aF. aF. - 1 month ago 14
Python Question

How to clone or copy a list?

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

Using

new_list = my_list
then modifies
new_list
every time
my_list
changes.

Why is this?

Answer

You have various possibilities:

  • 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[:]
c = list(a)
d = copy.copy(a)
e = copy.deepcopy(a)

# edit orignal list and instance 
a.append('baz')
foo.val = 5

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

Result:

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