python - How to clone or copy a list? -

what options clone or copy list in python?

using new_list = my_list modifies new_list every time my_list changes.
why this?

with new_list = my_list, don't have 2 lists. assignment copies reference list, not actual list, both new_list , my_list refer same list after assignment.

to copy list, have various possibilities:

• you can slice it:

  new_list = old_list[:] 

  alex martelli's opinion (at least back in 2007) is, it weird syntax , not make sense use ever. ;) (in opinion, next 1 more readable).

• you can use built in list() function:

  new_list = list(old_list) 

• you can use generic copy.copy():

  import copy new_list = copy.copy(old_list) 

  this little slower list() because has find out datatype of old_list first.

• if list contains objects , want copy them well, use generic copy.deepcopy():

  import copy new_list = copy.deepcopy(old_list) 

  obviously slowest , memory-needing method, unavoidable.

example:

import copy  class foo(object):     def __init__(self, val):          self.val = val      def __repr__(self):         return str(self.val)  foo = foo(1)  = ['foo', foo] b = a[:] c = list(a) d = copy.copy(a) e = copy.deepcopy(a)  # edit orignal list , 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]