python - 无法在使用 Fixtures 的 Pytest 函数中实例化 Python 数据类(冻结)
问题描述
我正在关注 Harry Percival 和 Bob Gregory 的 Python 架构模式。
在第三 (3) 章左右,他们介绍了测试 SQLAlchemy 的 ORM。
一个需要session
夹具的新测试,它AttributeError, FrozenInstanceError
由于以下原因而抛出cannot assign to field '_sa_instance_state'
可能需要注意的是,其他测试在创建 的实例时不会失败OrderLine
,但如果我只是将它们包含session
在测试参数中,它们就会失败。
无论如何,我会直接进入代码。
conftest.py
@pytest.fixture
def local_db():
engine = create_engine('sqlite:///:memory:')
metadata.create_all(engine)
return engine
@pytest.fixture
def session(local_db):
start_mappers()
yield sessionmaker(bind=local_db)()
clear_mappers()
模型.py
@dataclass(frozen=True)
class OrderLine:
id: str
sku: str
quantity: int
test_orm.py
def test_orderline_mapper_can_load_lines(session):
session.execute(
'INSERT INTO order_lines (order_id, sku, quantity) VALUES '
'("order1", "RED-CHAIR", 12),'
'("order1", "RED-TABLE", 13),'
'("order2", "BLUE-LIPSTICK", 14)'
)
expected = [
model.OrderLine("order1", "RED-CHAIR", 12),
model.OrderLine("order1", "RED-TABLE", 13),
model.OrderLine("order2", "BLUE-LIPSTICK", 14),
]
assert session.query(model.OrderLine).all() == expected
控制台错误pipenv run pytest test_orm.py
============================= test session starts =============================
platform linux -- Python 3.7.6, pytest-5.4.1, py-1.8.1, pluggy-0.13.1
rootdir: /home/[redacted]/Documents/architecture-patterns-python
collected 1 item
test_orm.py F [100%]
================================== FAILURES ===================================
____________________ test_orderline_mapper_can_load_lines _____________________
session = <sqlalchemy.orm.session.Session object at 0x7fd919ac5bd0>
def test_orderline_mapper_can_load_lines(session):
session.execute(
'INSERT INTO order_lines (order_id, sku, quantity) VALUES '
'("order1", "RED-CHAIR", 12),'
'("order1", "RED-TABLE", 13),'
'("order2", "BLUE-LIPSTICK", 14)'
)
expected = [
> model.OrderLine("order1", "RED-CHAIR", 12),
model.OrderLine("order1", "RED-TABLE", 13),
model.OrderLine("order2", "BLUE-LIPSTICK", 14),
]
test_orm.py:13:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
<string>:2: in __init__
???
../../.local/share/virtualenvs/architecture-patterns-python-Qi2y0bev/lib64/python3.7/site-packages/sqlalchemy/orm/instrumentation.py:377: in _new_state_if_none
self._state_setter(instance, state)
<string>:1: in set
???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
self = <[AttributeError("'OrderLine' object has no attribute '_sa_instance_state'") raised in repr()] OrderLine object at 0x7fd919a8cf50>
name = '_sa_instance_state'
value = <sqlalchemy.orm.state.InstanceState object at 0x7fd9198f7490>
> ???
E dataclasses.FrozenInstanceError: cannot assign to field '_sa_instance_state'
<string>:4: FrozenInstanceError
=========================== short test summary info ===========================
FAILED test_orm.py::test_orderline_mapper_can_load_lines - dataclasses.Froze...
============================== 1 failed in 0.06s ==============================
附加问题
我理解上面的逻辑以及这些文件在做什么,但如果我缺乏基本的理解,请纠正我。
conftest.py
(用于所有 pytest 配置)正在设置一个session
夹具,它基本上在内存中设置一个临时数据库 - 使用启动和清除映射器来确保 orm 模型定义绑定到数据库实例。model.py
只是一个用于表示原子OrderLine
对象的数据类。test_orm.py
用于 pytest 的类来提供session
固定装置,以便setup, execute, teardown
明确地为 db 运行测试。
解决方案
SqlAlchemy 允许您覆盖一些在使用映射类和表时应用的属性检测。特别是以下允许 sqla 将状态保存在已检测的冻结数据类上。这应该在调用mapper
关联数据类和 sql 表的函数之前应用。
from sqlalchemy.ext.instrumentation import InstrumentationManager
...
DEL_ATTR = object()
class FrozenDataclassInstrumentationManager(InstrumentationManager):
def install_member(self, class_, key, implementation):
self.originals.setdefault(key, class_.__dict__.get(key, DEL_ATTR))
setattr(class_, key, implementation)
def uninstall_member(self, class_, key):
original = self.originals.pop(key, None)
if original is not DEL_ATTR:
setattr(class_, key, original)
else:
delattr(class_, key)
def dispose(self, class_):
del self.originals
delattr(class_, "_sa_class_manager")
def manager_getter(self, class_):
def get(cls):
return cls.__dict__["_sa_class_manager"]
return get
def manage(self, class_, manager):
self.originals = {}
setattr(class_, "_sa_class_manager", manager)
def get_instance_dict(self, class_, instance):
return instance.__dict__
def install_state(self, class_, instance, state):
instance.__dict__["state"] = state
def remove_state(self, class_, instance, state):
del instance.__dict__["state"]
def state_getter(self, class_):
def find(instance):
return instance.__dict__["state"]
return find
OrderLine.__sa_instrumentation_manager__ = FrozenDataclassInstrumentationManager
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