Semi-newbie, rolling my own __deepcopy__

[Michael Spencer]

ladasky at my-deja.com wrote:
> Hi, folks,
> 
> First, the obligatory cheerleading -- then, my questions...
> 
> I love Python!  I am only an occasional programmer.  Still, the logic
> of the language is clear enough that I can retain pretty much all that
> I have learned from one infrequent programming session to the next.
> That's quite an accomplishment for a language this powerful.  Also, I'm
> finally beginning to grasp OOP.  I could never quite get the hang of it
> in C++ or Java.  Recently, I discovered __getitem__ and pickle.  Oh,
> yeah.
> 
> Anyway, my present problem is that I want to make copies of instances
> of my own custom classes.  I'm having a little trouble understanding
> the process.  Not that I think that it matters -- but in case it does,
> I'll tell you that I'm running Python 2.3.4 on a Win32 machine.
> 
> I started naively, thinking that I could just call copy.deepcopy() and
> be done with it.  After getting a TypeError from the interpreter, I
> read the deepcopy docs and discovered that I need to implement a
> __deepcopy__ method in my class.  But the docs are a bit vague here.
> What exactly should this __deepcopy__ do?  I tried looking for examples
> of __deepcopy__ code on the Net, but I'm not quite understanding what
> I'm finding there.  I guess that I'm getting deeper into the guts of
> Python than I planned.
> 
> AFAIK, I'm supposed to add a "def __deepcopy__(self, memo):" to my
> class definition.  This will get called when I invoke
> copy.deepcopy(myObject).  The object to be copied is self, I presume.
> What exactly is memo?  The docs say that it's a dictionary which "keeps
> track of what has already been copied."  Somewhere I remember reading
> that the namespace of an object is a dictionary.  So is memo identical
> to the dictionary of the new object that I'm trying to create?  What
> exactly do I add to memo?  I think that I should make shallow copies of
> methods, but deep copies of data structures (the contents of which I'm
> likely to change).  Do I iterate through and copy the items in
> dir(self)?  Do I update memo manually, or does passing memo into copy()
> or deepcopy() automatically update memo's contents?  Are there any
> items that I *shouldn't* copy from self to memo?  Should __deepcopy__
> return memo?
> 
> Sorry for all the confusion -- and thanks for your help!
> 
> --
> Rainforest laid low.
> "Wake up and smell the ozone,"
> Says man with chainsaw.
> John J. Ladasky Jr., Ph.D.
> 
If you google for:
  python __deepcopy__ cookbook
you will find a couple of examples of this method in use, among them: 
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/259179

class deque(object):

     def __init__(self, iterable=()):
         if not hasattr(self, 'data'):
             self.left = self.right = 0
             self.data = {}
         self.extend(iterable)

[...snip methods...]

     def __deepcopy__(self, memo={}):
         from copy import deepcopy
         result = self.__class__()
         memo[id(self)] = result
         result.__init__(deepcopy(tuple(self), memo))
         return result

HTH
Michael