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


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