def caractere_le_plus_frequent(chaine):
    d = {}
    for c in chaine:
        d[c] = d.get(c, 0) + 1
    
    caractere = ""
    frequence_max = -1
    for cle, val in d.items():
        if val > frequence_max:
            caractere = cle
            frequence_max = val
    return caractere


print(caractere_le_plus_frequent("abccccb"))

donnees = [
    {
        "address": {
            "building": "1449",
            "street": "Nostrand Avenue",
            "zipcode": "11226"
        },
        "grades": [
            {"date": "2014-10-21T02:00:00", "grade": "B"},
            # [...]
            {"date": "2011-12-14T01:00:00", "grade": "A"}
        ],
        "name": "Nostrand Donut Shop"
    },
    # [...]
]
mon_set = set()
for dico_resto in donnees:
    for evaluation in dico_resto.get("grades", []):
        if "grade" in evaluation.keys():
            mon_set.add(evaluation["grade"])
print(mon_set)