I recall in the past being able to evaluate rlocus for a single gain to obtain the closed-loop poles (as well as plotting), along the lines of:

L = tf([1,1],[1,2,3])
rr = rlocus(L, gains=np.array([1]))

but this fails giving the final message:
.../site-packages/control/pzmap.py:124
ValueError: zero-size array to reduction operation minimum which has no identity

While this does work:
rr = rlocus(L, gains=np.array([1,1]))
allowing for an evaluation at a single gain would be nice.