Creating a back-up list in python

Question

I want to create a back-up list of another list in python. Here is an example of the code.

x = [1,2,3]  
y = x  
x.pop(0)  

print y

This however yields the result y = [2,3] when I want it to yield [1,2,3] . How would I go about making the y list independent of the x list?

Answer 1

A common idiom for this is y = x[:] . This makes a shallow copy of x and stores it in y .

Note that if x contains references to objects, y will also contain references to the same objects. This may or may not be what you want. If it isn't, take a look at copy.deepcopy() .

Answer 2

Here is one way to do it:

import copy

x = [1,2,3]
y = copy.deepcopy(x)
x.pop(0)
print x
print y

from the docs here

Answer 3

While aix has the most parsimonious answer here, for completeness you can also do this:

y = list(x)

This will force the creation of a new list, and makes it pretty clear what you're trying to do. I would probably do it that way myself. But be aware- it doesn't make a deep copy (so all the elements are the same references).

If you want to make sure NOTHING happens to y, you can make it a tuple- which will prevent deletion and addition of elements. If you want to do that:

y = tuple(x)

As a final alternative you can do this:

y = [a for a in x]

That's the list comprehension approach to copying (and great for doing basic transforms or filtering). So really, you have a lot of options.