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?
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: