Python reading dll throws OSError: [WinError 126]

Question

I am clearly not the first one to have a problem to read a .dll file with python. Such example can be found there WindowsError: [Error 126] when loading a OS with ctypes , WindowsError: [Error 126] The specified module could not be found or https://github.com/apache/incubator-mxnet/issues/6313 . Knowing the extend of issues, I checked that the path to my dll is correct. I even did a small python script to minimally test it, adding as much as needed path I could think of:

import sys
import os
from ctypes import *

if __name__ == '__main__':
    print(sys.path)
    sys.path.append(r"C:\Program Files (x86)\OpenBLAS\bin")
    pathWin = os.environ["PATH"]
    pathWin = pathWin.split(";")
    sys.path = sys.path + pathWin
    print(sys.path)
    dllToLoad = "F:/installMxnet/mxnet/build/Debug/libmxnet.dll"
    cdll.libmxnet = cdll.LoadLibrary(dllToLoad)

I still end up with this error:

Traceback (most recent call last):
  File "C:\Program Files\JetBrains\PyCharm Community Edition 2018.1.1\helpers\pydev\pydevd.py", line 1664, in <module>
    main()
  File "C:\Program Files\JetBrains\PyCharm Community Edition 2018.1.1\helpers\pydev\pydevd.py", line 1658, in main
    globals = debugger.run(setup['file'], None, None, is_module)
  File "C:\Program Files\JetBrains\PyCharm Community Edition 2018.1.1\helpers\pydev\pydevd.py", line 1068, in run
    pydev_imports.execfile(file, globals, locals)  # execute the script
  File "C:\Program Files\JetBrains\PyCharm Community Edition 2018.1.1\helpers\pydev\_pydev_imps\_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "main.py", line 16, in <module>
    cdll.libmxnet = cdll.LoadLibrary(dllToLoad)
  File "C:\Users\educrocq\AppData\Local\Programs\Python\Python36\lib\ctypes\__init__.py", line 426, in LoadLibrary
    return self._dlltype(name)
  File "C:\Users\educrocq\AppData\Local\Programs\Python\Python36\lib\ctypes\__init__.py", line 348, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: [WinError 126] The specified module could not be found

According to what I read, it could be because the dll has a dependency that can't be found. (And I guess that the dependency that would not find its dependency would raise the same problem, and so on...). It seems that the problem comes from Windows which is not verbose on its output message.

But I need to know which dll can't be found in my situation, because this dll depends on lot of them... Is there a way to get which one is missing?

Answer 1

Alright, thanks to repeated help from @eryksun and information from the C++ equivalent problem explained here, DLL Load Library - Error Code 126 , I managed to find which dll was missing but not loaded. The sysinternal tool was a great help to know dynamically which one was missing. And one error I did at the time was to expect that every dll in the same folder would be loaded. In fact, the dll are loaded only when they are in the Windows path. Thus, creating a folder containing all the dll that were not solved my problem.

However, I deeply regret the lack of information provided by the error message to tell which dll was missing...