Visualizing Pointer Aliasing in Python

This is a concept that has annoyed many a newcomer to Python - including the author of this post. I will be talking briefly about this concept in my lecture next Thursday, and the excellent TA, +Jimmy Charnley Kromann, pointed me to an example he saw on pythontutor. The Visualize tool on pythontutor is a very explicit way to illustrate what happens under the hood of python: http://www.pythontutor.com/visualize.html

The Visualize tool on pythontutor is a very clever way to illustrate what happens under the hood of python: http://www.pythontutor.com/visualize.html

EDIT: There seems to be a problem with python tutor at the moment. Click this link, if the embedded code below fails to load:  Python Example

EDIT: embedded code not working - click link:

Python Example

 

If you want to avoid pointer aliasing,  you usually want to do one of these three things in order to copy a to b:

1)

b = a[:] 

This takes a shallow copy. Personally, I don't like this option, because the slicing notation is not clear to everyone. But it is definitely the shorter notation. This will fail if you want to copy a list of lists. Being a shallow copy, it does not copy lists of lists. It will merely  copy the list of pointers to other "sub"-lists.


2)

import copy
b = copy.copy(a)

This takes a shallow copy. It is more clear from the code what this does, so this is usually my preferred approach. Still a shallow copy, however.


3)
 
import copy 
b = copy.deepcopy(a)

This is the fail-safe method and takes a copy of everything, so use this if you're in doubt. Works pretty much in every case. The down side is that it's usually slower to copy everything with deepcopy.

Figure 1 : a and b contain to the  

word "banana" in two different ways.