Raise exception when field source is a built-in (#6766)

rest_framework/fields.py

@@ -47,10 +47,24 @@ class empty:
     pass
 
 
+class BuiltinSignatureError(Exception):
+    """
+    Built-in function signatures are not inspectable. This exception is raised
+    so the serializer can raise a helpful error message.
+    """
+    pass
+
+
 def is_simple_callable(obj):
     """
     True if the object is a callable that takes no arguments.
     """
+    # Bail early since we cannot inspect built-in function signatures.
+    if inspect.isbuiltin(obj):
+        raise BuiltinSignatureError(
+            'Built-in function signatures are not inspectable. '
+            'Wrap the function call in a simple, pure Python function.')
+
     if not (inspect.isfunction(obj) or inspect.ismethod(obj) or isinstance(obj, functools.partial)):
         return False
 
@@ -427,6 +441,18 @@ class Field:
         """
         try:
             return get_attribute(instance, self.source_attrs)
+        except BuiltinSignatureError as exc:
+            msg = (
+                'Field source for `{serializer}.{field}` maps to a built-in '
+                'function type and is invalid. Define a property or method on '
+                'the `{instance}` instance that wraps the call to the built-in '
+                'function.'.format(
+                    serializer=self.parent.__class__.__name__,
+                    field=self.field_name,
+                    instance=instance.__class__.__name__,
+                )
+            )
+            raise type(exc)(msg)
         except (KeyError, AttributeError) as exc:
             if self.default is not empty:
                 return self.get_default()

tests/test_fields.py

@@ -14,7 +14,9 @@ from django.utils.timezone import activate, deactivate, override, utc
 import rest_framework
 from rest_framework import exceptions, serializers
 from rest_framework.compat import ProhibitNullCharactersValidator
-from rest_framework.fields import DjangoImageField, is_simple_callable
+from rest_framework.fields import (
+    BuiltinSignatureError, DjangoImageField, is_simple_callable
+)
 
 # Tests for helper functions.
 # ---------------------------
@@ -86,6 +88,18 @@ class TestIsSimpleCallable:
 
         assert is_simple_callable(ChoiceModel().get_choice_field_display)
 
+    def test_builtin_function(self):
+        # Built-in function signatures are not easily inspectable, so the
+        # current expectation is to just raise a helpful error message.
+        timestamp = datetime.datetime.now()
+
+        with pytest.raises(BuiltinSignatureError) as exc_info:
+            is_simple_callable(timestamp.date)
+
+        assert str(exc_info.value) == (
+            'Built-in function signatures are not inspectable. Wrap the '
+            'function call in a simple, pure Python function.')
+
     def test_type_annotation(self):
         # The annotation will otherwise raise a syntax error in python < 3.5
         locals = {}
@@ -206,6 +220,18 @@ class TestSource:
 
         assert 'method call failed' in str(exc_info.value)
 
+    def test_builtin_callable_source_raises(self):
+        class BuiltinSerializer(serializers.Serializer):
+            date = serializers.ReadOnlyField(source='timestamp.date')
+
+        with pytest.raises(BuiltinSignatureError) as exc_info:
+            BuiltinSerializer({'timestamp': datetime.datetime.now()}).data
+
+        assert str(exc_info.value) == (
+            'Field source for `BuiltinSerializer.date` maps to a built-in '
+            'function type and is invalid. Define a property or method on '
+            'the `dict` instance that wraps the call to the built-in function.')
+
 
 class TestReadOnly:
     def setup(self):