Old relationship collection is modified when assigning a new list

I noticed this strange behavior in some code that logs changes and thus stores the old value before updating. Here's a simple case that reproduces it:

from sqlalchemy import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import *

Base = declarative_base()


class Foo(Base):
    __tablename__ = 'foo'
    id = Column(Integer, primary_key=True)

    def __repr__(self):
        return '<Foo {}>'.format(self.id)


class Bar(Base):
    __tablename__ = 'bar'
    id = Column(Integer, primary_key=True)
    foo_id = Column(Integer, ForeignKey('foo.id'))
    foo = relationship(Foo, lazy=False, backref=backref('bars', lazy=False))

    def __repr__(self):
        return '<Bar {}>'.format(self.id)


e = create_engine('sqlite:///', echo=False)
Base.metadata.create_all(e)
s = Session(e, autoflush=False)

s.add(Foo(bars=[Bar(), Bar()]))
s.commit()

foo = s.query(Foo).first()
print('initial', foo.bars)
old = foo.bars
foo.bars = [Bar(), Bar()]
print('old    ', old)
print('new    ', foo.bars)

s.expire_all()

print('\n--------\n')

foo = s.query(Foo).first()
print('initial', foo.bars)
old = foo.bars
foo.bars = [old[0], Bar()]
print('old    ', old)
print('new    ', foo.bars)

Output:

('initial', [<Bar 1>, <Bar 2>])
('old    ', [])
('new    ', [<Bar None>, <Bar None>])

--------

('initial', [<Bar 1>, <Bar 2>])
('old    ', [<Bar 1>])
('new    ', [<Bar 1>, <Bar None>])

I'd expect the list in 'old' to be the same as it is in 'initial'. After all, I'm assigning a new one so the old one should not be modified by that. Of course I could work around it by unconditionally calling copy() on the old attribute but it feels a bit like a bug to me.

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