In Python, a list comprehension is a construct that allows you to generate a new list by applying an expression to each item in an existing iterable and optionally filtering items with a condition. Apart from being briefer, list comprehensions often run faster.
A basic list comprehension consists of an expression followed by a for clause:
spam = [i * 2 for i in iterable]
The above uses the variable i to iterate over iterable. Each elements of the resulting list is obtained by evaluating the expression i * 2 at the current iteration.
In this step, you need to fill the empty list snake_cased_char_list using the list comprehension syntax.
Turn your empty list into a list comprehension that converts each character in pascal_or_camel_cased_string into a lowercase character and prepends an underscore to it (the code you commented out before may help you write the expression). Use char to iterate over pascal_or_camel_cased_string.