[Tutor] Deep Copy

[Kirby Urner]

>But, I would prefer not doing that every time (I am doing this alot).
>
>  - Tim

Fastest way to copy a list is:

newlist = oldlist[:]

That's a shallow copy, which is sufficient for the example
you gave, e.g. a list of integers or strings.

However, if oldlist contains objects and you want clones
of those objects as well, then you need the deepcopy
function in the copy module.

Here's some illustrative shell interaction:

  >>> class Foo:
         def __init__(self,var):
             self.value = var


  >>> o1 = Foo(2)
  >>> o2 = Foo(3)
  >>> oldlist = [o1,o2]
  >>> newlist = oldlist[:]
  >>> id(oldlist)
  11363440
  >>> id(newlist)  # ok, two distinct lists
  11371104
  >>> o1.value = 4
  >>> newlist[0].value  # oh oh, object o1 is same in both
  4

  >>> import copy       # copy to the rescue!
  >>> newlist = copy.deepcopy(oldlist)
  >>> o1.value = 44     # change o1's value property again
  >>> oldlist[0].value  # old list is effected
  44
  >>> newlist[0].value  # newlist has clone of o1, unchanged
  4
  >>> id(newlist[0])
  11406848
  >>> id(oldlist[0])    # confirming the difference
  11347248

Kirby