ModuleNotFoundError: No module named 'X' even though 'pip install X' says it's installed (or 'Requirement already satisfied'). Why, and how do I fix it?

Cause: almost always, pip installed the package into a different Python than the one running your code. Common ways this happens: several Pythons on the machine (system, Homebrew, pyenv, conda, the Windows Store build), a virtual environment that's active in one terminal but not in your IDE, Jupyter running a kernel from another environment, sudo pip installing for root's Python, or a pip command that belongs to an older Python version. How to check: - In the failing program, print the interpreter: import sys; print(sys.executable). - Compare with the pip you used: pip --version shows which Python it belongs to. - In Jupyter, run import sys; print(sys.executable) in a cell. Fix: 1. Install with the same interpreter that runs the code: <that-python> -m pip install X (for example python -m pip install X, py -m pip install X on Windows). Using python -m pip, as the pip and Python packaging docs recommend, ties pip to that exact interpreter. 2. Use a virtual environment per project (python -m venv .venv, then activate it) and point your IDE at it: VS Code 'Python: Select Interpreter', PyCharm project interpreter. 3. Jupyter: install the package from inside the notebook with %pip install X (installs into the kernel's environment), or register your venv as a kernel (python -m pip install ipykernel, then python -m ipykernel install --user --name myenv) and select it. 4. Check the import name: some packages install under a different import name (pip install pillow -> import PIL, opencv-python -> import cv2, scikit-learn -> import sklearn, PyYAML -> import yaml, beautifulsoup4 -> import bs4). 5. Make sure no local file shadows or breaks it (a file named like the package in your project folder, or a requests.py of your own). Verify: <same python> -c "import X; print(X.__file__)" works and shows a path inside the expected environment. Sources: https://pip.pypa.io/en/stable/user_guide/ https://packaging.python.org/en/latest/tutorials/installing-packages/