Output shape of numpy's rfft function
Check out the numpy docs ( https://docs.scipy.org/doc/numpy/reference/generated/numpy.fft.rfft.html )

To sum up, real DFT normally takes a real input of N samples and returns a complex output of N/2+1 values.

What you end up with are the positive frequencies (up to the Nyquist frequency) and the 
negative frequencies, which are just complex conjugates of their positive counterparts, are discarded or are not even computed.