Can anyone explain to me why b_list doesn't print c_list?

Question

def function_b(b_list, high_num):
    c_list = [0,0,0,0,0,0,0]
    i = 0
    for num in b_list:
        if num> high_num:
            c_list[i] = num
        i = i +1
    b_list = c_list
def main():
    b_list=[1,3,6,4,1,2,8]
    high_num=4
    function_b(b_list, high_num)
    print(b_list)
main()

Can anyone explain why this wouldn't print [0,0,6,0,0,0,8]? I thought that since lists were mutable objects that if we assign the parameter b_list =c_list in function_b, it would change b_list found within main. Why does this have no affect on the argument b_list we insert into function_b?

`b_list` becomes local to function as soon as you have an assignment to it, which is destroyed after the function scope is over. — Austin, Mar 05 '20 at 06:44
Does this answer your question? [Python modifying list within function](https://stackoverflow.com/questions/27264526/python-modifying-list-within-function) — Tayyab, Mar 05 '20 at 06:46
your function `function_b` is not returning anything and whatever Austin explained is also true. — Ashu007, Mar 05 '20 at 06:52

score 3 · Answer 1 · answered Mar 05 '20 at 06:44

use return from function_b

def function_b(b_list, high_num):
    c_list = [0,0,0,0,0,0,0]
    i = 0
    for num in b_list:
        if num> high_num:
            c_list[i] = num
        i = i +1
    return c_list

def main():
    b_list=[1,3,6,4,1,2,8]
    high_num=4
    b_list = function_b(b_list, high_num)
    print(b_list)
main()

score 2 · Accepted Answer · answered Mar 05 '20 at 06:42

Because in your function_b,b_list is a local variable.And it can not affect the local variable in the main function

Use global variable,like this:

def function_b(high_num):
    global b_list
    c_list = [0,0,0,0,0,0,0]
    i = 0
    for num in b_list:
        if num> high_num:
            c_list[i] = num
        i = i +1
    b_list = c_list
def main():
    global b_list
    b_list=[1,3,6,4,1,2,8]
    high_num=4
    function_b(high_num)
    print(b_list)
main()

Or use the return value:

def function_b(b_list, high_num):
    c_list = [0,0,0,0,0,0,0]
    i = 0
    for num in b_list:
        if num> high_num:
            c_list[i] = num
        i = i +1
    return c_list
def main():
    b_list=[1,3,6,4,1,2,8]
    high_num=4
    b_list = function_b(b_list, high_num)
    print(b_list)
main()

Or call `b_list.clear()` and `b_list.extend(c_list)` – Ed Ward Mar 05 '20 at 06:48 — Ed Ward, Mar 05 '20 at 06:48

Can anyone explain to me why b_list doesn't print c_list?

2 Answers2