pythontutor

import string
# takes a non-empty string and returns a lowercase string that contains the most frequently
# occurring letter(s) in s in alphabetical order (ignoring case in the original string)
# you will likely need a nested loop for this...
def mostFrequentLetter(s):
    d = {}
    for c in s.lower():
        #Ignore other non-alphabetic characters.
        if c in string.ascii_lowercase:
            d[c] = d.get(c, 0) + 1
    #Make a list of tuples.
    t = [(v, k) for k, v in d.items()]
    #Sort the list by the elements in tuples.
    t.sort(key = lambda x: (-x[0], x[1]))
    if not t:
        return ''
    max_count = t[0][0]
    return ''.join(c[1] for c in t if c[0] == max_count)