Standard Deviation One-liner

[Billy Mays]

I'm trying to shorten a one-liner I have for calculating the standard 
deviation of a list of numbers.  I have something so far, but I was 
wondering if it could be made any shorter (without imports).


Here's my function:

a=lambda d:(sum((x-1.*sum(d)/len(d))**2 for x in d)/(1.*(len(d)-1)))**.5


The functions is invoked as follows:

 >>> a([1,2,3,4])
1.2909944487358056