Python installation change on Mac

[snippsat]

Doing it Right

python3 will launch the homebrew-installed Python 3 interpreter.
Homebrew version of Python 3 is installed then pip will point to Python 3.

However when I run a simple script that worked with 2.7 it fails importing import matplotlib and numpy with "ModuleNotFoundError" error. What else is necessary for the install to find the packages.

You most pip install into Python 3.
Check that pip or pip3 point to python 3.

mint@mint ~ $ pip -V

# Install
pip install numpy
pip install -U matplotlib

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Alternative pyenv is the best version management to use,
and safe it do not mess with system Python installation.
Examle Linux,work the same on Mac.

# Install
mint@mint ~ $ pyenv install 3.6.4
Downloading Python-3.6.4.tar.xz...
-> https://www.python.org/ftp/python/3.6.4/Python-3.6.4.tar.xz
Installing Python-3.6.4...
Installed Python-3.6.4 to /home/mint/.pyenv/versions/3.6.4

# Set python and pip to point to 3.6.4
mint@mint ~ $ pyenv global 3.6.4
mint@mint ~ $ python -V
Python 3.6.4
mint@mint ~ $ pip -V
pip 9.0.1 from /home/mint/.pyenv/versions/3.6.4/lib/python3.6/site-packages (python 3.6)
# Finish