When the computer has other ideas

Python does not seem to exist

1. Try python --version, then python3 --version, then py --version.
2. Close and reopen the terminal after installation.
3. If the command is still not found, the issue is often PATH-related rather than a total install failure.

python --version
python3 --version
py --version

A package installed, but import still fails

1. Confirm the environment is activated.
2. Run python -c "import sys; print(sys.executable)".
3. Install the package again inside that same environment.
4. If using Jupyter, confirm the notebook kernel matches the environment you expect.

python -c "import sys; print(sys.executable)"
python -m pip list

Jupyter is behaving strangely

1. Restart the kernel.
2. Run all cells from the top.
3. Check whether a variable from an earlier run is still lingering in memory.
4. If needed, build a fresh notebook with the smallest working example.

jupyter lab
# then inside Jupyter:
# Kernel -> Restart Kernel
# Run -> Run All Cells

napari or the plugin will not launch

1. Confirm you are in the environment where napari was installed.
2. Reinstall the plugin in that same environment.
3. If the environment feels messy, create a fresh one rather than layering more fixes onto the old one.