Python3.4 统计字符个数

求以下问题要怎么写啊：
Define a function which, given a string argument, counts the total number of times every word appears in the string. For this problem, take a very simplified definition of a word as a contiguous subsequence of letters. Any non-letter character is taken as a word boundary. Upper and lower case letters are equivalent.
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定义一个函数，给定一个字符串参数，计算每个单词出现在字符串总次数 不区分大小写 。还有其他什么要求没看懂
##python 2.7 windows
words =raw_input (' please enter words:')
countdict={}
for word in words.strip():

word=word.lower()
if countdict.has_key(word):
countdict[word]+=1
else:
countdict[word]=1
print countdict
--------------------------

>>>
please enter words:asdasdnm,.nm.zxcnm,
{'a': 2, 'c': 1, 'z': 1, 'd': 2, 'm': 3, ',': 2, 'n': 3, 's': 2, 'x': 1, '.': 2}
>>>

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