'ImportError: DLL load failed' when importing 2 modules with a similar dll file name

[mrrezaie]

Bug report

Bug description:

Hi,

I need to use two modules, let's say A and B. Both having a specific dll file in their path (Lib/site-packages/A/xxx.dll and Lib/site-packages/B/xxx.dll). B is the original package in C++, and is a compiled and distributed by A as well.

Both work well independently. However, I need to import both and work with them simultaneously. The order of import matters; the later one ends up with:
ImportError: DLL load failed while importing _ezc3d: The specified procedure could not be found.

I tried removing any reference from below, followed by importing the later one; but failed again.

globals()
locals()
os.environ['PATH']
sys.modules
sys.path

I also tried adding the path for the later one, as follows, failed again.

ctypes.util.find_library
sys.path.insert
ctypes.windll.LoadLibrary
os.environ['PATH']
os.add_dll_directory

I was wondering if there is any solution. Thank you in advance.
(Just in case, A is opensim, and B is ezc3d)

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[a12k]

Oh I haven't used Windows in a long while, but remember this "DLL Hell." I suspect what is happening is that when you import opensim and ezc3d, both try to load a DLL with the same name (maybe a shared dependency). The problem is that Windows identifies DLLs by their filename, not their full path. So it loads the first DLL it sees from the first import, then doesn't load the second because the first is already in memory based on the filename alone, which seems to have a different API, so the procedure isn't found. You can potentially load these manually and solve your issue (which is a Windows issue, not a Python issue) ike this:

import os
import ctypes

opensim_dll = r"C:\path\to\site-packages\opensim\xyz.dll"
ezc3d_dll = r"C:\path\to\site-packages\ezc3d\xyz.dll"

#preload the paths
ctypes.WinDLL(opensim_dll)
ctypes.WinDLL(ezc3d_dll)

# now import
import opensim
import ezc3d

That may work, or may need a little tweaking, but should be close. Good luck!

[aisk]

Hi, I don't think this is a bug report to CPython but a request for help, so this may not be the right place. Please go to https://discuss.python.org/, or contact the maintainers of these packages to ask for help.

Feel free to re-open this issue if someone thinks I'm wrong.

[mrrezaie]

Hi @a12k and @aisk; thanks for the response.

The snippet already failed, and there is actually no solution. That's why I think this is potentially a bug, and requires improvements. Here are what ChatGPT says (as @a12k mentioned):

On Windows, a DLL is loaded once per process, keyed by its filename, not by full path.
So if both DLLs are named "ezc3d.dll", then:

• The first one loaded wins
• The second WinDLL() call is silently ignored
• Both packages now use the same DLL, even if paths differ

What actually happens here:

1. ctypes.WinDLL(opensim_dll) loads ezc3d.dll (version A)
2. ctypes.WinDLL(ezc3d_dll) → Windows says ezc3d.dll already loaded
3. import opensim binds to version A
4. import ezc3d now also binds to version A

It would be great if there was a separate link to libraries for each module. I am not a developer and have no idea whether it's possible or not 🙃. Thanks.