Where the newly defined attributes are stored?

Question

Hey! Formally talking, each new object has its own environment, which inherits class's default attributes or initialize the local attributes.

In the case with the 'setattr', when we define a new attribute, where it is stored after and how we can access it or keep a track of it? I hope I put my question in the clear way. If I think abstractly, then I imagine that the new attributes, which were set on fly, hang somewhere in the space (but definitely in the space of an environment of the new instance). The solution I might think of is creating a storage manually (in the form of the list or dictionary) where we place whatever new was defined.

Can somebody explain it please from the scientific point of view? Thank you.

Answer 1 · 2015-03-17T03:07:23Z

March 17, 2015 3:07am

Attributes are tracked in an object's "namespace". "Class" Attributes are defined as part of the class namespace. "Instance" attributes are defined as part of the individual instance namespace. They behave like python dictionaries. An attribute is first looked for in the instance namespace, then the class namespace. You can list the contents of the namespaces using dir(some_object)

An example:

def MyClass():
    class_var = 1

# See class variable
MyClass.class_var
1

# See class namespace:
dir(MyClass)
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__',
 '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__',
 '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
 '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__',
 'class_var']


# Create instance
my1 = MyClass()

# See instance namespace (currently similar to class)
dir(my1)
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__',
 '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__',
 '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
 '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__',
 'class_var']

# See class_var value
my1.class_var
1

# Set class_var locally (becomes instance var)
my1.class_var = "thing"
my1.class_var
'thing'
MyClass.class_var
1

# Create new instance
my2 = MyClass()
my2.class_var
1

# Change class variable
MyClass.class_var = "Whoa!"
MyClass.class_var
'Whoa!'

# ready for this?
my2.class_var
'Whoa!'  # <-- class attribute changes with class
my1.class_var
'thing' # <--still local instance attribute

# delete local instance variable
del my1.class_var
# what is it now?
my1.class_var
'Whoa!' # <-- falls back to the class attribute

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Where the newly defined attributes are stored?

1 Answer

Chris Freeman

Chris Freeman

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