如何在 Python 中覆盖属性获取器

Question

I have a class with a property called sessionID. From that class I've derived a new class and would like to implement some other functionality in the property getter and property setter. I seem to have the property setter working correctly, but for some strange reason the getter always executes the getter on the base class. Any of you guys have an idea what I'm doing wrong, or what I could change "

Base Class :

class workflowerAPIBase(object):
    _sessionID = None
    _IP        = None

    @property
    def sessionID(self):
        return self._sessionID

    @sessionID.setter
    def sessionID(self, value):
        self._sessionID = value

Descendant Class :

class workflowerAPI(workflowerAPIBase):
    session = None

    @property
    def sessionID(self):
        # Try to get the previous sessionID from the Session ... 
        # we will still have to check if that sessionID is valid though
        if ( 'sessionID' in self.session ):
            self._sessionID = self.session['sessionID']

        # If no sessionID was found, try to use a default one for now
        # by generating a loginPrintersCloud using default login info
        if ( self._sessionID == None ) or ( self._sessionID == '' ):
            result = self.defaultlogin
            self._sessionID = result['sessionID']

        return self._sessionID

    @workflowerAPIBase.sessionID.setter
    def sessionID(self, value):
        # Store the new sessionID in our session for later use
        if ( self.session != None ):
            self.session['sessionID'] = value

        # call the inherited sessionID setter
        workflowerAPIBase.sessionID.fset( self, value )
        #super(workflowerAPI, self).sessionID(self, value)
#        self._sessionID = value

    def __init__(self, request):
        if request != None and request.session != None and request.session.has_key('sessionID'):
            self.session   = request.session
            self.sessionID = request.session['sessionID']

        super(workflowerAPI, self).__init__()

I'm not sure what I'm doing wrong, but each time the value of the sessionID property gets read it seems to call the getter on workflowerAPIBase although I'm making an instance of worflowerAPI.

Answer 1

Had the same issue here (using Python 3.8):

class C:
    def getx(self):
        print('getx inside C')
        return None

    x = property(getx, None)

class D(C):
    def getx(self):
        print('getx inside D')
        return None

c = C()
_ = c.x
d = D()
_ = d.x

For some reason this prints:

getx inside C
getx inside C

So the property always uses the getter of the base class. However with an intermediate function that you derive you can get around this:

class C:
    def getx(self):
        self.getx_intermediate()
        return None

    def getx_intermediate(self):
        print('getx inside C')

    x = property(getx, None)

class D(C):
    def getx_intermediate(self):
        print('getx inside D')

c = C()
_ = c.x
d = D()
_ = d.x

This now prints:

getx inside C
getx inside D

Same solution you could use for the setter.

Answer 2

In Python you don't override methods, you just hide the original method by a new one in the derived class. On the other side, decorators are only syntactical sugar for funcion calls. So

@workflowerAPIBase.sessionID.setter
def sessionID(self, value):
    ...

is short for

def set_sessionID(self, value):
    ...
sessionID = workflowerAPIBase.sessionID.setter(set_sessionID)

You set sessionID with the getter from the base class and the setter from the derived class. Correct is simply:

@sessionID.setter
def sessionID(self, value):
    ...