"backtracking" variables in python del function

Question

The del statement in python, when applied to a list, does something very strange. It not only deletes the entry in the list in question, but also "backtracks" to delete the same element in whatever list the variable was derived from.

For instance:

>>> x
[1, 2, 3, 4, 5]
>>> z=x
>>> del z[1]
>>> z
[1, 3, 4, 5]
>>> x
[1, 3, 4, 5]

In other words, even though I apply delete to z, the deletion is also applied to x. This "backtracking" doesn't come up with any other commands or functions, e.g.

>>> x=[1,2,3,4,5]
>>> z=x
>>> z+[6]
[1, 2, 3, 4, 5, 6]
>>> x
[1, 2, 3, 4, 5]

applying the add function to z leaves x unaffected, as it very well should.

This is causing me all sorts of headaches with the delete 'NA' scrip that I'm working on. Do you know what's going on with the del statement, and how can I get around this problem so that if I set z=x and then apply del to z, x remains unchanged?

Read this: python.net/~goodger/projects/pycon/2007/idiomatic/… — FogleBird
– FogleBird, Commented Nov 11, 2012 at 2:57

FogleBird · Accepted Answer · 2012-11-12 09:39:33Z

9

z = x does not make a copy of the list. z and x now point to the same list. Any modification of that list will be visible through both names, z and x.

With z + [6], you did not assign the result back to z. Try printing z after doing that. It will be unchanged, just like x.

z += [6] would be necessary to assign the result back to z and it would have the same behavior as your first example.

You can copy the list using one of these methods:

z = x[:]
z = list(x)

edited Nov 12, 2012 at 9:39

answered Nov 11, 2012 at 2:59

FogleBird

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6 Comments

Miles Over a year ago

Close, but there's an important distinction between z += [6] and z = z + [6] that you're missing.

miki725 Over a year ago

@Miles What is the difference between z += [6] and z = z + [6]. Seems to be the same thing. Am I missing something?

Eryk Sun Over a year ago

@miki725: z += [6] is an in-place operation on the list. z + [6] creates a new list.

Miles Over a year ago

Right, so if z and x point to the same list, the former will update the list that both variables point to, while z = z + [6] will assign z to a new longer list and leave x pointing to the original list.

FogleBird Over a year ago

Thanks for the clarification, I should've known better!

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