Python exec() command working in IDLE, not in program

Question

I want to re-assign the value of a variable chosen by the user. The idea is that, if the user chooses this variable, the value of that variable is set to zero.

The code looks like this:

v1 = 8.4
v2 = 12.1
v3 = 36.2

list = ['', 'v1=', 'v2=', 'v3=']

print("What v do you want to be zero?")
print("    1 = v1")
print("    2 = v2")
print("    3 = v3")
a = eval(input("Enter your choice: "))

(the user enters 1)

b = list[a] + '0'
exec(b)
print(v1, v2, v3)

When I enter this code in IDLE, line by line, I get:

0 12.1 36.2

But, when I run the identical code within a function (*so the exec() and print(v1,v2,v3) statements are inside the same function), I get:

8.4 12.1 36.2

Can anyone explain this? I think it's probably a terrible way to do what I'm doing (I'm new to coding), and I welcome your suggestions on how to do it better, maybe a loop or a series of 'if/elif' statements? However, what I'd really like to know is why this code works when it's entered line-by-line, but not as a function.

(* = edit)

This is a program that does not re-assign any variable:

# delete me.py

def main():

    v1 = 8.4
    v2 = 12.1
    v3 = 36.2

    list = ['', 'v1=', 'v2=', 'v3=']

    print("What v do you want to be zero?")
    print("    1 = v1")
    print("    2 = v2")
    print("    3 = v3")
    a = eval(input("Enter your choice: "))

    b = list[a] + '0'
    exec(b)

    print(v1, v2, v3)

main()

Answer 1

The easiest way to understand this is that you aren't allowed to modify variables through exec. It will sometimes work, but its not really allowed, and its never a good idea.

For this piece of code, you should write:

v = [8.4, 12.1, 36.2]
print("What v do you want to be zero?")
print("    1 = v1")
print("    2 = v2")
print("    3 = v3")
a = int(input("Enter your choice: "))
v[a - 1] = 0
print(v)

Basically, if you need to lookup a variable by user input, it should be in a list or dictionary. You should never need to do this for local variables.

Answer 2

This is because you are updating the local scope of the function inside which you exec is run.

Compare this:

v = 42
def f(): v = 43 #which is inside you exec
f()
print(v)