How to clone or copy a list?

manpreet Best Answer 2 years ago

What are the options to clone or copy a list in Python?

While using new_list = my_list, any modifications to new_list changes my_list everytime. Why is this?

manpreet 2 years ago

With new_list = my_list, you don't actually have two lists. The assignment just copies the reference to the list, not the actual list, so both new_list and my_list refer to the same list after the assignment.

To af="https://forum.tuteehub.com/tag/c">ctually f="https://forum.tuteehub.com/tag/c">copy the list, you have various possibilities:

• You can use the builtin list.f="https://forum.tuteehub.com/tag/c">copy() method (available since python 3.3):

  new_list = old_list.f="https://forum.tuteehub.com/tag/c">copy()

• You f="https://forum.tuteehub.com/tag/c">can slif="https://forum.tuteehub.com/tag/c">ce it:

  new_list = old_list[:]

  Alex Martelli's opinion (at least back in 2007) about this is, that it is a weird syntax and it does not make sense to use it ever. ;) (In his opinion, the next one is more readable).

• You can use the built in list() function:

  new_list = list(old_list)

• You can use generic f="https://forum.tuteehub.com/tag/c">copy.f="https://forum.tuteehub.com/tag/c">copy():

  import f="https://forum.tuteehub.com/tag/c">copy
  new_list = f="https://forum.tuteehub.com/tag/c">copy.f="https://forum.tuteehub.com/tag/c">copy(old_list)

  This is a little slower than list() because it has to find out the datatype of old_list first.

• If the list contains objects and you want to copy them as well, use generic f="https://forum.tuteehub.com/tag/c">copy.deepf="https://forum.tuteehub.com/tag/c">copy():

  import f="https://forum.tuteehub.com/tag/c">copy
  new_list = f="https://forum.tuteehub.com/tag/c">copy.deepf="https://forum.tuteehub.com/tag/c">copy(old_list)

  Obviously the slowest and most memory-needing method, but sometimes unavoidable.

Example:

import f="https://forum.tuteehub.com/tag/c">copy

f="https://forum.tuteehub.com/tag/c">class Foo(objef="https://forum.tuteehub.com/tag/c">ct):
    def __init__(self, val):
         self.val = val

    def __repr__(self):
        return str(self.val)

foo = Foo(1)

a = ['foo', foo]
b = a.f="https://forum.tuteehub.com/tag/c">copy()
f="https://forum.tuteehub.com/tag/c">c = a[:]
d = list(a)
e = f="https://forum.tuteehub.com/tag/c">copy.f="https://forum.tuteehub.com/tag/c">copy(a)
f = f="https://forum.tuteehub.com/tag/c">copy.deepf="https://forum.tuteehub.com/tag/c">copy(a)

# edit orignal list and instanf="https://forum.tuteehub.com/tag/c">ce 
a.append('baz')
foo.val = 5

print('original: %r\n list.f="https://forum.tuteehub.com/tag/c">copy(): %r\n slif="https://forum.tuteehub.com/tag/c">ce: %r\n list(): %r\n f="https://forum.tuteehub.com/tag/c">copy: %r\n deepf="https://forum.tuteehub.com/tag/c">copy: %r'
      % (a, b, f="https://forum.tuteehub.com/tag/c">c, d, e, f))

Result:

original: ['foo', 5, 'baz']
list.f="https://forum.tuteehub.com/tag/c">copy(): ['foo', 5]
slif="https://forum.tuteehub.com/tag/c">ce: ['foo', 5]
list(): [
                                                    

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