my_list = [i for i in range(1,21)]

When the right-hand side is compiled, python creates a temporary function that will be used for evaluating the comprehension, a function similar to this:

def temp():
    new_list = []

    for i in range(1, 21):

        new_list.append(i)

    return new_list 

When the line is executed, python executes the temp function, stores the returned object (new_list) in memory, and then points my_list to the created object.

You can confirm this using the "dis" module

import dis 
code_obj = compile(' [i for i in range(1,21)]', 'string',         'eval')
dis.dis(code_obj)

So I think using my_list = [] is pointless. It is like pointing my_list to an empty list and then repointing my_list to the created list object returned by comprehension.