Another nice loop! And with a thing where you want to have a table of data. Do you remove things from the set, or validate a hash-table later? Let’s do the latter.

def is_pangram(s):
    seen = set([a.lower() for a in s])
    for l in "abcdefghijklmnopqrstuvwxyz":
        if l not in seen:
            return False
    return True