<s>[INST] 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?
 [/INST] 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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