Michael Spencer wrote:
[EMAIL PROTECTED] wrote:
[snip]
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.
[snip]
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
1 class deque(object): 2 3 def __init__(self, iterable=()): 4 if not hasattr(self, 'data'): 5 self.left = self.right = 0 6 self.data = {} 7 self.extend(iterable)
[...snip methods...]
8 def __deepcopy__(self, memo={}): 9 from copy import deepcopy 10 result = self.__class__() 11 memo[id(self)] = result 12 result.__init__(deepcopy(tuple(self), memo)) 13 return result
So, entering deepcopy, I encounter the first new concept (for me) on line 10. We obtain the class/type of self. On line 11 we create a dictionary item in memo, [id(self):type(self)]. So now I'm confused as to the purpose of memo. Why should it contain the ID of the *original* object?
Note that you obtain *and then call* the class/type of self:
py> i, s = 100, 'string' py> i.__class__, s.__class__ (<type 'int'>, <type 'str'>) py> i.__class__(), s.__class__() (0, '')
So you've created a new object calling the constructor with no arguments. So 'result' is a new instance of the class.
Things get even stranger for me on line 12. Working from the inside of the parentheses outward, an attempt is made to convert self to a tuple. Shouldn't this generate a TypeError when given a complex, non-iterable item like the deque class? I just tried running one of my programs, which assigns the name "x" to one of my custom objects, and when I execute tuple(x), a TypeError is what I get.
tuple(x) works on any object that defines __iter__:
py> class C(object): ... def __iter__(self): ... yield 1 ... yield 2 ... yield 5 ... py> tuple(C()) (1, 2, 5)
So line 12 is making a tuple of the values in the object, and then calling result's __init__ again, this time with (a deepcopied) tuple as an argument. This adds the content to the previously uninitialized object.
HTH,
STeVe -- http://mail.python.org/mailman/listinfo/python-list
