python module dlls

Ok it turns out python always loads the dll in the same directory as the pyd file, regardless of what the python and os paths are set to.

So I needed to copy the _sqlite3.pyd from python/v2.5/DLLS to my apps directory where the new sqlite3.dll is, making it load my new dll, rather than the one that comes with python (since the pyd files seem to follow the PYTHONPATH, even though the actual dlls themselves don't).

The answer with modifying os.environ['PATH'] is right but it didn't work for me because I use python 3.9. Still I was getting an error:

ImportError: DLL load failed while importing module X: The specified module could not be found.

Turned out since version python 3.8 they added a mechanism to do this more securely. Read documentation on os.add_dll_directory https://docs.python.org/3/library/os.html#os.add_dll_directory

Specifically see python 3.8 what's new:

DLL dependencies for extension modules and DLLs loaded with ctypes on Windows are now resolved more securely. Only the system paths, the directory containing the DLL or PYD file, and directories added with add_dll_directory() are searched for load-time dependencies. Specifically, PATH and the current working directory are no longer used, and modifications to these will no longer have any effect on normal DLL resolution. If your application relies on these mechanisms, you should check for add_dll_directory() and if it exists, use it to add your DLLs directory while loading your library.

So now this is the new way to make it work in python 3.8 and later:

import os
os.add_dll_directory('my-app-dir')

Again, the old way is still correct and you will have to use it in python 3.7 and older:

import os
os.environ['PATH'] = 'my-app-dir' + os.pathsep + os.environ['PATH']

After that my module with a dll dependency has been successfully loaded.

If you're talking about Python module DLLs, then simply modifying sys.path should be fine. However, if you're talking about DLLs linked against those DLLs; i.e. a libfoo.dll which a foo.pyd depends on, then you need to modify your PATH environment variable. I wrote about doing this for PyGTK a while ago, but in your case I think it should be as simple as:

import os
os.environ['PATH'] = 'my-app-dir' + os.pathsep + os.environ['PATH']

That will insert my-app-dir at the head of your Windows path, which I believe also controls the load-order for DLLs.

Keep in mind that you will need to do this before loading the DLL in question, i.e., before importing anything interesting.

sqlite3 may be a bit of a special case, though, since it is distributed with Python; it's obviously kind of tricky to test this quickly, so I haven't checked sqlite3.dll specifically.