# Copying data

# Copy a dictionary

A dictionary object has the method copy. It performs a shallow copy of the dictionary.

>>> d1 = {1:[]}
>>> d2 = d1.copy()
>>> d1 is d2
False
>>> d1[1] is d2[1]
True

A shallow copy is a copy of a collection without performing a copy of its elements.

>>> import copy
>>> c = [[1,2]]
>>> d = copy.copy(c)
>>> c is d
False
>>> c[0] is d[0]
True

If you have nested lists, it is desireable to clone the nested lists as well. This action is called deep copy.

>>> import copy
>>> c = [[1,2]]
>>> d = copy.deepcopy(c)
>>> c is d
False
>>> c[0] is d[0]
False

You can create shallow copies of lists using slices.

>>> l1 = [1,2,3]
>>> l2 = l1[:]     # Perform the shallow copy.
>>> l2
[1,2,3]
>>> l1 is l2
False

Sets also have a copymethod. You can use this method to perform a shallow copy.

>>> s1 = {()}
>>> s2 = s1.copy()
>>> s1 is s2
False
>>> s2.add(3)
>>> s1
{[]}
>>> s2
{3,[]}