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I was just reading the Python documentation about classes; it says, in Python "classes themselves are objects". How is that different from classes in C#, Java, Ruby, or Smalltalk? What advantages and disadvantages does this type of classes have compared with those other languages?
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A class is a user-defined type that describes what a certain type of object will look like. A class description consists of a declaration and a definition. Usually these pieces are split into separate files. An object is a single instance of a class. You can create many objects from the same class type.
A class isn't an object, you can think of it as a 'blueprint' for an object. It describes the shape and behaviour of that object. Objects are instances of a class.
Because in Java, a variable of type abc doesn't contain an abc object. A variable of type abc contains a reference to an abc object. Your reasoning would be valid in say C++. But a class can have static object of self type.
An object is simply a collection of data (variables) and methods (functions) that act on those data. Similarly, a class is a blueprint for that object. We can think of a class as a sketch (prototype) of a house. It contains all the details about the floors, doors, windows, etc.
In Python, classes are objects in the sense that you can assign them to variables, pass them to functions, etc. just like any other objects. For example
>>> t = type(10)
>>> t
<type 'int'>
>>> len(t.__dict__)
55
>>> t() # construct an int
0
>>> t(10)
10
Java has Class objects which provide some information about a class, but you can't use them in place of explicit class names. They aren't really classes, just class information structures.
Class C = x.getClass();
new C(); // won't work
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