从另一个目录导入Cython公开的类

Question

I have a Python project in which I want to make use of a C++ class that I exposed through Cython (really, I just need a specific instance of the class, as the code below will demonstrate). Because there were a bunch of files associated with the class I decided to put it in its own package.

In the __init__.py file of this package, I have what amounts to the following code:

from foo import Foo  # import the class

bar = Foo(some_parameters)
__all__ = ["bar"]

This works fine when I run __init__.py by itself. However, when I try to access it from outside the directory:

from qux import bar  # inside main.py in the parent directory

I get the error traced back to the same __init__.py :

File "D:\path\to\qux\\__init__.py", line 2, in <module>
from foo import Foo
ImportError: No module named 'foo'

Recall that foo is a Cython file, not pure Python code.

The directory structure looks like this:

main_project\
  main.py
  (more supporting files here)
  qux\
    __init__.py
    cy_foo.cpp
    cy_foo.pyx
    foo.cpp
    foo.h
    foo.cp35-win_amd64.pyd
    (more supporting files here)

What's going on?

Answer 1

I don't think this has anything to do with Cython per se, rather, this issue is due to the fact that when you execute main.py in the top level directory, Python will execute __init__.py and search in the same directory failing to locate the foo module inside qux .

As a solution, change the import statement in __init__.py to:

from qux.foo import Foo

If for some reason you still need to run __init__.py as the __main__ script, you can use the oh so familiar if clause to check the __name__ :

if __name__ == "__main__":
    from foo import Foo
else:
    from qux.foo import Foo

bar = Foo("arguments")
__all__ = ["bar"]

Now, if run as the __main__ module, __init__.py will find foo , if not, it allows others to find it.