sqlalchemy中的值更改后如何触发事件

Question

I am defining a mapped class using sqlalchemy. It contains three user defined values ( a , b , c ) and one dependend value ( abc ), which is calculated using a , b and c . I want to attach a listener to a , b and c , which updates abc , when their values are changed. Using event.listen(variable.a, 'set', updateFunction) calls the function, which should update abc . Unfortunately the event is fired before the new value is stored in the variable. Therefore abc will be calculated using the old values.

Example:

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, Numeric, Sequence
from sqlalchemy import event

base = declarative_base()

class demo(base):
    __tablename__ = 'demo'

    id = Column(Integer, Sequence('some_id_seq'), primary_key=True)
    a = Column(Integer)
    b = Column(Integer)
    c = Column(Integer)
    abc = Column(Numeric) #calculated from a, b, and c

    def __init__(self, A, B, C):
        self.a = A
        self.b = B
        self.c = C

        self.calcABC() #initial calculation of abc

        event.listen(demo.a, 'set', self.calcABCEvent, named=True)
        event.listen(demo.b, 'set', self.calcABCEvent, named=True)
        event.listen(demo.c, 'set', self.calcABCEvent, named=True)

    #super complicated formular
    def calcABC(self):
        self.abc = ( self.a + self.b ) * self.c

    #This event ist called, before the new value is set
    #Thus the calculation is not correct
    def calcABCEvent(self, target, value, oldvalue, initiator):
        print("\nevent called")
        print(f"self.a \t\t{self.a}")
        print(f"target.a \t{target.a}")
        print(f"oldvalue \t{oldvalue}")
        print(f"value \t\t{value}") #The new value only appears in value

        self.calcABC()

    def __str__(self):
        return f"({self.a} + {self.b}) * {self.c} = {self.abc}"

print("init")
obj = demo(1,2,3)
print(obj) #abc should be (1+2)*3 = 9

print("\nchange a to 4")
obj.a = 4

print("\nchanged object")
print(obj) #abc should be (4+2)*3 = 18

Output:

init
(1 + 2) * 3 = 9

change a to 4

event called
self.a      1
target.a    1
oldvalue    1
value       4

changed object
(4 + 2) * 3 = 9

How can i update abc using only one function? In my application, abc is calculated using seven variables, and i want to avoid writing seven update functions. Is there something like event.listen(variable.a, 'after_set', updateFunction) ?

Answer 1

Found a similiar quastion after learning about properties: Python getter and setter via @property within SqlAlchemy

My solution looks like this:

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, Numeric, Sequence
from sqlalchemy.ext.hybrid import  hybrid_property

base = declarative_base()

class demo(base):
    __tablename__ = 'demo'

    id = Column(Integer, Sequence('some_id_seq'), primary_key=True)
    _a = Column("a", Integer)
    _b = Column("b", Integer)
    _c = Column("c", Integer)
    abc = Column(Numeric) #calculated from a, b, and c

    def __init__(self, A, B, C):
        self._a = A
        self._b = B
        self._c = C
        self.calcABC() #inital calculation of abc

    #super complicated formular
    def calcABC(self):
        self.abc = ( self._a + self._b ) * self._c

    def __str__(self):
        return f"({self._a} + {self._b}) * {self._c} = {self.abc}"

    @hybrid_property
    def a(self):
        return self._a

    @a.setter
    def a(self, a):
        self._a = a
        self.calcABC()


    @hybrid_property
    def b(self):
        return self._b

    @b.setter
    def b(self, b):
        self._b = b
        self.calcABC()


    @hybrid_property
    def c(self):
        return self._c

    @c.setter
    def c(self, c):
        self._c = c
        self.calcABC()

print("init")
obj = demo(1,2,3)
print(obj) #abc should be (1+2)*3 = 9

print("\nchange a to 4")
obj.a = 4
print(obj) #abc should be (4+2)*3 = 18

print("\nchange b to 5")
obj.b = 5
print(obj) #abc should be (4+5)*3 = 27

print("\nchange c to 6")
obj.c = 5
print(obj) #abc should be (4+5)*6 = 54

producing the following output

init
(1 + 2) * 3 = 9

change a to 4
(4 + 2) * 3 = 18

change b to 5
(4 + 5) * 3 = 27

change c to 6
(4 + 5) * 6 = 54

Process finished with exit code 0