これを見てください:
>>> from collections import Counter
>>> mystr = "AaAaaEeEiOoouuu"
>>> a,b = Counter(c for c in mystr.lower() if c in "aeiou").most_common(1)[0]
>>> "the most frequent vowel is {} occurring {} times".format(a.upper(), b)
'the most frequent vowel is A occurring 5 times'
>>>
についての参考資料collections.Counter
です。
編集:
以下は、何が起こっているかの段階的なデモンストレーションです。
>>> from collections import Counter
>>> mystr = "AaAaaEeEiOoouuu"
>>> Counter(c for c in mystr.lower() if c in "aeiou")
Counter({'a': 5, 'o': 3, 'e': 3, 'u': 3, 'i': 1})
>>> # Get the top three most common
>>> Counter(c for c in mystr.lower() if c in "aeiou").most_common(3)
[('a', 5), ('o', 3), ('e', 3)]
>>> # Get the most common
>>> Counter(c for c in mystr.lower() if c in "aeiou").most_common(1)
[('a', 5)]
>>> Counter(c for c in mystr.lower() if c in "aeiou").most_common(1)[0]
('a', 5)
>>> a,b = Counter(c for c in mystr.lower() if c in "aeiou").most_common(1)[0]
>>> a
'a'
>>> b
5
>>>