Fix is_simple_callable with variable args, kwargs (#4622)

rest_framework/fields.py

@@ -54,12 +54,17 @@ if six.PY3:
         """
         True if the object is a callable that takes no arguments.
         """
-        if not callable(obj):
+        if not (inspect.isfunction(obj) or inspect.ismethod(obj)):
             return False
 
         sig = inspect.signature(obj)
         params = sig.parameters.values()
-        return all(param.default != param.empty for param in params)
+        return all(
+            param.kind == param.VAR_POSITIONAL or
+            param.kind == param.VAR_KEYWORD or
+            param.default != param.empty
+            for param in params
+        )
 
 else:
     def is_simple_callable(obj):

tests/test_fields.py

@@ -37,6 +37,9 @@ class TestIsSimpleCallable:
             def valid_kwargs(self, param='value'):
                 pass
 
+            def valid_vargs_kwargs(self, *args, **kwargs):
+                pass
+
             def invalid(self, param):
                 pass
 
@@ -45,11 +48,13 @@ class TestIsSimpleCallable:
         # unbound methods
         assert not is_simple_callable(Foo.valid)
         assert not is_simple_callable(Foo.valid_kwargs)
+        assert not is_simple_callable(Foo.valid_vargs_kwargs)
         assert not is_simple_callable(Foo.invalid)
 
         # bound methods
         assert is_simple_callable(Foo().valid)
         assert is_simple_callable(Foo().valid_kwargs)
+        assert is_simple_callable(Foo().valid_vargs_kwargs)
         assert not is_simple_callable(Foo().invalid)
 
     def test_function(self):
@@ -59,13 +64,31 @@ class TestIsSimpleCallable:
         def valid(param='value', param2='value'):
             pass
 
+        def valid_vargs_kwargs(*args, **kwargs):
+            pass
+
         def invalid(param, param2='value'):
             pass
 
         assert is_simple_callable(simple)
         assert is_simple_callable(valid)
+        assert is_simple_callable(valid_vargs_kwargs)
         assert not is_simple_callable(invalid)
 
+    def test_4602_regression(self):
+        from django.db import models
+
+        class ChoiceModel(models.Model):
+            choice_field = models.CharField(
+                max_length=1, default='a',
+                choices=(('a', 'A'), ('b', 'B')),
+            )
+
+            class Meta:
+                app_label = 'tests'
+
+        assert is_simple_callable(ChoiceModel().get_choice_field_display)
+
     @unittest.skipUnless(typings, 'requires python 3.5')
     def test_type_annotation(self):
         # The annotation will otherwise raise a syntax error in python < 3.5