CODEWITHSUNDEEP
pythondescriptor
FlorianK
FlorianK

Reputation: 33

after own descriptor, the attribute is missing in __dict__

i would like to add to attributes of an object in Python some information. Later i would like to store this data in a DB, therefore it would be neat to know its format in the tables. I think the solution from SQLAlchemy is good and i would like to do it like that.

First a short example: 

class MyAttribute(object):

    def __init__(self, AttrType):
        self.val = None
        self.AttrType = AttrType

    def __get__(self, obj, objtype):
        return self.val

    def __set__(self, obj, val):
        self.val = val


class MyClass(object):

    Attr1 = MyAttribute('int(11)')

    def __init__(self):
        self.Attr1 = 44
        self.Attr2 = 55


x = MyClass()

print("Attr1: %s"%x.Attr1)
print("Attr2: %s"%x.Attr2)

print(x.__dict__)

The output of this script is:

Attr1: 44
Attr2: 55

{'Attr2': 55}

Now my problem: in __dict__, there is Attr2, but not Attr1. Why? How can I add it? Which function is responsible for that? (My read is __set__, correct?) What do I have to add where to add the attribute object to the dict?

Upvotes: 2

Views: 746

Answers (1)

Eric
Eric

Reputation: 97681

Try MyClass.__dict__:

>>> MyClass.__dict__['Attr1']
<__main__.MyAttribute object at 0x000000000296A5F8>

Descriptors are stored in the class object, not the instance object. You're actually in trouble here, for that reason:

>>> x = MyClass()
>>> x.Attr1
44
>>> y = MyClass()
>>> y.Attr1 = 66
>>> y.Attr1
66
>>> x.Attr1
66 # uh oh

It's the job of MyAttribute to keep track of which data belongs to which instance.

Upvotes: 1

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