[bocklund]

Cython infers a module's qualified name from the structure implied by __init__.py. Old CMake build copied the whole symengine/ package into the build tree (file(COPY symengine/ ...) in the top-level CMakeLists.txt), so those files were present.

Since the move to scikit-build-core builds compile the wrapper with the module name symengine_wrapper instead of symengine.lib.symengine_wrapper and Symbol('x').__module__ == "symengine_wrapper" that breaks pickles across process.

This only occurs on non-editable installs and builds outside the source tree that have __init__.py files in the right structure and my proposed fix is to create dummy __init__.py files to produce the correct import structure if they don't already exist (which they do for in-tree builds).

To reproduce (before the fix):

# create.py
import pickle
import symengine
with open('x.pkl', 'wb') as fp:
  fp.write(pickle.dumps(symengine.Symbol('x')))

# read.py
import pickle
with open("x.pkl", "rb") as fp:
  print(pickle.load(fp))

Running

> python create.py
> python read.py
Traceback (most recent call last):
  File "/.../read.py", line 3, in <module>
    print(pickle.load(fp))
          ~~~~~~~~~~~^^^^
ModuleNotFoundError: No module named 'symengine_wrapper'

I'm not sure how to test this in CI since CI does in-tree builds and the module name resolves correctly.

It'd be great to get this in before 0.15 since we depend on this for PyCalphad and have been stuck pinned <0.14 for awhile