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`ModuleNotFoundError` means that in running your program, Python attempted to import some module `xyz` but it was not present on your system.

It could be that you forgot to install the module (forgot to `pip install -r requirements.txt`), misspelled the module name, or tried to import a module that doesn't exist. But that would be somewhat obvious so let's assume it's not that, and the module name is correct. However, do note that some packages on Pypa for example have surprising import names - sometimes you `pip install xyz` but `import pyxyz` etc - this should normally be covered in the package's documentation.

The first useful thing is to find a way to test the import easily. If you got `ModuleNotFoundError: No module named 'xyz'`, you can reproduce it by running `python -c 'import xyz` in your shell: If the import succeeds, it will print nothing and exit with code 0. If the import fails, you will see the `ModuleNotFoundError` again.

Often, package imports fail due to an environment mismatch. The package is actually installed, but in another Python venv, and not in the current one. It's of course hard to find putative "other* venvs on your computer, but you can look in obvious places like a `venv/` dir in the project directory. `pip list` will show the packages installed in the *current* environment, so that can help identify them. You can of course just install them again in the current environment as an alternative to looking for the other one.

The environment stuff often comes up due to using "helper" programs, like conda or pyenv. IDEs with integrated Python consoles, like VS Code's Python tab, can also start it with a different environment than your terminal. It's useful to be mindful of which helper programs you are using and how they handle environments, and avoid helpers that are hard to understand.

As a brute force solution, you can try searching for the package or module `xyz` on your computer: `rg xyz /` (`rg` is https://github.com/BurntSushi/ripgrep). Because of Python's import system and conventions, if there is an importable `xyz`, there is probably a file `xyz.py` somewhere. You can analyze its path to figure out what environment it is.