William M. answered 12/15/19
STEM Tutor (Ph.D, Northwestern) | Algebra–Calc, Bio, Chem, Physics, CS
There is no need to shut down the shell and reopen it (although that will work).
Just call workon environment_name to activate your environment,
and call deactivate to deactivate it.
If you have set up the virtual environment correctly, you will see the name of the virtual environment in parentheses when you call "workon".
e.g., if your virtual environment is called cv, then you activate it by calling:
your_prompt $ workon cv
(cv) your_prompt $ python
>>>quit()
(cv) your_prompt $ deactivate
your_prompt $