Providing meaningful error message if you forget many=True. Closes #1914.

rest_framework/serializers.py

@@ -70,6 +70,7 @@ class BaseSerializer(Field):
         self.partial = kwargs.pop('partial', False)
         self._context = kwargs.pop('context', {})
         kwargs.pop('many', None)
+        kwargs.pop('many_init', None)
         super(BaseSerializer, self).__init__(**kwargs)
 
     def __new__(cls, *args, **kwargs):
@@ -77,6 +78,14 @@ class BaseSerializer(Field):
         # `ListSerializer` classes instead when `many=True` is set.
         if kwargs.pop('many', False):
             return cls.many_init(*args, **kwargs)
+        if not kwargs.pop('many_init', False):
+            if not issubclass(cls, ListSerializer):
+                instance = kwargs.get('instance', args[0] if args else None)
+                if isinstance(instance, (list, tuple, models.QuerySet)):
+                    msg = (
+                        'You have passed a %s as `instance` argument but did '
+                        'not set `many=True`.' % instance.__class__.__name__)
+                    raise AssertionError(msg)
         return super(BaseSerializer, cls).__new__(cls, *args, **kwargs)
 
     @classmethod
@@ -87,7 +96,7 @@ class BaseSerializer(Field):
         control which keyword arguments are passed to the parent, and
         which are passed to the child.
         """
-        child_serializer = cls(*args, **kwargs)
+        child_serializer = cls(many_init=True, *args, **kwargs)
         list_kwargs = {'child': child_serializer}
         list_kwargs.update(dict([
             (key, value) for key, value in kwargs.items()

tests/test_serializer.py

@@ -1,3 +1,4 @@
+from django.db.models.query import QuerySet
 from rest_framework import serializers
 import pytest
 
@@ -42,6 +43,18 @@ class TestSerializer:
         with pytest.raises(AttributeError):
             serializer.data
 
+    def test_missing_many_kwarg(self):
+        self.Serializer([], many=True)
+        with pytest.raises(AssertionError):
+            self.Serializer([])
+        with pytest.raises(AssertionError):
+            self.Serializer(())
+        with pytest.raises(AssertionError):
+            self.Serializer(QuerySet())
+        self.Serializer(None)
+        self.Serializer(object())
+        self.Serializer({})
+
 
 class TestValidateMethod:
     def test_non_field_error_validate_method(self):