ctypes.util.find_library does not respect os.add_dll_directory

Bug report

Bug description:

os.add_dll_directory's documentation states that "This search path is used when resolving dependencies for imported extension modules (the module itself is resolved through sys.path), and also by ctypes.", yet paths added there do not affect ctypes.util.find_library (one can easily verify that the implementation of the latter simply walks PATH:

cpython/Lib/ctypes/util.py

Lines 55 to 68 in b4c215e

    
           def find_library(name): 
        
               if name in ('c', 'm'): 
        
                   return find_msvcrt() 
        
               # See MSDN for the REAL search order. 
        
               for directory in os.environ['PATH'].split(os.pathsep): 
        
                   fname = os.path.join(directory, name) 
        
                   if os.path.isfile(fname): 
        
                       return fname 
        
                   if fname.lower().endswith(".dll"): 
        
                       continue 
        
                   fname = fname + ".dll" 
        
                   if os.path.isfile(fname): 
        
                       return fname 
        
               return None

I do realize that find_library's doc does state "The purpose of the find_library() function is to locate a library in a way similar to what the compiler or runtime loader does (on platforms with several versions of a shared library the most recent should be loaded), while the ctypes library loaders act like when a program is run, and call the runtime loader directly." which might justify the difference in behavior; still, it would be nice if ctypes provided a function (or changed find_library) to return the path of the library that would be loaded while respecting add_dll_directory (right now a workaround would be to call GetModuleFileNameEx ourselves on the handle returned by WinDLL).

CPython versions tested on:

3.12

Operating systems tested on:

Windows

Did you ever find a solution to this?

Using Python 3.12.6 and Windows 11 64-bit, I am also struggling with ctypes.util.find_library() not finding DLLs in directories added by os.add_dll_directory(), but will locate them if the full path to the DLL is provided to find_library(). Unfortunately this breaks an imported module that uses find_library() with the DLL name to locate a DLL.

Test folder structure:

tester.py:

import os
from pathlib import Path
from ctypes.util import find_library

# Add the directory this script is in (also contains vxlapi64.dll)
add_obj = os.add_dll_directory(os.path.dirname(__file__))

# Check for library using DLL name
if find_library("vxlapi64"):
    print("vxlapi64.dll from add_dll_directory found")
else:
    print("vxlapi64.dll from add_dll_directory NOT FOUND")

# Check for library using DLL path
if find_library(os.path.dirname(__file__) + "/vxlapi64"):
    print("vxlapi64.dll abs path found")
else:
    print("vxlapi64.dll abs path NOT FOUND")

Output of python tester.py:

vxlapi64.dll from add_dll_directory NOT FOUND
vxlapi64.dll abs path found

I'm proposing to soft-deprecate ctypes.util.find_library: https://discuss.python.org/t/106232

It is now soft-deprecated. It cannot be made to work correctly and consistently on all platforms, and any change runs a chance to break whatever workarounds people already have in place.
I'm prioritizing stability over new features, and encourage solving this outside ctypes itself.

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ctypes.util.find_library does not respect os.add_dll_directory #111104

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Operating systems tested on:

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	def find_library(name):
	if name in ('c', 'm'):
	return find_msvcrt()
	# See MSDN for the REAL search order.
	for directory in os.environ['PATH'].split(os.pathsep):
	fname = os.path.join(directory, name)
	if os.path.isfile(fname):
	return fname
	if fname.lower().endswith(".dll"):
	continue
	fname = fname + ".dll"
	if os.path.isfile(fname):
	return fname
	return None

ctypes.util.find_library does not respect os.add_dll_directory #111104

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

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davidva-ecco commented on Apr 9, 2025

encukou commented on Feb 23, 2026

encukou commented on Mar 20, 2026

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