Moved EmpytResultSet to django.core.exceptions.

This removes the need for some inner imports.

django/core/exceptions.py

@@ -187,3 +187,8 @@ class ValidationError(Exception):
 
     def __repr__(self):
         return 'ValidationError(%s)' % self
+
+
+class EmptyResultSet(Exception):
+    """A database query predicate is impossible."""
+    pass

django/db/models/expressions.py

 import copy
 import datetime
 
-from django.core.exceptions import FieldError
+from django.core.exceptions import EmptyResultSet, FieldError
 from django.db.backends import utils as backend_utils
 from django.db.models import fields
 from django.db.models.query_utils import Q
@@ -836,7 +836,6 @@ class Case(Expression):
         return c
 
     def as_sql(self, compiler, connection, template=None, case_joiner=None, **extra_context):
-        from django.db.models.sql.datastructures import EmptyResultSet
         connection.ops.check_expression_support(self)
         if not self.cases:
             return compiler.compile(self.default)

django/db/models/lookups.py

@@ -2,6 +2,7 @@ import math
 import warnings
 from copy import copy
 
+from django.core.exceptions import EmptyResultSet
 from django.db.models.expressions import Func, Value
 from django.db.models.fields import DateTimeField, Field, IntegerField
 from django.db.models.query_utils import RegisterLookupMixin
@@ -276,7 +277,6 @@ class In(FieldGetDbPrepValueIterableMixin, BuiltinLookup):
                 rhs = self.rhs
 
             if not rhs:
-                from django.db.models.sql.datastructures import EmptyResultSet
                 raise EmptyResultSet
 
             # rhs should be an iterable; use batch_process_rhs() to

django/db/models/sql/__init__.py

-from django.db.models.sql.datastructures import EmptyResultSet
+from django.core.exceptions import EmptyResultSet
 from django.db.models.sql.query import *  # NOQA
 from django.db.models.sql.subqueries import *  # NOQA
 from django.db.models.sql.where import AND, OR

django/db/models/sql/compiler.py

 import re
 from itertools import chain
 
-from django.core.exceptions import FieldError
+from django.core.exceptions import EmptyResultSet, FieldError
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.expressions import OrderBy, Random, RawSQL, Ref
 from django.db.models.query_utils import QueryWrapper, select_related_descend
 from django.db.models.sql.constants import (
     CURSOR, GET_ITERATOR_CHUNK_SIZE, MULTI, NO_RESULTS, ORDER_DIR, SINGLE,
 )
-from django.db.models.sql.datastructures import EmptyResultSet
 from django.db.models.sql.query import Query, get_order_dir
 from django.db.transaction import TransactionManagementError
 from django.db.utils import DatabaseError

django/db/models/sql/datastructures.py

@@ -2,13 +2,11 @@
 Useful auxiliary data structures for query construction. Not useful outside
 the SQL domain.
 """
+# for backwards-compatibility in Django 1.11
+from django.core.exceptions import EmptyResultSet  # NOQA: F401
 from django.db.models.sql.constants import INNER, LOUTER
 
 
-class EmptyResultSet(Exception):
-    pass
-
-
 class MultiJoin(Exception):
     """
     Used by join construction code to indicate the point at which a

django/db/models/sql/where.py

@@ -2,7 +2,7 @@
 Code to manage the creation and SQL rendering of 'where' constraints.
 """
 
-from django.db.models.sql.datastructures import EmptyResultSet
+from django.core.exceptions import EmptyResultSet
 from django.utils import tree
 from django.utils.functional import cached_property
 

docs/ref/exceptions.txt

@@ -24,6 +24,19 @@ Django core exception classes are defined in ``django.core.exceptions``.
     See :meth:`~django.db.models.query.QuerySet.get()` for further information
     on :exc:`ObjectDoesNotExist` and :exc:`~django.db.models.Model.DoesNotExist`.
 
+``EmptyResultSet``
+------------------
+
+.. exception:: EmptyResultSet
+
+    ``EmptyResultSet`` may be raised during query generation if a query won't
+    return any results. Most Django projects won't encounter this exception,
+    but it might be useful for implementing custom lookups and expressions.
+
+    .. versionchanged:: 1.11
+
+        In older versions, it's only importable from ``django.db.models.sql``.
+
 ``FieldDoesNotExist``
 ---------------------
 

tests/queries/tests.py

@@ -6,11 +6,10 @@ import unittest
 from collections import OrderedDict
 from operator import attrgetter
 
-from django.core.exceptions import FieldError
+from django.core.exceptions import EmptyResultSet, FieldError
 from django.db import DEFAULT_DB_ALIAS, connection
 from django.db.models import Count, F, Q
 from django.db.models.sql.constants import LOUTER
-from django.db.models.sql.datastructures import EmptyResultSet
 from django.db.models.sql.where import NothingNode, WhereNode
 from django.test import TestCase, skipUnlessDBFeature
 from django.test.utils import CaptureQueriesContext