Pull Last 3 Months

[John Machin]

On 18/10/2007 10:33 AM, Ben Finney wrote:
> Paul Hankin <paul.hankin at gmail.com> writes:
> 
>> import datetime
>>
>> months = 'Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec'.split()
>>
>> def last_months(n):
>>     month = datetime.date.today().month
>>     return [months[(month - i - 1) % 12] for i in range(n)]
>>
>> print last_months(3)
> 
> Heck you don't even need the magic number 12 in there.
> 
>     import datetime
> 
>     months = 'Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec'.split()

Heck if you really want to be anal, you could even guard against a typo 
(or one of those spaces actually being '\xA0' [seen it happen]) by 
adding in here:
       MONTHS_IN_YEAR = 12
       assert len(months) == MONTHS_IN_YEAR

>     def last_months(n):
>         month = datetime.date.today().month
>         return [months[(month - i - 1) % len(months)
>                 for i in range(n)]
> 
> In general I try to avoid magic numbers: always be explicit about the
> semantic purpose of the number, either by binding a meaningful name to
> it and only using that reference thereafter, or showing how that value
> is derived.
> 

It's a bit hard to see how anybody could imagine that in the expression
     [months[(month - i - 1) % 12] for i in range(n)]
the number 12 referred to anything but the number of months in a year.