pypy-comprehensions

def covariance(X,Y, X_mean, Y_mean):
    if __debug__:
        assert X_mean is not None
        assert Y_mean is not None
        assert type(X_mean)==float
        assert type(Y_mean)==float
        assert len(X) == len(Y)
        assert len(X) > 0


    L = len(X)
    slower = True
    if slower:
        s = sum( x*y for x,y in zip(X,Y))
    else:
        s = 0
        for i in range(L):
            s += X[i] * Y[i]
    m = float(s)/float(L)

    return m - X_mean * Y_mean