Ordering a list by date substring

[pystarter]

I am trying to write some code so it orders the list by the date in the substring.

I am trying to use 'sorted', or possibly .sort

Can anyone give me some pointers?

mylist=["word_number_25_05_2011_XX", "word_number_14_04_2011_YY", "word_number_17_04_2011_XY"]

sorted(mylist, key=lambda x: x[2:5])
print mylist

I'm trying to get it so it will print:

word_number_14_ 04_2011_YY
word_number_17_ 04_2011_XY
word_number_25_ 05_2011_XX

[bvdet]

Sort on the character positions (12:) or split the string on the "_" character.

>>> mylist=["word_number_25_05_2011_XX", "word_number_14_04_2011_YY", "word_number_17_04_2011_XY"]
>>> mylist = sorted(mylist, key=lambda x: x[12:])
>>> mylist
['word_number_14_04_2011_YY', 'word_number_17_04_2011_XY', 'word_number_25_05_2011_XX']
>>> mylist=["word_number_25_05_2011_XX", "word_number_14_04_2011_YY", "word_number_17_04_2011_XY", "word_number_17_03_2010_XY"]
>>> mylist = sorted(mylist, key=lambda x: x.split("_")[2:5])
>>> mylist
['word_number_14_04_2011_YY', 'word_number_17_03_2010_XY', 'word_number_17_04_2011_XY', 'word_number_25_05_2011_XX']
>>>

Note that built-in function sorted() returns the sorted list and does not sort the list in place.

[pystarter]

But this then only sorts by the day, and does not take into account month and year?

[bvdet]

You may want to sort on year, month and day in that order. Following are two ways of accomplishing that:

def comp(a, b, *keys):
    if len(keys)<2:
        return cmp(a,b)
    a1 = a.split("_")
    b1 = b.split("_")
    for i in keys[:-1]:
        x = cmp(a1[i],b1[i])
        if x:
            return x
    return cmp(a1[keys[-1]], b1[keys[-1]])

mylist = ["word_number_25_05_2011_XX",
          "word_number_14_04_2011_YY",
          "word_number_17_04_2011_XY",
          "word_number_17_03_2011_XY",
          "word_number_17_03_2010_XY",
          "word_number_17_02_2010_XY",
          "word_number_16_02_2010_XY"]

mylist1 = sorted(mylist, lambda x,y: comp(x,y,4,3,2))
print
print mylist1



from operator import itemgetter

mylist = ["word_number_25_05_2011_XX",
          "word_number_14_04_2011_YY",
          "word_number_17_04_2011_XY",
          "word_number_17_03_2011_XY",
          "word_number_17_03_2010_XY",
          "word_number_17_02_2010_XY",
          "word_number_16_02_2010_XY"]

mylist=sorted([item.split("_") for item in mylist], key=itemgetter(4,3,2))
mylist = ["_".join(item) for item in mylist]
print mylist

There is probably a better way. Can someone improve on the above?