Best way to remove unused pip installed modules/module dependencies from a virtual env?

[Malcolm Greene]

[Reformatted as original post got mangled]

Looking for advice on the best way to remove unused modules from a Python virtual environment. My setup is Python 3.6.6 running on macOS although I believe my use case is OS independent.

Background: Long running project that, over the course of time, pip installed modules that are no longer used by the code. I'm looking for a way to identity unused modules and remove them. 

Here's my back-of-napkin strategy to do this. Wondering if there are holes in this approach or if there's an off-the-shelf solution for
my use case?

1. pip freeze > modules.txt

2. build a list of all import statements, extract out module names

3. remove these module names from modules.txt and add to used-modules.txt

4. modules that remain in modules.txt are either module dependencies of directly imported modules or no longer used

5. remove my virtual env and recreate it again to start with a fresh env

6. reinstall each directly imported module (from list in used-modules.txt); this will pull in dependencies again

7. pip freeze > requirements.txt <--- this should be the exact modules used by our code

Thank you,
Malcolm