Python gives error when importing simple C extension module

Question

On windows I have built a very simple "hello world" C extension (the file hello.c from this site https://gist.github.com/physacco/2e1b52415f3a964ad2a542a99bebed8f). Using VS2015 I successfully obtain hello.dll. The problem is that I can't figure out how to import this file/module.

In the python shell (python 3.7) I have made sure that I'm in the same folder as the hello.dll. I have also made sure that sys.path() contains the folder path. But when I write "import hello" I get an error "ModuleNotFoundError: No module named 'hello'"

Does anyone has an idea of what is wrong is this very simple setup?

Update: When trying to import a module that does not exist the ModuleNotFoundError is reported. After renaming the hello.dll to hello.pyd an ImportError is returned. So it seems like it tries to actually load the module.

From what I understand; you built on VS2015, and obtained the (.dll). Now on Python side, you need to set it up as an Extension (see: distutils.core.Extension), for which you'll have to execute python3 setup.py build. FYI: this setup.py file is available in the same link you posted. — dvlper
– dvlper, Commented Jul 6, 2020 at 15:59
@dvlper I know it may seem strange, but I'm trying to avoid using setup.py. And because the example is so simple I hoped it would be easy to just use the module without running other tools. — cosas
– cosas, Commented Jul 6, 2020 at 16:19

DavidW · Accepted Answer · 2020-07-06 18:49:48Z

4

Python compiled modules on Windows have the extension .pyd, not .dll. If you'd built it using setup.py the file would be built with the correct name. However, you built it yourself and gave it a name that Python doesn't recognise as a module.

In terms of the build command: you have to link it with libpython. You don't look to be doing this. The error you report is definitely one that you can get if the module is not linked against all its dependencies.

I know you don't want to use setup.py, however I'd use it at least once just to see what it does and if it works. You'll then at least have a command that you can copy with a working set of options.

edited Jul 6, 2020 at 18:49

answered Jul 6, 2020 at 15:59

DavidW

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11 Comments

cosas Over a year ago

Ah, I see. When I rename hello.dll to hello.pyd I get a different error ">>> import hello Traceback (most recent call last): File "<stdin>", line 1, in <module> ImportError: DLL load failed: The specified module could not be found."

DavidW Over a year ago

It's found the DLL but something else is wrong - it's failing at a later stage of the process.

DavidW Over a year ago

I'd suggest that the exact command you used to build it would probably be the next thing to look at.

DavidW Over a year ago

How can the command you used to build it "not ring a bell"? Is something else generating the build options? This is all stuff that should be in the question...

DavidW Over a year ago

Python can be slightly picky about how you build it so it's worth starting from a known working point. If you're linking against Python 3.5 but trying to run in Python 3.7 that's definitely wrong though! The version does need to match.

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