This is a function which produces the sum of primes beneath (not including) a certain number (using a variant on the sieve of Eratosthenes).
erastosum = lambda x: sum([i for i in xrange(2, x) if i == 2 or i == 3 or reduce(lambda y,z: y*z, [i%j for j in xrange(2,int(i ** 0.5) + 1)])])
Excessive use of lambda? Perhaps. Beautification would be nice, but I'm looking for performance optimizations. Sadly, I'm not sure if there is any further way to optimize the setup I've got right now, so any suggestions (on how to, or what else to do) would be nice.