While running in tmux session 0, window 3, I run a script with the following contents:
MWE:

import libtmux as tmux
t       = tmux.Server()
sess    = t.new_session('uut')
win0    = sess.select_window(0)

Expected Result: tmux session 'uut' has window 0 selected, and a handle to it is returned in win0
Actual Result: tmux session 0 switches to window 0, but win0 is also a handle to window 0 in session 'uut'.

Relevant configuration: macOS 10.14
tmux: 2.4
python: 2.7.15 and 3.7.0 (issue occurs in both)
libtmux: cloned from pypi on 16 Nov. 2018.