I stumbled over some fairly obvious things when importing a Windows dll in python this morning. I’m writing this post to shorten the amount of time I spend reading Stack Overflow next time.
The code I will be using for this post is:
from ctypes import cdll lib = cdll.LoadLibrary("mydll.dll")
Are you using the static version of the library instead of the dynamic version?
ctypes can only import dynamic libraries. If you attempt to load a static library, you will get the error:
File "Python35\Lib\ctypes\__init__.py", line 425, in LoadLibrary return self._dlltype(name) File "Python35\Lib\ctypes\__init__.py", line 347, in __init__ self._handle = _dlopen(self._name, mode) OSError: [WinError 193] %1 is not a valid Win32 application
Make sure the dynamic .dll file is loaded, not the static .lib file. If only static libraries are provided, it might be possible to recompile as a dynamic library, but I did not try this.
Are you using 32 bit python with a 64 bit library?
Using a 64-bit dll with 32 bit python results in the error:
File "Python35\lib\ctypes\__init__.py", line 429, in LoadLibrary return self._dlltype(name) File "Python35\lib\ctypes\__init__.py", line 351, in __init__ self._handle = _dlopen(self._name, mode) OSError: [WinError 193] %1 is not a valid Win32 application
To solve this, download the Windows x86-64 version of python, and configure your IDE to use this python interpreter.
Are you using the 32 bit version of ctypes with a 64 bit version of python?
If you install the 64 bit version of python alongside the 32 bit version, it is likely that your environment variables will still be set up to point the PYTHONPATH to the 32 bit versions of the python libraries.
This will result in the error:
File "example.py", line 1, in <module> from ctypes import cdll File "Python35\lib\ctypes\__init__.py", line 7, in <module> from _ctypes import Union, Structure, Array ImportError: DLL load failed: %1 is not a valid Win32 application.
To fix this, I set the PYTHONPATH in my IDE to be Python35\Lib\;Python35\libs; Python35\DLLs
On Windows, with python > 3.5, it is important to add the DLL folder; the _ctypes module lives in there now.