from string import ascii_lowercase
def checkio(text):
letters = [x for x in text.lower() if x in ascii_lowercase]
counts = sorted({x : letters.count(x) for x in letters}.items(), key=lambda x:x[1], reverse=True)
wanted_count = counts[0][1]
wanted = sorted([x[0] for x in counts if x[1] == wanted_count])[0]
return wanted
if __name__ == '__main__':
#These "asserts" using only for self-checking and not necessary for auto-testing
assert checkio("Hello World!") == "l", "Hello test"
assert checkio("How do you do?") == "o", "O is most wanted"
assert checkio("One") == "e", "All letter only once."
assert checkio("Oops!") == "o", "Don't forget about lower case."
assert checkio("AAaooo!!!!") == "a", "Only letters."
assert checkio("abe") == "a", "The First."
print("Start the long test")
assert checkio("a" * 9000 + "b" * 1000) == "a", "Long."
print("The local tests are done.")
http://www.checkio.org/mission/most-wanted-letter/