Checking if a String is a Pangram?

from string import ascii_lowercase, ascii_uppercase


def pangram(s):
    L1 = list(s)
    L2, str2 = [], ""
    k = 0

    while k < len(L1):
        for i in L1:
            if i in ascii_lowercase:
                if i not in L2:
                    L2.append(i)

                for j in L1:
                    if (i == j) or (i == j.lower()):
                        del L1[L1.index(j)]

            elif i in ascii_uppercase:
                if i.lower() not in L2:
                    L2.append(i.lower())

                for j in L1:
                    if (i == j) or (i == j.upper()):
                        del L1[L1.index(j)]

        k += 1

    L2.sort()

    for i in L2:
        str2 += i

    if str2 == ascii_lowercase:
        return "pangram"
    else:
        return "not pangram"


val = pangram("The Quick Brown Fox Jumps Over The Lazy Dog")
print(val)