Is there a reference/alias/pointer in Python?

James Henderson james at logicalprogression.net
Fri Jul 23 04:45:42 CEST 2004


Daniel Eloff wrote:

> class UserOptionsClass(object): ? what does inheriting from object do?
> Enable you to use properties?

That is correct.  Specifically it makes it a new-style class, see:

http://www.python.org/doc/newstyle.html

> Additionally if I return Users['current_users_username'] from a method
> isn't it also ref counted? Probably not since
> Users['current_users_username'] must not return a ref-counted object...
> Python is still all very confusing to me :) Would you do me a big favor
> and help me figure out where to expect a rec-counted instance or a
> reference?

I can't give you the full story now (assuming I can at all) because it 
3.30am my time, but I'm putting the discussion on list so that others 
can contribute.

Basically every Python object is a pointer, a PyObject* in the C 
implementation.  So if you assign a variable to 
Users['current_users_username'] you have two references to the same 
object.  However, you can't use this fact to change the value of one 
variable through the other because you can't dereference and change the 
value of either.  All you can do is make the variable point to a new object.

All this is assuming that the variables are strings, which are immutable 
  in Python, meaning that they cannot be changed in place.  If you have 
two indentifiers pointing to the same mutable object, such as a list, 
then you can change the value of one through the other by changing it in 
place (e.g. assigning to an element of the list) rather than by 
re-assigning the variable.

If this doesn't make sense, you know why. :)

James




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