Python 3.8 not loading C dll

Question

My working code broke after upgrading to Python 3.8.1. Following line used to work, but now returns an error message:

cdll.LoadLibrary('mydllpath/mydll.dll')
==> Could not find module 'mydll.dll'. Try using the full path with constructor syntax.

Someone suggested adding the dll folder to the dll search path. So I changed the code to:

    os.chdir('mydllpath')
    print(os.listdir())  # Proof of success
    os.add_dll_directory('mydllpath')
    cdll.LoadLibrary('my.dll')
    # cdll.LoadLibrary('mydllpath/my.dll')  # Also unsuccessful

Well, the print() on second line of code lists mydll.dll as one of the files in the directory, but Python/cdll still returns the exact same error message: "Could not find ..."
Looking for ideas/suggestions. Thanks ...

Hard to tell what's wrong, but I'd guess that some dependencies are missing since the upgrade. You might have to recompile. If that doesn't work, extract and provide a minimal reproducible example and provide the full (!) error message including the backtrace. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Mar 3, 2020 at 14:16

Babar-Baig · Accepted Answer · 2020-03-22 18:13:37Z

2

OK, I figured-out the issue: Python returned error message: "Could not find module 'mydll.dll'" whether is had a problem locating mydll, or another dll that mydll depends upon. Python 3.8.2 also makes the error message a bit more descriptive.
In my case, mydll.dll depends on a widely used dll located one level-up the directory tree. Once I added that folder to my search path using os.add_dll_directory(), the program started working again.

answered Mar 22, 2020 at 18:13

Babar-Baig

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2 Comments

ISquared Over a year ago

Did you have problems Python throwing you an AttributeError: module 'os' has no attribute 'add_dll_directory' ? This is what I get when I do os.add_dll_directory(dllpath) - dllpath of course being the main folder where all my driver dlls are

Babar-Baig Over a year ago

Sorry, that's a new one for me. Idea: Look into the module "os" to make sure the error message is correct.

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