Numpy.dot TypeError: Cannot cast array data from dtype('float64') to dtype('S32') according to the rule 'safe'

Just taking the input from BrenBarn and Warren Weckesser to provide a code snippet which should run (by converting your strings to float):

a = map(lambda x: float(x),a)
b = map(lambda x: float(x),b)
np.dot(a,b.T)

or simpler as suggested by @JLT

a = map(float,a)
b = map(float,b)
np.dot(a,b.T)

But as Warren Weckesser already said, you should check the types of the array, most likely one already contains floats.