dll_paths.md

Adding & Removing DLL Search Paths in Python

Ensuring that DLLs can be found be python (and DLLs loaded by the process) may need to be solved differently depending on

       (a) Python version
       (b) The DLL loading logic implemented in the package(s) using DLLS.

Therefore, simple functions called ensure_dll and forget_dll were created to handle the different cases in a simple way.

Example of using ensure_dll and forget_dll

from pathtub import ensure_dll, forget_dll
import usb.backend.libusb1

#try loading library without adding the dlls to search path -> LibraryNotFoundException
usb.backend.libusb1._load_library()

dll_folder32 = r'C:\path to\libusb-1.0.21\MS32\dll'
ensure_dll(dll_folder32)

usb.backend.libusb1._load_library()
# <WinDLL 'C:\path to\libusb-1.0.21\MS32\dll\libusb-1.0.dll', handle 62690000 at 0x7297580>

• Now, if for some reason we want to forget a DLL path, we can use

forget_dll(dll_folder32)

# -> LibraryNotFoundException
usb.backend.libusb1._load_library()

• Note that ensure_dll always adds DLL folder just for the duration of the Process, and therefore "forgetting" is not needed.
• There might be some special cases (for example, wanting different versions of DLL with same name to be loaded), where forget_dll is needed

Appendix

DLL search Python 3.8 onwards

• On Python 3.8+ searching for DLLs is done a bit differently.
  • PATH and current working directory are not used.
• New function called os.add_dll_directory was added.
• Seems that it is not the case always, even in Python 3.8 (depends on python package dll inclusion logic). See, for example fohrloop#2.

DLL Search Prior Python 3.8

• Could just add DLL folder to Process PATH and DLL's would be loaded.