Question

I’m trying to implement caching of value returned from database:

class Foo

...

    def getTag(self):
        value = self._Db.get(self._f[F_TAG])

        setattr(self, 'tag', value)

        return value

    def _setTag(self, tag):
        self._Db.set(self._f[F_TAG], tag)


    tag = property(getTag)

...

x = Foo()        

x._setTag("20")
print(x.tag)
x._setTag("40")
print(x.tag)

When I first time handle tag property, it must get value from DB and override class field tag with instance field for following use, but error occurs:

Traceback (most recent call last):
  File "/home/altera/www/autoblog/core/dbObject.py", line 99, in <module>
    print(x.tag)
  File "/home/altera/www/autoblog/core/dbObject.py", line 78, in getTag
    setattr(self, 'tag', value)
AttributeError: can't set attribute

Answer 1

Unfortunately it’s impossible to override a @property. This is because the @property is attached to the class, not the instance.

You can either make your @property getter slightly more complex:

@property
def tag(self):
    try:
        return self._db_values["tag"]
    except KeyError:
        pass
   val = self._db.get("tag")
   self._db_values["tag"] = val
   return val

Or create a descriptor which will do the caching for you:

Undefined = object()

class DBValue(object):
    def __init__(self, column_name):
        self.column_name = column_name
        self.value = Undefined

    def __get__(self, instance, owner):
        if self.value is Undefined:
            self.value = instance._db.get(self.column_name)
        return self.value

class Foo(object):
    tag = DBValue("tag")