Python SQLAlchemy：如何通过事件侦听器有条件地添加到集合？

在sqlalchemy中，我可以监听集合上的

append

事件，以拦截并可能更改要追加的值。例如，当我想用任意条件实现类似于

集的行为时，如何静默地删除该值（而不是追加）

MCVE（除了sqlalchemy，没有任何依赖项，只需复制和粘贴）：

在这里回答我自己，因为没有人感兴趣

我现在认为，在添加事件侦听器时删除元素是不可能的。因此，我最终实现了一个（这并不是很受鼓励的），结果是非常简单的：

from sqlalchemy import create_engine, Integer, Text
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.schema import Column, ForeignKey

engine = create_engine('sqlite:///:memory:')
Base = declarative_base()

_next_id = 1


def get_id():
    global _next_id
    _ = _next_id
    _next_id += 1
    return _


class NamedThingCollection(object):

    def __init__(self):
        self.data = []

    def __contains__(self, thing):
        return next((True for x in self.data if x.name == thing.name), False)

    def append(self, item):
        if item not in self:
            self.data.append(item)

    def remove(self, item):
        self.data.remove(item)

    def extend(self, items):
        self.data.extend(items)

    def __iter__(self):
        return iter(self.data)

    def __len__(self):
        return len(self.data)


class A1(Base):
    __tablename__ = 'a1'
    id = Column(Integer, primary_key=True, default=get_id)
    a2 = relationship(
        'A2', back_populates='a', collection_class=NamedThingCollection)


class A2(Base):
    __tablename__ = 'a2'
    id = Column(Integer, primary_key=True, default=get_id)
    a_id = Column(Integer, ForeignKey('a1.id'))
    a = relationship('A1', back_populates='a2')

    name = Column(Text)


Base.metadata.create_all(engine)
Session = sessionmaker()
Session.configure(bind=engine)
_session = Session()

a2_1 = A2(name='a2_1')
a2_2 = A2(name='a2_2')
a2_3 = A2(name='a2_1')

a1 = A1()

for thing in (a2_1, a2_2, a2_3, a1):
    _session.add(thing)

for a in (a2_1, a2_2, a2_3, a2_1):
    a1.a2.append(a)

_session.flush()

assert len(a1.a2) == 2

当然，用户必须根据需要实现进一步的行为，例如用于索引的
\uuu getitem\uuuuuuuuuu（）
/
\uuuuuuuu setitem\uuuuuuuuu（）

from sqlalchemy import create_engine, Integer, Text
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.schema import Column, ForeignKey

engine = create_engine('sqlite:///:memory:')
Base = declarative_base()

_next_id = 1


def get_id():
    global _next_id
    _ = _next_id
    _next_id += 1
    return _


class NamedThingCollection(object):

    def __init__(self):
        self.data = []

    def __contains__(self, thing):
        return next((True for x in self.data if x.name == thing.name), False)

    def append(self, item):
        if item not in self:
            self.data.append(item)

    def remove(self, item):
        self.data.remove(item)

    def extend(self, items):
        self.data.extend(items)

    def __iter__(self):
        return iter(self.data)

    def __len__(self):
        return len(self.data)


class A1(Base):
    __tablename__ = 'a1'
    id = Column(Integer, primary_key=True, default=get_id)
    a2 = relationship(
        'A2', back_populates='a', collection_class=NamedThingCollection)


class A2(Base):
    __tablename__ = 'a2'
    id = Column(Integer, primary_key=True, default=get_id)
    a_id = Column(Integer, ForeignKey('a1.id'))
    a = relationship('A1', back_populates='a2')

    name = Column(Text)


Base.metadata.create_all(engine)
Session = sessionmaker()
Session.configure(bind=engine)
_session = Session()

a2_1 = A2(name='a2_1')
a2_2 = A2(name='a2_2')
a2_3 = A2(name='a2_1')

a1 = A1()

for thing in (a2_1, a2_2, a2_3, a1):
    _session.add(thing)

for a in (a2_1, a2_2, a2_3, a2_1):
    a1.a2.append(a)

_session.flush()

assert len(a1.a2) == 2