will then be called with the `serializer_field`
 or `serializer` as an additional argument.
 
-    def __call__(self, value, serializer_field):
+    requires_context = True
-        # Determine if this is an update or a create operation.
-        is_update = serializer_field.parent.instance is not None
 
-        pass  # implementation of the validator that uses `is_update`
+    def __call__(self, value, serializer_field):
+        ...
 
 [cite]: https://docs.djangoproject.com/en/stable/ref/validators/

rest_framework/fields.py

@@ -250,18 +250,29 @@ class CreateOnlyDefault:
     for create operations, but that do not return any value for update
     operations.
     """
+    requires_context = True
+
     def __init__(self, default):
         self.default = default
 
-    def set_context(self, serializer_field):
+    def __call__(self, serializer_field):
-        self.is_update = serializer_field.parent.instance is not None
+        is_update = serializer_field.parent.instance is not None
-        if callable(self.default) and hasattr(self.default, 'set_context') and not self.is_update:
+        if is_update:
-            self.default.set_context(serializer_field)
-
-    def __call__(self):
-        if self.is_update:
             raise SkipField()
         if callable(self.default):
+            if hasattr(self.default, 'set_context'):
+                warnings.warn(
+                    "Method `set_context` on defaults is deprecated and will "
+                    "no longer be called starting with 3.12. Instead set "
+                    "`requires_context = True` on the class, and accept the "
+                    "context as an additional argument.",
+                    DeprecationWarning, stacklevel=2
+                )
+                self.default.set_context(self)
+
+            if getattr(self.default, 'requires_context', False):
+                return self.default(serializer_field)
+            else:
                 return self.default()
         return self.default
 
@@ -270,11 +281,10 @@ class CreateOnlyDefault:
 
 
 class CurrentUserDefault:
-    def set_context(self, serializer_field):
+    requires_context = True
-        self.user = serializer_field.context['request'].user
 
-    def __call__(self):
+    def __call__(self, serializer_field):
-        return self.user
+        return serializer_field.context['request'].user
 
     def __repr__(self):
         return '%s()' % self.__class__.__name__
@@ -490,8 +500,20 @@ class Field:
             raise SkipField()
         if callable(self.default):
             if hasattr(self.default, 'set_context'):
+                warnings.warn(
+                    "Method `set_context` on defaults is deprecated and will "
+                    "no longer be called starting with 3.12. Instead set "
+                    "`requires_context = True` on the class, and accept the "
+                    "context as an additional argument.",
+                    DeprecationWarning, stacklevel=2
+                )
                 self.default.set_context(self)
+
+            if getattr(self.default, 'requires_context', False):
+                return self.default(self)
+            else:
                 return self.default()
+
         return self.default
 
     def validate_empty_values(self, data):
@@ -549,22 +571,20 @@ class Field:
         Test the given value against all the validators on the field,
         and either raise a `ValidationError` or simply return.
         """
-        from rest_framework.validators import ContextBasedValidator
-
         errors = []
         for validator in self.validators:
             if hasattr(validator, 'set_context'):
                 warnings.warn(
                     "Method `set_context` on validators is deprecated and will "
-                    "no longer be called starting with 3.11. Instead derive the "
+                    "no longer be called starting with 3.12. Instead set "
-                    "validator from `rest_framwork.validators.ContextBasedValidator` "
+                    "`requires_context = True` on the class, and accept the "
-                    "and accept the context as an additional argument.",
+                    "context as an additional argument.",
                     DeprecationWarning, stacklevel=2
                 )
                 validator.set_context(self)
 
             try:
-                if isinstance(validator, ContextBasedValidator):
+                if getattr(validator, 'requires_context', False):
                     validator(value, self)
                 else:
                     validator(value)

rest_framework/validators.py

@@ -30,23 +30,14 @@ def qs_filter(queryset, **kwargs):
         return queryset.none()
 
 
-class ContextBasedValidator:
+class UniqueValidator:
-    """Base class for validators that need a context during evaluation.
-
-    In extension to regular validators their `__call__` method must not only
-    accept a value, but also an instance of a serializer.
-    """
-    def __call__(self, value, serializer):
-        raise NotImplementedError('`__call__()` must be implemented.')
-
-
-class UniqueValidator(ContextBasedValidator):
     """
     Validator that corresponds to `unique=True` on a model field.
 
     Should be applied to an individual field on the serializer.
     """
     message = _('This field must be unique.')
+    requires_context = True
 
     def __init__(self, queryset, message=None, lookup='exact'):
         self.queryset = queryset
@@ -90,7 +81,7 @@ class UniqueValidator(ContextBasedValidator):
         )
 
 
-class UniqueTogetherValidator(ContextBasedValidator):
+class UniqueTogetherValidator:
     """
     Validator that corresponds to `unique_together = (...)` on a model class.
 
@@ -98,6 +89,7 @@ class UniqueTogetherValidator(ContextBasedValidator):
     """
     message = _('The fields {field_names} must make a unique set.')
     missing_message = _('This field is required.')
+    requires_context = True
 
     def __init__(self, queryset, fields, message=None):
         self.queryset = queryset
@@ -174,9 +166,10 @@ class UniqueTogetherValidator(ContextBasedValidator):
         )
 
 
-class BaseUniqueForValidator(ContextBasedValidator):
+class BaseUniqueForValidator:
     message = None
     missing_message = _('This field is required.')
+    requires_context = True
 
     def __init__(self, queryset, field, date_field, message=None):
         self.queryset = queryset