Merge "Deprecate negative slice indexes"

3 files changed, 25 insertions, 5 deletions

diff --git a/lib/sqlalchemy/orm/dynamic.py b/lib/sqlalchemy/orm/dynamic.py
index af67b4772..48161a256 100644
--- a/lib/sqlalchemy/orm/dynamic.py
+++ b/lib/sqlalchemy/orm/dynamic.py
@@ -510,7 +510,9 @@ class AppenderQuery(Generative):
                 attributes.PASSIVE_NO_INITIALIZE,
             ).indexed(index)
         else:
-            return orm_util._getitem(self, index)
+            return orm_util._getitem(
+                self, index, allow_negative=not self.session.future
+            )
 
     @_generative
     def limit(self, limit):
diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py
index c6e6f6466..42987932d 100644
--- a/lib/sqlalchemy/orm/query.py
+++ b/lib/sqlalchemy/orm/query.py
@@ -2495,7 +2495,11 @@ class Query(
         self._compile_options += {"_enable_single_crit": False}
 
     def __getitem__(self, item):
-        return orm_util._getitem(self, item)
+        return orm_util._getitem(
+            self,
+            item,
+            allow_negative=not self.session or not self.session.future,
+        )
 
     @_generative
     @_assertions(_no_statement_condition)
diff --git a/lib/sqlalchemy/orm/util.py b/lib/sqlalchemy/orm/util.py
index 27b14d95b..c8d639e8c 100644
--- a/lib/sqlalchemy/orm/util.py
+++ b/lib/sqlalchemy/orm/util.py
@@ -1811,12 +1811,26 @@ def randomize_unitofwork():
     ) = session.set = mapper.set = dependency.set = RandomSet
 
 
-def _getitem(iterable_query, item):
+def _getitem(iterable_query, item, allow_negative):
     """calculate __getitem__ in terms of an iterable query object
     that also has a slice() method.
 
     """
 
+    def _no_negative_indexes():
+        if not allow_negative:
+            raise IndexError(
+                "negative indexes are not accepted by SQL "
+                "index / slice operators"
+            )
+        else:
+            util.warn_deprecated_20(
+                "Support for negative indexes for SQL index / slice operators "
+                "will be "
+                "removed in 2.0; these operators fetch the complete result "
+                "and do not work efficiently."
+            )
+
     if isinstance(item, slice):
         start, stop, step = util.decode_slice(item)
 
@@ -1827,11 +1841,10 @@ def _getitem(iterable_query, item):
         ):
             return []
 
-        # perhaps we should execute a count() here so that we
-        # can still use LIMIT/OFFSET ?
         elif (isinstance(start, int) and start < 0) or (
             isinstance(stop, int) and stop < 0
         ):
+            _no_negative_indexes()
             return list(iterable_query)[item]
 
         res = iterable_query.slice(start, stop)
@@ -1841,6 +1854,7 @@ def _getitem(iterable_query, item):
             return list(res)
     else:
         if item == -1:
+            _no_negative_indexes()
             return list(iterable_query)[-1]
         else:
             return list(iterable_query[item : item + 1])[0]