Closes #3937. Support callable as the value of initial for any serializer.Field

docs/api-guide/fields.md

@@ -81,7 +81,13 @@ A text string that may be used as a description of the field in HTML form fields
 
 ### `initial`
 
-A value that should be used for pre-populating the value of HTML form fields.
+A value that should be used for pre-populating the value of HTML form fields. You may pass a callable to it, just as
+you may do with any regular Django `Field`:
+
+    def user_default_color():
+        return 'blue'
+
+    color = serializers.CharField(initial=user_default_color)
 
 ### `style`
 

rest_framework/fields.py

@@ -370,6 +370,8 @@ def get_initial(self):
         Return a value to use when the field is being returned as a primitive
         value, without any object instance.
         """
+        if callable(self.initial):
+            return self.initial()
         return self.initial
 
     def get_value(self, dictionary):

tests/test_fields.py

@@ -191,6 +191,24 @@ def test_initial(self):
         }
 
 
+class TestInitialWithCallable:
+    def setup(self):
+        def initial_value():
+            return 123
+
+        class TestSerializer(serializers.Serializer):
+            initial_field = serializers.IntegerField(initial=initial_value)
+        self.serializer = TestSerializer()
+
+    def test_initial_should_accept_callable(self):
+        """
+        Follows the default ``Field.initial`` behaviour where they accept a
+        callable to produce the initial value"""
+        assert self.serializer.data == {
+            'initial_field': 123,
+        }
+
+
 class TestLabel:
     def setup(self):
         class TestSerializer(serializers.Serializer):