Python list help

Question

simple Python question:

Example list: A = [1,2,3,4,5]

I need to generate another list B ~~which is a shallow copy of list A~~ such that B is a new list containing the same elements in the same order (so that I can substitute one of B's elements w/o affecting A). How can I do this?

clarification: I want to do something like

def some_func(A)
   B = {what do I do here to get a copy of A's elements?}
   B[0] = some_other_func(B[0])
   yet_another_func(B)

based on all your answers + the Python docs, a better way to do what I want is the following:

def some_func(A)
   B = [some_other_func(A[0])] + A[1:]
   yet_another_func(B)

thanks for pointing me in the right direction!

You want a deep copy as JoshD mentioned. Really threw us off there! — wkl
– wkl, Commented Oct 22, 2010 at 19:43
Just added a clarification. I'm pretty sure I want shallow, not a deep copy. — Jason S
– Jason S, Commented Oct 22, 2010 at 19:43
@Jason S: your clarification doesn't help clarify what kind of copy you need. Look at the various answers below. They may reveal what you want. — JoshD
– JoshD, Commented Oct 22, 2010 at 19:45
hmm, then I'm off about what "shallow copy" means. I don't need the deep copy, but a straight "B=A" doesn't copy anything, it just assigns the same list to A and B. I want to copy all of the individual elements of A, but not any subelements. — Jason S
– Jason S, Commented Oct 22, 2010 at 19:49
@Jason S: OK. That would be a deep copy of the list, but a shallow copy of the elements in the list. B = A[:] will do just that. I believe copy.deepcopy will make deep copies of the elements, too. — JoshD
– JoshD, Commented Oct 22, 2010 at 19:52

JoshD · Accepted Answer · 2010-10-22 19:55:59Z

8

That would be a deep copy, not a shallow one.

Lists copy shallow by default. That's why there's a deepcopy command in the copy module.

B = copy.deepcopy(A)

Optionally, B = A[:] will do. But keep deepcopy in mind for future. More complex data types can benefit from it.

Added Info about copy:

A shallow copy:

b = [1,2]
a = b
b[0] = 11
print a  // [1,11]

A deep copy:

b = [1,2]
a = b[:]
b[0] = 11
print a  // [1,2]

But, furthermore:

>>> a = [[1,2]]
>>> b = a[:]
>>> b
[[1, 2]]
>>> a
[[1, 2]]
>>> a[0][0] = 11
>>> a
[[11, 2]]
>>> b
[[11, 2]]
>>>

So, the elements themselves are shallow copies in this case.

edited Oct 22, 2010 at 19:55

answered Oct 22, 2010 at 19:40

JoshD

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1 Comment

JoshD Over a year ago

@Jason S, thanks. I learned something with this question. So +1 to you, too!

Steven Rumbalski · Accepted Answer · 2010-10-22 19:45:46Z

5

Here are 3 ways to make a copy of list A:

Use slice notation:

copy_of_A = A[:]

Use the list constructor:

copy_of_A = list(A)

Use the copy module:

from copy import copy
copy_of_A = copy(A)

As you requested these copies are all shallow copies. To learn about the difference between shallow copy and deep copy read the documentation of the copy module.

answered Oct 22, 2010 at 19:45

Steven Rumbalski

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JoshD Over a year ago

@Jason S, I guess I should have been more explicit with my link :) Also +1. nice answer.

Roxy Light · Accepted Answer · 2010-10-22 19:41:06Z

3

Like this:

B = A[:]

answered Oct 22, 2010 at 19:41

Roxy Light

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Brandon E Taylor · Accepted Answer · 2010-10-22 19:42:51Z

3

You can perform that copy in the following way:

B = A[:]

answered Oct 22, 2010 at 19:42

Brandon E Taylor

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unutbu · Accepted Answer · 2010-10-22 19:53:02Z

3

B=A[:] suffices:

In [22]: A=[1,2]

In [23]: B=A[:]

In [24]: B[0]=100

In [25]: A
Out[25]: [1, 2]

In [26]: B
Out[26]: [100, 2]

A[:] uses slice notation to get the slice with all the elements of A. Since slices of Python lists always return new lists, you get a copy of A.

Note that the elements inside B are identical to the elements inside A. If the elements are mutable, mutating them through B will affect A.

edited Oct 22, 2010 at 19:53

answered Oct 22, 2010 at 19:40

unutbu

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3 Comments

JoshD Over a year ago

I think we all need to clarify when we mean a shallow copy of the list and a shallow copy of the elements in the list. When I hear shallow copy of a list, I think of a=b. What you've done, I would call a deep copy, even though the elements themselves are shallow copy, the list is a deep copy.

unutbu Over a year ago

@JoshD: I think you are right, I was misusing the term "shallow copy" as defined here: en.wikipedia.org/wiki/Shallow_copy#Shallow_copy

JoshD Over a year ago

It's an awkward situation because both are happening. A is a deep copy of B, but A[0] is a shallow copy of B[0]. Your current answer is excellent and clears this up perfectly. +1

Sujoy · Accepted Answer · 2010-10-22 19:42:48Z

2

import copy
A=[1,2,3,4,5]
B=copy.copy(A)
B[0]=9999
print B[0]
print A[0]

import copy and use copy.copy() for copying. see this for reference.

answered Oct 22, 2010 at 19:42

Sujoy

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