CDLL __init__ no longer supports name being passed as None when the handle is not None

[DavidHeffernan]

BPO	39243
Nosy	@pfmoore, @tjguk, @zware, @eryksun, @zooba, @jonnyhsu

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-01-07.10:53:45.831>
labels = ['easy', 'type-bug', '3.8', '3.9', 'ctypes', 'OS-windows']
title = 'CDLL __init__ no longer supports name being passed as None when the handle is not None'
updated_at = <Date 2020-06-07.06:19:59.024>
user = 'https://bugs.python.org/DavidHeffernan'

bugs.python.org fields:

activity = <Date 2020-06-07.06:19:59.024>
actor = 'Jonathan Hsu'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Windows', 'ctypes']
creation = <Date 2020-01-07.10:53:45.831>
creator = 'David Heffernan'
dependencies = []
files = []
hgrepos = []
issue_num = 39243
keywords = ['easy', '3.8regression', 'newcomer friendly']
message_count = 7.0
messages = ['359501', '359536', '359539', '359540', '359543', '359565', '359607']
nosy_count = 7.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'eryksun', 'steve.dower', 'David Heffernan', 'Jonathan Hsu']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39243'
versions = ['Python 3.8', 'Python 3.9']

Linked PRs

• gh-83424: Allow empty name if handle is non-null when create ctypes.CDLL #136878
• [3.14] gh-83424: Allow empty name if handle is non-null when create ctypes.CDLL on Windows (GH-136878) #138546
• [3.13] gh-83424: Allow empty name if handle is non-null when create ctypes.CDLL on Windows (GH-136878) #138547

[DavidHeffernan]

When creating an instance of CDLL (or indeed WinDLL) for a DLL that is already loaded, you pass the HMODULE in the handle argument to the constructor.

In older versions of ctypes you could pass None as the name argument when doing so. However, the changes in 2438cdf now mean that such code fails with a NoneType is not iterable error.

The relevant change is in __init__ for CDLL. The code inside the if _os.name == "nt" block sets up mode, but this is pointless is handle is not None. Because the mode variable is never used, rightly so because the DLL is already loaded.

The issue could be resolved by changing

if _os.name == "nt":

to

if _os.name == "nt" and handle is not None:

The following program demonstrates the issue:

import ctypes

handle = ctypes.windll.kernel32._handle
print(handle)
lib = ctypes.WinDLL(name=None, handle=handle)
print(lib._handle)

This runs to completion in Python 3.7 and earlier, but fails in Python 3.8 and later:

Traceback (most recent call last):
  File "test.py", line 5, in <module>
    lib = ctypes.WinDLL(name=None, handle=handle)
  File "C:\Program Files (x86)\Python\38\lib\ctypes\__init__.py", line 359, in __init__
    if '/' in name or '\\' in name:
TypeError: argument of type 'NoneType' is not iterable

[zooba]

Good catch.

We should probably make the line 351 check "if name and ('/' in name ..." like the others in the same function.