Can a class contain an instance of itself as a data container?

This won't work, for the reason already given:

1. Python sees A(2) and calls A.__init__.
2. A.__init__ calls A(val).
3. A(val) calls A.__init__.
4. GOTO 2

I assume you're doing this so that you have a log of what val has been; that is, if sometime later you decide that you want val to be 3 instead, you don't throw away the original value 2. How about:

Code

class A( object ):
    @property
    def val( self ):
        return self.history[ -1 ]

    @val.setter
    def val( self, value ):
        self.history.append( value )

    def __init__( self, val ):
        self.history = [ ]
        self.val = val

Explanation

• A( object ): classes should now inherit from object. Just because, basically.
• @property: this tells python that every time we ask for A.val, it should call A.val() and return the result. This is a decorator; look up the property builtin function for more information.
• @val.setter: this is similar to the above, but tells Python that every time we try to assign to A.val it should call the following function instead. Instead of setting A.val, it appends the value to the history list.

Yes, a class can contain an instance of itself, you just can't create it on initiation for the reasons described by others.

For example this class will do it,

class A:
    def __init__(self,value):
        self.value=value
    def setProperty(self,subvalue):
        self.innerInstance=A(subvalue)

You can then instantiate it and set its inner copy of itself like this:

>>>OuterInstance=A(123)
>>>OuterInstance.setProperty(456)

And verify it worked with:

>>>OuterInstance.innerInstance.value
456