[Tutor] about copy.copy

[Kent Johnson]

Luke Paireepinart wrote:
> linda.s wrote:
>   
>> what is the difference between b and c in the following code?
>>     
> #---copyexample.py
> import copy
> a=[1,4,5]
> b=a
> c=copy.copy(a)
>
> del(a[0])
> print a
> print b
> print c
>
> #---eof
>
> output:
> [4, 5]
> [4, 5]
> [1, 4, 5]
>
>
> I.E. 'b' is just a reference to 'a', so modifying 'a' by deleting an 
> element changed the value of 'b' (since they are really both just 
> references to the
> same data structure in memory.)
>   
Assignment makes a new reference to the same object. When you say
  b = a
b and a now refer to the same object, in this case a list. Another way 
to say this is, a and b are aliases of the same object. If the object is 
mutable (can be changed), then any change to the object through either 
reference is equivalent.

Note that some objects, like ints and strings, are immutable - they 
can't be changed. If you say
  a = 1
  b = a
  a = a + 1
you are actually making a refer to a new integer with the value two, so 
b doesn't change.

When you use copy.copy(), you create a /shallow copy/ of a. A shallow 
copy only copies one level deep. So when a is a list, a shallow copy 
makes a new list populated with references to the same objects as 
contained in the original list.

This is a situation very similar to a=b, except it is pushed down one 
level. If the list elements themselves are mutable then changes to the 
elements in one list will be reflected in the other list. In your case, 
you made a list of lists. Copying with copy.copy() makes a new list 
whose elements are references to the same lists as contained in the 
original list. Changing one of the elements is reflected in both lists.

Try copy.deepcopy() if you want to make copies all the way down.

This article might help you understand the semantics of assignment in 
Python:
http://effbot.org/zone/python-objects.htm

Kent
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