Alternative to processing a Python requirements file

check_requirements.py

#!/usr/bin/env python3

"""Checks the installation of packages, as recommended by the authors of
   `Tiny Python Projects` (MEAP 6.0.0, Manning Publications 2020) to
   learn by doing while enduring the covid-19 `stay safer @ home` orders.
   
   See https://www.manning.com/books/tiny-python-projects?query=tiny.
"""

import shlex
import subprocess


def verify_requirements():
    """ Verifies the following user installed packages"""
    tools = [
        "black",
        "flake8",
        "ipython",
        "mypy",
        "pylint",
        "pytest",
        "yapf",
        "dephell",
    ]

    for tool in tools:
        # probe whereabouts within site package folder(s)?
        if len(subprocess.getoutput(f"python3 -m pip list | grep {tool}")) == 0:
            # perhaps tool is managed by homebrew?
            if len(subprocess.getoutput(f"ls /usr/local/{tool}")) == 0:
                # install tool
                command_string = f"python3 -m pip install -U {tool}"
                arguments = shlex.split(command_string)
                process = subprocess.Popen(arguments)

                # see ref 2
                if process.returncode != 0:
                    message = utils.decode_utf8(process.stdout)
                    if len(message) == 0:
                        message = utils.decode_utf8(process.stderr)
                    raise CalledProcessError(message)
                else:
                    print(f"Installed {tool}, in site packages.")
            else:
                print(f"Found {tool}, managed by homebrew.")
        else:
            print(f"Found {tool}, in site packages.")


if __name__ == "__main__":
    try:
        verify_requirements()
    except subprocess.CalledProcessError as err:
        print(err.output)


# Terminal stdout:
# Found black, managed by homebrew.
# Found flake8, in site packages.
# Found ipython, managed by homebrew.
# Found mypy, in site packages.
# Found pylint, in site packages.
# Found pytest, in site packages.
# Found yapf, in site packages.
# Found dephell, managed by homebrew.

# Could have run `python3 -m pip install -r requirements.txt` as an alternative,
# but what fun is there in that?

# References:
# 1. Python 3.8.1 API Subprocess management:
#    https://docs.python.org/3/library/subprocess.html
# 2. Python Example: subprocess.CalledProcessError():
#    https://www.programcreek.com/python/example/2244/subprocess.CalledProcessError