putting date strings in order

[Peter Otten]

noydb wrote:

> On May 12, 12:26 pm, John Machin <sjmac... at lexicon.net> wrote:
>> On May 13, 1:58 am, Jaime Fernandez del Rio <jaime.f... at gmail.com>
>> wrote:
>>
>>
>>
>>
>>
>> > On Tue, May 12, 2009 at 5:02 PM, MRAB <goo... at mrabarnett.plus.com>
>> > wrote:
>> > > John Machin wrote:
>>
>> > >> MRAB <google <at> mrabarnett.plus.com> writes:
>>
>> > >>> Sort the list, passing a function as the 'key' argument. The
>> > >>> function should return an integer for the month, eg 0 for 'jan', 1
>> > >>> for 'feb'. If you want to have a different start month then add
>>
>> > >> and if you don't like what that produces, try subtract :-)
>>
>> > > Oops!
>>
>> > >>> the appropriate
>> > >>> integer for that month (eg 0 for 'jan', 1 for 'feb') and then
>> > >>> modulo 12 to make it wrap around (there are only 12 months in a
>> > >>> year), returning the result.
>>
>> > > Actually, subtract the start month, add 12, and then modulo 12.
>>
>> > Both on my Linux and my Windows pythons, modulos of negative numbers
>> > are properly taken, returning always the correct positive number
>> > between 0 and 11. I seem to recall, from my distant past, that Perl
>> > took pride on this being a language feature. Anyone knows if that is
>> > not the case with python, and so not adding 12 before taking the
>> > modulo could result in wrong results in some implementations?
>>
>> If that happens, it's a
>> bug.http://docs.python.org/reference/expressions.html#binary-arithmetic-
o...
>>
>> If you look at function i_divmod() in the 2.x branch's Objects/
>> intobject.c, you'll be reassured to see that it doesn't just take
>> whatever C serves up :-)- Hide quoted text -
>>
>> - Show quoted text -
> 
> Thanks to those who provided suggestions.  I ended up using code
> similar to what Jaime provided above first -- truly eloquent and
> simple, especially compared to my original thoughts of several messy
> loops.  I knew it could be done way better.  Thanks very much Jaime!!
> That was a good learning experience for me.
> 
> fairly finished portion of code:
> 
> ordered_raster_list = []
> 
> pRasters = gp.ListRasters("precip_*", "All") # an enumeration object,
> arcgis method
> pRast = pRasters.next()
> while pRast:
> ##    month = pRast[-3:]
> ##    print month
>     print pRast
>     ordered_raster_list.append(pRast)
>     pRast = pRasters.next()
> 
> 
> print ordered_raster_list #unordered at this point
> 
> # create a dictionary dictating the order of the the precip_<months>
> rasters
> monthsD = {"precip_jan" : 1, "precip_feb" : 2, "precip_mar" : 3,
> "precip_apr" : 4, "precip_may" : 5, "precip_jun" : 6,
>            "precip_jul" : 7, "precip_aug" : 8, "precip_sep" : 9,
> "precip_oct" : 10, "precip_nov" : 11, "precip_dec" : 12}
> 
> # sort the list based on the dictionary
> ordered_raster_list.sort(None, lambda x : monthsD[x])
> 
> print ordered_raster_list #ordered
> 
> start = 2 #user to define, starting month
> 
> ordered_raster_list = ordered_raster_list[start - 1:] +
> ordered_raster_list[:start - 1]
> 
> print ordered_raster_list #ordered but starting in the middle, feb in
> this case, ending with jan

Hm, if ordered_raster_list is guaranteed to contain one string item for 
every month the above can be simplified to

months = [
    'precip_jan', 'precip_feb', 'precip_mar', 'precip_apr', 
    'precip_may', 'precip_jun', 'precip_jul', 'precip_aug', 
    'precip_sep', 'precip_oct', 'precip_nov', 'precip_dec']

start = 2
ordered_raster_list = months[start-1:] + months[:start-1]

print ordered_raster_list

What am I missing?

Peter