python - How to clone or copy a list?

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?

Answer

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 actually copy the list, you have various possibilities:

• You can use the builtin list.copy() method (available since Python 3.3):

  
  
  
  

  new_list = old_list.copy()
  




• You can slice 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 copy.copy():

  
  
  

  import copy
  new_list = copy.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 copy.deepcopy():

  
  
  
  

  import copy
  new_list = copy.deepcopy(old_list)
  

  
  
  

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

Example:

import copy

class Foo(object):
    def __init__(self, val):
         self.val = val

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

foo = Foo(1)


a = ['foo', foo]
b = a.copy()
c = a[:]
d = list(a)
e = copy.copy(a)
f = copy.deepcopy(a)

# edit orignal list and instance 
a.append('baz')

foo.val = 5

print('original: %r\n list.copy(): %r\n slice: %r\n list(): %r\n copy: %r\n deepcopy: %r'
      % (a, b, c, d, e, f))

Result:

original: ['foo', 5, 'baz']
list.copy(): ['foo', 5]

slice: ['foo', 5]
list(): ['foo', 5]
copy: ['foo', 5]
deepcopy: ['foo', 1]