Python 3.6 / my function changes my global variables - python-3.x

Question

I am trying really hard for my function not to mess my global 'b' value because I wish to re-use that list in another similar function using sets but it seems that even not using the same name (y), they (b and y) are still bound together...
most of the print lines are for debugging only as I was not understanding what was happening.
What am I doing wrong?
import random
a = random.sample(range(1,25),8)
b = random.sample(range(1,25),11)
a.sort()
b.sort()
def list_rdup(x,y):
print('Loop remove duplicates:')
print('x:',x)
print('y:',y)
for i in x:
y.append(i)
y.sort()
print('y modified:',y)
c = []
for i in y:
if i in c:
pass
else:
c.append(i)
return c
print('a:',a)
print('b:',b)
print(list_rdup(a,b))
print('a:',a)
print('b:',b)
Output: we see a and b in their original state.. then I run the function and
print a and b again to show that b was modified in the process...
a: [1, 6, 10, 11, 12, 13, 17, 22]
b: [1, 2, 3, 7, 13, 16, 17, 19, 20, 21, 24]
Loop remove duplicates:
x: [1, 6, 10, 11, 12, 13, 17, 22]
y: [1, 2, 3, 7, 13, 16, 17, 19, 20, 21, 24]
y modified: [1, 1, 2, 3, 6, 7, 10, 11, 12, 13, 13, 16, 17, 17, 19, 20, 21, 22, 24]
[1, 2, 3, 6, 7, 10, 11, 12, 13, 16, 17, 19, 20, 21, 22, 24]
a: [1, 6, 10, 11, 12, 13, 17, 22]
b: [1, 1, 2, 3, 6, 7, 10, 11, 12, 13, 13, 16, 17, 17, 19, 20, 21, 22, 24]

Answer 1

The call list_rdup(a,b) simply passes the reference of a and b which are stored in x and y. So, any change in x and y will change a and b. If you do not want a and b to change make a copy by using b_copy = b.copy().

Answer 2

To avoid altering the elements, copy the array.
import random
a=[1, 6, 10, 11, 12, 13, 17, 22]
b=[1, 2, 3, 7, 13, 16, 17, 19, 20, 21, 24]
#a.sort()
#b.sort()
def list_rdup(x,y):
print('Loop remove duplicates:')
print('x:',x)
print('y:',y)
for i in x:
y.append(i)
y.sort()
print('y modified:',y)
c = []
for i in y:
if i in c:
pass
else:
c.append(i)
return c
print('a:',a)
print('b:',b)
print(list_rdup(a[:], b[:]))
print('a:',a)
print('b:',b)
a: [1, 6, 10, 11, 12, 13, 17, 22]
b: [1, 2, 3, 7, 13, 16, 17, 19, 20, 21, 24]
Loop remove duplicates:
x: [1, 6, 10, 11, 12, 13, 17, 22]
y: [1, 2, 3, 7, 13, 16, 17, 19, 20, 21, 24]
y modified: [1, 1, 2, 3, 6, 7, 10, 11, 12, 13, 13, 16, 17, 17, 19, 20, 21, 22, 24]
[1, 2, 3, 6, 7, 10, 11, 12, 13, 16, 17, 19, 20, 21, 22, 24]
a: [1, 6, 10, 11, 12, 13, 17, 22]
b: [1, 2, 3, 7, 13, 16, 17, 19, 20, 21, 24]
Also consider deep copying if you have non-primitive values in the array.