forever function does not have write-access on top-level variables in Python

Describe the bug
In python:
The function passed to basic.forever can read top-level variables, but not modify. Trying to do so makes the function read the variable as NaN.

To Reproduce

my_var = 2
def f():
    basic.show_number(my_var)
    my_var = my_var+1      # Offending line
    basic.show_number(my_var)
    basic.pause(1000000)
basic.forever(f)

Running as is will print NaN twice. Removing the offending line prints the correct value twice. This error occurs even when the offending line is changed to my_var = 1, i.e. simple assignment.

Expected behavior
The above code to print 2, then 3.

Desktop (please complete the following information):

• OS: Linux
• Browser: Mozilla Firefox 80.0.1

Additional context
Happens on master branch, and in live website.

Start by tracing how Python code passed to basic.forever is translated and how assignments to top-level variables are handled. Use the reproduction in this issue, including both my_var = my_var + 1 and simple assignment, and consider the work done when the function can read and update my_var so it displays 2 and then 3.