Drop set_context() (#7062)

* Do not persist the context in validators

Fixes #5760

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

Author: tomchristie

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. This works the same way as for [validators](validators.md#using-set_context).
+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

@@ -291,13 +291,17 @@ To write a class-based validator, use the `__call__` method. Class-based validat
                 message = 'This field must be a multiple of %d.' % self.base
                 raise serializers.ValidationError(message)
 
-#### Using `set_context()`
+#### 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 declaring a `set_context` method on a class-based validator.
+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 setting
+a `requires_context = True` attribute on the validator. The `__call__` method
+will then be called with the `serializer_field`
+or `serializer` as an additional argument.
 
-    def set_context(self, serializer_field):
-        # Determine if this is an update or a create operation.
-        # In `__call__` we can then use that information to modify the validation behavior.
-        self.is_update = serializer_field.parent.instance is not None
+    requires_context = True
+
+    def __call__(self, value, serializer_field):
+        ...
 
 [cite]: https://docs.djangoproject.com/en/stable/ref/validators/

rest_framework/fields.py

@@ -5,6 +5,7 @@
 import inspect
 import re
 import uuid
+import warnings
 from collections import OrderedDict
 from collections.abc import Mapping
 
@@ -249,31 +250,41 @@ 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):
-        self.is_update = serializer_field.parent.instance is not None
-        if callable(self.default) and hasattr(self.default, 'set_context') and not self.is_update:
-            self.default.set_context(serializer_field)
-
-    def __call__(self):
-        if self.is_update:
+    def __call__(self, serializer_field):
+        is_update = serializer_field.parent.instance is not None
+        if is_update:
             raise SkipField()
         if callable(self.default):
-            return 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
 
     def __repr__(self):
         return '%s(%s)' % (self.__class__.__name__, repr(self.default))
 
 
 class CurrentUserDefault:
-    def set_context(self, serializer_field):
-        self.user = serializer_field.context['request'].user
+    requires_context = True
 
-    def __call__(self):
-        return self.user
+    def __call__(self, serializer_field):
+        return serializer_field.context['request'].user
 
     def __repr__(self):
         return '%s()' % self.__class__.__name__
@@ -489,8 +500,20 @@ def get_default(self):
             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)
-            return self.default()
+
+            if getattr(self.default, 'requires_context', False):
+                return self.default(self)
+            else:
+                return self.default()
+
         return self.default
 
     def validate_empty_values(self, data):
@@ -551,10 +574,20 @@ def run_validators(self, value):
         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 3.12. Instead set "
+                    "`requires_context = True` on the class, and accept the "
+                    "context as an additional argument.",
+                    DeprecationWarning, stacklevel=2
+                )
                 validator.set_context(self)
 
             try:
-                validator(value)
+                if getattr(validator, 'requires_context', False):
+                    validator(value, self)
+                else:
+                    validator(value)
             except ValidationError as exc:
                 # If the validation error contains a mapping of fields to
                 # errors then simply raise it immediately rather than