python - 混合 Cython 类和 SqlAlchemy

Question

摘要:

我有一个代表业务单元的 cython 类。此类以纯 cython 风格声明。

在一个项目中，我需要将业务单元映射到数据库。为此，我想导入 .pxd 文件并使用 SQLAlchemy“映射”它。

赛通定义

让我们假设类 Equipment。该类定义为 .pyx 和 .pxd 中的类接口(interface)(因为我需要在其他模块中导入它)。

设备.pxd

cdef class Equipment:
    cdef readonly int x
    cdef readonly str y

设备.pyx

cdef class Equipment:
    def __init__(self, int x, str y):
        self.x = x
        self.y = y

我编译所有内容并得到一个 equipment.pyd 文件。到目前为止，没问题。 此文件包含业务逻辑模型，不得更改。

映射

然后在一个应用程序中，我导入 equipment.pyd 并用 SQLAlchemy 映射它。

from sqlalchemy import Table, Column, Integer, String
from sqlalchemy.orm import mapper
from equipment import Equipment

metadata = MetaData()

# Table definition
equipment = Table(
    'equipment', metadata,
    Column('id', Integer, primary_key=True),
    Column('x', Integer),
    Column('y', String),
)

# Mapping the table definition with the class definition
mapper(Equipment, equipment)

TypeError: can't set attributes of built-in/extension type 'equipment.Equipment'

确实，SQLAlchemy 正在尝试创建 Equipment.c.x、Equipment.c.y，...这在 Cython 中是不可能的，因为它未在 .pxd 中定义...

那么如何将 Cython 类映射到 SQLAlchemy？

不满意的解决方案

如果我在 .pyx 文件中以 python 模式定义设备类，它会起作用，因为最后，它只是 cython 类定义中的“python”对象。

设备.pyx

class Equipment:
    def __init__(self, x, y):
        self.x = x
        self.y = y

但我失去了很多功能，这就是为什么我需要纯 Cython。

谢谢! :-)

-- 编辑部分 --

半满意的解决方案

保留 .pyx 和 .pxd 文件。从 .pyd 继承。尝试映射。

映射.py

from sqlalchemy import Table, Column, Integer, String
from sqlalchemy.orm import mapper
from equipment import Equipment

metadata = MetaData()

# Table definition
equipment = Table(
    'equipment', metadata,
    Column('id', Integer, primary_key=True),
    Column('x', Integer),
    Column('y', String),
)

# Inherit Equipment to a mapped class
class EquipmentMapped(Equipment):
    def __init__(self, x, y):
        super(EquipmentMapped, self).__init__(x, y)

# Mapping the table definition with the class definition
mapper(EquipmentMapped, equipment)

from mapping import EquipmentMapped

e = EquipmentMapped(2, 3)

print e.x

## This is empty!

为了让它工作，我必须将每个属性定义为属性!

设备.pxd

cdef class Equipment:
    cdef readonly int _x
    cdef readonly str _y

设备.pyx

cdef class Equipment:
    def __init__(self, int x, str y):
        self.x = x
        self.y = y
    property x:
        def __get__(self):
            return self._x
        def __set__(self, x):
            self._x = x
    property y:
        def __get__(self):
            return self._y
        def __set__(self, y):
            self._y = y

这并不令人满意，因为 :lazy_programmer_mode on: 我在业务逻辑中有很多更改要做... :lazy_programmer_mode off:

Answer 1

我认为最基本的问题是，当你调用 mapper 时，它会(除其他外)

Equipment.x = ColumnProperty(...) # with some arguments
Equipment.y = ColumnProperty(...)

当 ColumnProperty 是一个 sqlalchemy 定义的属性时，所以当您执行 e.x = 5 时，它会注意到该值已更改，因为它周围的所有数据库相关的东西.

这显然不能与您试图用来控制存储的下面的 Cython 类配合得很好。

就个人而言，我怀疑唯一真正的答案是定义一个包含 Cython 类和 sqlalchemy 映射类的包装类，并拦截所有属性访问和方法调用以保持它们同步。下面是一个粗略的实现，它应该适用于简单的情况。虽然它几乎没有经过测试，所以几乎可以肯定有它遗漏的错误和极端情况。当心!

def wrapper_class(cls):
    # do this in a function so we can create it generically as needed
    # for each cython class
    class WrapperClass(object):
        def __init__(self,*args,**kwargs):
            # construct the held class using arguments provided
            self._wrapped = cls(*args,**kwargs)

        def __getattribute__(self,name):
            # intercept all requests for attribute access.
            wrapped = object.__getattribute__(self,"_wrapped")
            update_from = wrapped
            update_to = self
            try:
                o = getattr(wrapped,name)
            except AttributeError:
                # if we can't find it look in this class instead.
                # This is reasonable, because there may be methods defined
                # by sqlalchemy for example
                update_from = self
                update_to = wrapped
                o = object.__getattribute__(self,name)
            if callable(o):
                return FunctionWrapper(o,update_from,update_to)
            else:
                return o

        def __setattr__(self,name,value):
            # intercept all attempt to write to attributes
            # and set it in both this class and the wrapped Cython class
            if name!="_wrapped":
                try:
                    setattr(self._wrapped,name,value)
                except AttributeError:
                    pass # ignore errors... maybe bad!
            object.__setattr__(self,name,value)
    return WrapperClass

class FunctionWrapper(object):
    # a problem we have is if we call a member function.
    # It's possible that the member function may change something
    # and thus we need to ensure that everything is updated appropriately
    # afterwards
    def __init__(self,func,update_from,update_to):
        self.__func = func
        self.__update_from = update_from
        self.__update_to = update_to

    def __call__(self,*args,**kwargs):
        ret_val = self.__func(*args,**kwargs)

        # for both Cython classes and sqlalchemy mappings
        # all the relevant attributes exist in the class dictionary
        for k in self.__update_from.__class__.__dict__.iterkeys():
            if not k.startswith('__'): # ignore private stuff
                try:
                    setattr(self.__update_to,k,getattr(self.__update_from,k))
                except AttributeError:
                    # There may be legitmate cases when this fails
                    # (probably relating to sqlalchemy functions?)
                    # in this case, replace raise with pass
                    raise
        return ret_val

要使用它，您需要执行以下操作:

class EquipmentMapped(wrapper_class(Equipment)):
    # you may well have to define __init__ here
    # you'll have to check yourself, and see what sqlalchemy does...
    pass

mapper(EquipmentMapped,equipment)

请记住，这是一个可怕的工作区，它基本上只是在两个地方复制所有数据，然后拼命尝试保持同步。

---

编辑:这个的原始版本提供了一种机制来自动执行一系列查询 OP 曾尝试过但决定手动完成很多工作(在 Cython 类上定义属性，它只会成功在覆盖 sqlalchemy 的跟踪更改机制中)进一步测试确认它不起作用。如果您对不该做什么感到好奇，请查看编辑历史记录!