Python IsInstance() for classes and subclasses

If SizedString is an instance of Descriptor ( based on Inheritance hierarchy- String , Typed , MaxSized, Descriptor ), then String and Integer also should satisfy the If condition right ? because at the end it is also the subclass of ( typed , Descriptor ) ?

We have to look carefully at what is being passed into the check_attributes function. Look closer at what the value of the name and share keyword arguments are:

@check_attributes(name=String,shares=Integer,place=SizedString('tester',size=8))

Notice the lack of parenthesis after the String and Integer class names? This means the String and Integer class object themselves are being passed into check_attributes, not an instance of either class. And since the String class object and the Integer class object are not sub-classes of Descriptor, isinstance(value, Descriptor) fails.

What is value(key) in setattr(cls, key, value(key)) means , cant understand what is value(key ) means ?

Think about it. Since value has the value of whatever keyword argument was passed into check_attributes, and the value is not an instance of the Descriptor class, then value must be referring to a class object. (If you don't understand why this is, refer back to my answer to your first question). So calling value(key) is creating an instance of some class, and passing in the key value as a constructor argument.