MutableDict |= and MutableList *= do not emit change events; changes are not persisted

Describe the bug

When modifying a MutableDict using the in-place union operator (|=) or a MutableList using in-place multiplication (*=), change events are not emitted. The underlying instance is not marked as modified (session.is_modified(instance) == False), and subsequent session.commit() or session.flush() silently skips persisting the changes to the database.

MutableDict does not implement __ior__ (introduced for Python dictionaries in PEP 584 / Python 3.9), and MutableList does not implement __imul__. As a result, the built-in dict.__ior__ and list.__imul__ run without calling self.changed().

In ticket #3853, upstream added __ior__, __iand__, __ixor__, and __isub__ to MutableSet, and __iadd__ (+=) to MutableList for the exact same reason. dict |= and list *= appear to have been overlooked.

To Reproduce

from sqlalchemy import JSON, create_engine, select
from sqlalchemy.ext.mutable import MutableDict, MutableList
from sqlalchemy.orm import DeclarativeBase, Mapped, Session, mapped_column


class Base(DeclarativeBase):
    pass


class Document(Base):
    __tablename__ = "doc"
    id: Mapped[int] = mapped_column(primary_key=True)
    payload: Mapped[dict] = mapped_column(MutableDict.as_mutable(JSON))
    tags: Mapped[list] = mapped_column(MutableList.as_mutable(JSON))


engine = create_engine("sqlite://")
Base.metadata.create_all(engine)

with Session(engine) as session:
    session.add(Document(id=1, payload={"a": 1}, tags=[1, 2]))
    session.commit()

    doc = session.get(Document, 1)
    # In-place mutations
    doc.payload |= {"b": 2}
    doc.tags *= 2

    print("In memory:", doc.payload, doc.tags)
    print("session.is_modified(doc):", session.is_modified(doc))  # False!

    session.commit()

with Session(engine) as session:
    reloaded = session.execute(select(Document.payload, Document.tags)).one()
    print("In DB after commit:      ", reloaded)  # ({'a': 1}, [1, 2]) -> changes silently lost!

Error

No error is raised; changes made via |= and *= are silently ignored by the unit of work and discarded upon session close.

Output of the snippet above:

In memory: {'a': 1, 'b': 2} [1, 2, 1, 2]
session.is_modified(doc): False
In DB after commit:       ({'a': 1}, [1, 2])

Possible fix

Implement __ior__ in MutableDict and __imul__ in MutableList:

--- a/lib/sqlalchemy/ext/mutable.py
+++ b/lib/sqlalchemy/ext/mutable.py
@@ -840,6 +840,10 @@
         dict.update(self, *a, **kw)
         self.changed()
 
+    def __ior__(self, other: Any) -> MutableDict[_KT, _VT]:
+        self.update(other)
+        return self
+
     if TYPE_CHECKING:
@@ -949,6 +953,11 @@
         self.extend(x)
         return self
 
+    def __imul__(self, x: SupportsIndex) -> MutableList[_T]:
+        list.__imul__(self, x)
+        self.changed()
+        return self
+
     def insert(self, i: SupportsIndex, x: _T) -> None:

Environment

SQLAlchemy: 2.1.2 (main at 749086a) / 2.0.x
Python: CPython 3.14.7 / 3.13 / 3.12
OS: Linux x86_64

claude on behalf of zzzeek

Reproduced: yes. Using the ORM with sqlite in-memory, I tried each in-place operator on the built-in mutable collections:

op	is_modified	persisted
`MutableDict \|=`	False	no
`MutableList *=`	False	no
`MutableList +=`	True	yes
`MutableSet \|=`, `&=`, `-=`, `^=`	True	yes

The only gaps are the two the report names. MutableDict has no __ior__, so dict.__ior__ runs without calling changed(). MutableList has no __imul__, so list.__imul__ doesn't call it either. The obj.attr op= x form then assigns the same object back to the attribute, and that assignment isn't counted as a change.

Versions: main (2.1.4.dev) and rel_2_0 (2.0.55) give identical results, so this is not a 2.1 regression. The operators were missed when #3853 added MutableSet __ior__/__iand__/__ixor__/__isub__ and MutableList.__iadd__. Milestone is set to 2.0.x.

Severity: low-to-moderate. Changes are silently lost, but neither operator is used much on a mapped JSON/pickle attribute. dict |= (PEP 584, Python 3.9+) is the likelier one to show up in real code. list *= is rare. The workaround is .update() for dicts, or flag_modified().

Tests: these go in test/ext/test_mutable.py:

_MutableDictTestBase: add a test_operator_update next to test_update. It runs through MutableWithScalarPickleTest, MutableWithScalarJSONTest, MutableColumnCopyJSONTest, MutableAssociationScalar*Test, etc.
_MutableListTestBase: add a test_operator_multiply next to test_operator_extend. It runs through MutableListWithScalarPickleTest and MutableColumnCopyArrayTest.

The existing tests don't catch this because they only cover the explicit methods plus the operators added in #3853. Nothing calls |= on a dict or *= on a list.

Fix: in lib/sqlalchemy/ext/mutable.py:

MutableDict.__ior__(self, other): call self.update(other) and return self, the same pattern as MutableSet.__ior__.
MutableList.__imul__(self, n): call list.__imul__(self, n), then self.changed(), and return self.

The diff proposed in the report is essentially right. Typing will probably need the same # type: ignore[override,misc] comments the sibling methods use.

Thanks for the quick triage!

Just to clarify regarding the robots involved label: I investigated the issue and wrote the reproducer myself, but used an LLM to help translate and format the report since English is not my native language.

Happy to submit a PR with the fix and tests if this is open for pull requests.

Mike Bayer has proposed a fix for this issue in the main branch:

*emit change events for MutableDict |= and MutableList = https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/7030

Mike Bayer has proposed a fix for this issue in the rel_2_0 branch:

*emit change events for MutableDict |= and MutableList = https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/7032

emit change events for MutableDict |= and MutableList *=

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MutableDict |= and MutableList *= do not emit change events; changes are not persisted #13643

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MutableDict |= and MutableList *= do not emit change events; changes are not persisted #13643

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zzzeek commented on Oct 4, 2026

ashm-dev commented on Oct 4, 2026

sqla-tester commented on Oct 5, 2026

sqla-tester commented on Oct 5, 2026

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