Python: operation being unexpectedly propagated along parent lists

Question

I am inserting and removing elements from a list that has been copied from another one I want to preserve unchanged. However, after applying the operations to the former, the later results also changed. How can I avoid that?

This is an example of what is happening:

a = range(11)

b = []

for i in a:
    b.append(i+1)

print b
#Out[10]: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

c = b
# Here, what I expect is to safely save "b" and work on its copy.

c.insert(-1,10.5)

print c
#Out[13]: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10.5, 11]

c.remove(11)
print c

#Out[15]: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10.5]

# So far everything is right: I inserted and removed what I wanted.
# But then, when I check on my "backed-up b", it has been modified:

print b
#Out[16]: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10.5]

# On the other hand, "a" remains the same; it seems the propagation does not affect
# "loop-parenthood":

print a
# Out[17]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

I do not understand why the operation propagates over the parent list. How can I avoid that? Should I save the list as an arrange, or should I create the list copy by using a loop?

Answer 1

You need to make a deepcopy:

import copy 

a = range(11)

b = []

for i in a:
    b.append(i+1)

print b
#Out[10]: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

c = copy.deepcopy(b)
# Here, what I expect is to safely save "b" and work on its copy.

c.insert(-1,10.5)

print c
#Out[13]: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10.5, 11]

c.remove(11)
print c

#Out[15]: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10.5]

# So far everything is right: I inserted and removed what I wanted.
# But then, when I check on my "backed-up b", it has been modified:

print b
#Out[16]: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10.5]

# On the other hand, "a" remains the same; it seems the propagation do not affect
# "loop-parenthood":

print a

Because without making a copy of the array, you are just creating a reference to the array. Here is how to use copy:

http://docs.python.org/2/library/copy.html

Answer 2

You need to make either a shallow copy or a deep copy (if you need to recursively copy inner objects) using c = b[:] or copy.deepcopy(b). Doing c = b, just creates a reference pointed to the same object as by b, so, if b changes or c changes, both the variables reflect those changes.

>>> a = range(11)
>>> b = []
>>> for i in a:
        b.append(i+1)


>>> print b
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
>>> c = b[:]
>>> c.insert(-1, 10.5)
>>> print c
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10.5, 11]
>>> c.remove(11)
>>> print c
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10.5]
>>> print b
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

Answer 3

Use

new_list = old_list[:]

or

new_list = list(old_list)

How to clone or copy a list?