Debug: Python ModuleNotFoundError after pip install

You pip install a package but Python says ModuleNotFoundError when you import it.

Common causes and fixes:

1. Wrong Python version: pip installed for Python 3.9 but you're running 3.11

Fix: Use python3 -m pip install <package> to ensure pip matches your Python

1. Not in virtualenv: Installed globally but running in a venv (or vice versa)

Fix: which python && which pip — they should point to the same env

1. Package name differs from import name: pip install Pillow, but import PIL

Fix: Check PyPI page for the correct import name

1. Jupyter kernel mismatch: Notebook uses a different Python than terminal

Fix: !which python inside the notebook to verify

Python has multiple installation locations and the pip you run might not correspond to the python you run.