Module discovery

Slotscheck needs to import files in order to check them. This process usually behaves as you would expect. However, there are some complications that you may need to be aware of.

Summary

You should generally be fine if you follow these rules:

  • To check files in your current directory, you should run slotscheck as python -m slotscheck.

  • To check files elsewhere, you may need to set the $PYTHONPATH environment variable.

Whether you run python -m slotscheck or just slotscheck has an impact on which files will be imported and checked. This is not a choice by slotscheck, but simply the way Python works. When running python -m slotscheck, the current working directory is added to sys.path, so any modules in the current directory can be imported. This is not the case when running bare slotscheck.

So if you run slotscheck foo.py, foo will not be importable. In fact, if foo happens to be the name of an installed module, import foo will import that instead! In that case slotscheck will refuse to run, and print an informative message. An alternative way to ensure the correct files can be imported is with the $PYTHONPATH environment variable.

To illustrate all this, imagine the following file tree:

src/
  foo/
    __init__.py
    bar.py

In this example:

  • slotscheck src/foo/bar.py will result in an error, because src is not in sys.path.

  • slotscheck -m foo.bar will result in an error, because src is not in sys.path.

  • cd src && slotscheck foo/bar.py will also result in an error, because the current working directory is not in sys.path.

  • cd src && slotscheck -m foo.bar will also result in an error, because the current working directory is not in sys.path.

  • cd src && python -m slotscheck foo/bar.py will scan the foo.bar module as expected, because the current working directory is in the import path.

  • cd src && python -m slotscheck -m foo.bar will scan the foo.bar module as expected, because the current working directory is in the import path.

  • env PYTHONPATH=src slotscheck src/foo/bar.py will scan the foo.bar module as expected, because src is added to sys.path by environment variable.

  • env PYTHONPATH=src slotscheck -m foo.bar will scan the foo.bar module as expected, because src is added to sys.path by environment variable.

Once the foo module is installed into site-packages, the following behavior will change: if foo/ files are passed, but the installed module would be imported instead, slotscheck will report an error.

Why doesn’t slotscheck just add the right paths to sys.path for me?

Automatically changing sys.path is a global change that could have unintended consequences. In this case it’s best not to assume this is the user’s intention. Since you’ll probably only need to add a single entry to Python’s path, it’s easy to define $PYTHONPATH explicitly.