Use a list comprehension like this:
main_list = [item for item in list_2 if item not in list_1]
Output:
>>> list_1 = ["a", "b", "c", "d", "e"]
>>> list_2 = ["a", "f", "c", "m"]
>>>
>>> main_list = [item for item in list_2 if item not in list_1]
>>> main_list
['f', 'm']
Edit:
Like mentioned in the comments below, with large lists, the above is not the ideal solution. When that's the case, a better option would be converting list_1 to a set first:
set_1 = set(list_1) # this reduces the lookup time from O(n) to O(1)
main_list = [item for item in list_2 if item not in set_1]