In order for a python project to be importable when it consists of both .py files and compiled extensions, you have to do one of:

• have them be different top-level importable modules, no shared namespace

They are separate importable modules. However, the pure-Python module cannot find the C extension. This is the source tree after the mason-python build:

.+- build/|  +- cp311/|     +- ...|     +- src/|        +- ext/|           +- myext.cpython-311-x86_64-linux-gnu.so+- src|  +- ext|     +- myext.c+- module|  +- __init__.py+- main.py

main.py importsmodule needs to importmyext. However,module can't findmyext even withPYTHONPATH set.

• install the project (at least in editable mode) and merge the source and build trees together

I am not sure what this means. I am using editable builds but do you mean that after the build I would have to copy all of the pure-Python files into the build directorybuild/cp311?

If PYTHONPATH contains both:

• $PWD (to execute the statementimport module)
• $PWD/build/cp11/src/ext (to execute the statementimport myext)

Then in principle I can't see what the problem would be when running directly from the source tree. Have you tried adding e.g. a debug print to main.py to have it show what it sees as sys.path when the failure occurs?

I think in this example it would work, but if you modify it slightly to:

• src/mymodule/__init__.py
• src/mymodule/ext.c

then the issue is that you wind up with two conflicting locations for the same module:

• src/mymodule (contains the py files)
• build/src/mymodule (contains the shared libraries)

You can add both to yourPYTHONPATH, but python stops searching once it thinks it knows where the module lives. So, for example,

frommymodule.extimportfoo

will fail because python seessrc/mymodule/__init__.py and decides that "mymodule" lives atsrc/mymodule.

If I drop the src location fromPYTHONPATH, I can import the extension, and if I drop the build location, I can import__init__.py, but getting both to work at the same time is an open problem.

@eli-schwartz Does it means that we cannot run test on a wheel (because i don't think it is possible to install a wheel in editable)?

Not completely sure what you mean by that. "Editable installs" per the specification is just a wheel that doesn't contain your code, but instead contains a small import hook that locates your code from your build tree.

It's possible to test a non-editable wheel too. But your tests have to be separately importable from outside the wheel. For example, you can have

myproject/tests/

Or you can have

src/myproject/tests/

But what you cannot do is:

src/myproject/src/myproject/tests/

(which is a bad idea anyway), because the tests cannot be imported in the same python process as the project itself.