[Tutor] Splitting a number into even- and odd- numbered digits

[Python]

On Wed, 2006-04-19 at 17:17 -0700, Carroll, Barry wrote:
> Greetings:
> 
> I am writing a function that accepts a string of decimal digits,
> calculates a checksum and returns it as a single character string.  
> The first step in the calculation is to split the input into two
> strings: the even- and odd- numbered digits, respectively.  The least
> significant digit is defined as odd.  
> 
This sounds like credit card checksum processing.  This is my code for
that:

def isbad(cardnumber):
    factors = ([2,1] * 8)[-len(cardnumber):]        # alternating factors of 2 and 1 ending with 1
    chkprods = [int(d)*f for (d,f) in zip(cardnumber,factors)]
    return sum(chkprods) % 10

This deviates quite a bit from your description and the description of
the algorithm that I was working from.  It was only after I had coded up
separate even/odd character lists and looked at what was going on that I
realized there was a much simpler way to describe the rules.

Hopefully, I am not off in left field here.

> The following code fragment does the job but seems sort of brutish and inelegant to me:
> 
> >>>>>>>
> >>> s = '987654321'
> >>> odd = ''
> >>> for c in s[::-2]:
> ...     odd = c + odd
> ...     
> >>> s = s[:-1]
> >>> even = ''
> >>> for c in s[::-2]:
> ...     even = c + even
> ...     
> >>> odd
> '97531'
> >>> even
> '8642'
> >>>>>>>
> 
> Is there a better (i.e. more Pythonic) way to do this?  
> 
> Thanks in advance for all your help.
> 
> Regards,
>  
> Barry
> barry.carroll at psc.com
> 541-302-1107
> ________________________
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> 
> 
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