python subfolder import chaos

Question

suppose I have a folder structure that looks like this:

.
├── A
│   ├── a.py
│   └── b.py
└── main.py

The files have the following content:

b.py:

class BClass:
    pass

a.py:

from b import BClass

main.py:

from A import a

If I run python3.3 A/a.py or python3.3 B/b.by, there are no errors. However, if I run python3.3 main.py, the following error occurs:

Traceback (most recent call last):
  File "main.py", line 1, in <module>
    from A import a
  File "/tmp/python_imports/A/a.py", line 1, in <module>
    from b import BClass
ImportError: No module named 'b'

Changing the import-line in a.py to import A.b works, but obviously python3.3 A/a.py will fail then. I am not actually interested in running python3.3 A/a.py but I want the module to be importable from multiple locations. Therefore a.py should import b.py regardless of where a.py is imported.

How can this issue be resolved?

Answer 1

Besides the __init__.py I mentioned in my comment which is mandatory for packages, you need to import the sibling module relatively:

from .b import BClass

Then it also works in Python 3.

Alternatively you can of course import the full name:

from A.b import BClass

But then your module isn't relocatable as easily within your package tree.

In neither way, though, you are able to use a.py as a standalone. To achieve this you would need to surround the import statement with try/except and try a different version in case the first one fails:

try:
  from .b import BClass
except ValueError:
  from b import BClass

But that is understandable. In a larger system, modules might depend on other modules somewhere in the package, otherwise they maybe should not be part of a package but standalone. And if there are such dependencies, using a module as if it was a standalone will of course be a problem.

Answer 2

You need an __init__.py file (empty will be just fine) in the A directory. Otherwise, python won't recognize it as a package.

Now you're A is a package, you should use either absolute imports or explicit relative imports. In this case, in A/a.py either use from A.b import BClass or from .b import BClass.