beyond top level package error in relative import

Question

It seems there are already quite some questions here about relative import in python 3, but after going through many of them I still didn't find the answer for my issue. so here is the question.

I have a package shown below

package/
   __init__.py
   A/
      __init__.py
      foo.py
   test_A/
      __init__.py
      test.py

and I have a single line in test.py:

from ..A import foo

now, I am in the folder of package, and I run

python -m test_A.test

I got message

"ValueError: attempted relative import beyond top-level package"

but if I am in the parent folder of package, e.g., I run:

cd ..
python -m package.test_A.test

everything is fine.

Now my question is: when I am in the folder of package, and I run the module inside the test_A sub-package as test_A.test, based on my understanding, ..A goes up only one level, which is still within the package folder, why it gives message saying beyond top-level package. What is exactly the reason that causes this error message?

Answer 1

EDIT: There are better/more coherent answers to this question in other questions:

• Sibling package imports
• Relative imports for the billionth time

---

Why doesn't it work? It's because python doesn't record where a package was loaded from. So when you do python -m test_A.test, it basically just discards the knowledge that test_A.test is actually stored in package (i.e. package is not considered a package). Attempting from ..A import foo is trying to access information it doesn't have any more (i.e. sibling directories of a loaded location). It's conceptually similar to allowing from ..os import path in a file in math. This would be bad because you want the packages to be distinct. If they need to use something from another package, then they should refer to them globally with from os import path and let python work out where that is with $PATH and $PYTHONPATH.

When you use python -m package.test_A.test, then using from ..A import foo resolves just fine because it kept track of what's in package and you're just accessing a child directory of a loaded location.

Why doesn't python consider the current working directory to be a package? NO CLUE, but gosh it would be useful.