How do implicit relative imports work in Python?

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Packages are directories with a __init__.py file, yes, and are loaded as a module when found on the module search path. So pkg is only a package that you can import and treat as a package if the parent directory is on the module search path.

But by running the pkg/core/main.py file as a script, Python added the pkg/core directory to the module search path, not the parent directory of pkg. You do have a __init__.py file on your module search path now, but that’s not what defines a package. You merely have a __main__ module, there is no package relationship to anything else, and you can’t rely on implicit relative imports.

You have three options:

  1. Do not run files inside packages as scripts. Put a script file outside of your package, and have that import your package as needed. You could put it next to the pkg directory, or make sure the pkg directory is first installed into a directory already on the module search path, or by having your script calculate the right path to add to sys.path.

  2. Use the -m command line switch to run a module as if it is a script. If you use python -m pkg.core Python will look for a __main__.py file and run that as a script. The -m switch will add the current working directory to your module search path, so you can use that command when you are in the right working directory and everything will work. Or have your package installed in a directory already on the module search path.

  3. Have your script add the right directory to the module search path (based on os.path.absolute(__file__) to get a path to the current file). Take into account that your script is always named __main__, and importing pkg.core.main would add a second, independent module object; you’d have two separate namespaces.

I also strongly advice against using implicit relative imports. You can easily mask top-level modules and packages by adding a nested package or module with the same name. pkg/time.py would be found before the standard-library time module if you tried to use import time inside the pkg package. Instead, use the Python 3 model of explicit relative module references; add from __future__ import absolute_import to all your files, and then use from . import <name> to be explicit as to where your module is being imported from.

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