[Tutor] group txt files by month

[questions anon]

I think what I am trying to do is relatively easy but can't get my head
around how to do it.
I have a list of txt files that contain daily rainfall for many years. I
would like to produce a list that contains the year-month and the max, min
and mean of rainfall for each month.
My main question at this stage is how to group the files by each month for
each year?
They are set out like:
r20110101.txt
r20110102.txt
r20110103.txt
r20110104.txt
r20110105.txt
r20110106.txt
r20110107.txt
r20110108.txt
r20110109.txt
r20110110.txt
r20110111.txt
r20110112.txt
r20110113.txt
r20110114.txt
r20110115.txt
r20110116.txt
r20110117.txt
r20110118.txt

and so on for each day for many years.

so far I am able to open each file and calculate the max, min and mean for
each file (see below) but not sure about grouping to monthly for each year.

MainFolder=r"E:/Rainfalldata/"
outputFolder=r"E:/test/"
for (path, dirs, files) in os.walk(MainFolder):
    path=path+'/'
    for fname in files:
        if fname.endswith('.txt'):
            filename=path+fname
            f=np.genfromtxt(filename, skip_header=6)
            print f.max(), f.min(), f.mean()

the ideal output would be something like:
year-month      max   min   mean
2010-12         100      0      50
2011-01          200     0      100
2011-02          50       0      25


any feedback will be greatly appreciated.
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