Freeze when closing handle from __del__ during python exit

[Ariakenom]

I tried to add a __del__ method for cleanup but it freezes if python exits while the object is alive.

Python 3.6.3 (v3.6.3:2c5fed8, Oct 3 2017, 17:26:49) [MSC v.1900 32 bit (Intel)] on win32 pywinusb 0.4.2

import pywinusb.hid

class C:
  def __init__(self,h):
    self.h = h
  def __del__(self):
    print("closing")
    self.h.close()
    print("closed")

h = pywinusb.hid.HidDeviceFilter().get_devices()[0]
h.open()
c = C(h)
# del c  # works if do this before python exits

[rene-aguirre]

This is a known Python behavior. Example alternative: https://www.quora.com/Python-My-class-defines-del__-but-it-is-not-called-when-I-delete-the-object

[Ariakenom]

The problem wasn't that the __del__method doesn't run, that would be ok behavior. It was that the python process doesn't exit, it freezes.

[rene-aguirre]

Makes sense.

[mcuee]

No issue here under Windows 10 with latest git.

(py39venv) C:\work\hid\pywinusb [master ≡ +1 ~0 -0 !]> python
Python 3.9.6 (tags/v3.9.6:db3ff76, Jun 28 2021, 15:26:21) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pywinusb.hid
>>> class C:
...   def __init__(self,h):
...     self.h = h
...   def __del__(self):
...     print("closing")
...     self.h.close()
...     print("closed")
...
>>> h = pywinusb.hid.HidDeviceFilter().get_devices()[0]
>>> h.open()
>>> c = C(h)
>>> del c
closing
closed

[mcuee]

@Ariakenom Maybe you can test with the latest git and up-to-date Python 3 versions.

[Ariakenom]

del c is commented in the example

[mcuee]

del c is commented in the example

I know. But please look at my run log, I have the 'del c' in the run log.

[Ariakenom]

It's not supposed to be in the run log.

That's why the statement is commented out. With a comment that says that it works as expected if we run the statement. But without it it does not work as expected and we get the freeze.

[mcuee]

Thanks for the clarification. But it does not freeze for me.

(py39venv) C:\work\python> python
Python 3.9.6 (tags/v3.9.6:db3ff76, Jun 28 2021, 15:26:21) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pywinusb.hid
>>> class C:
...   def __init__(self,h):
...     self.h = h
...   def __del__(self):
...     print("closing")
...     self.h.close()
...     print("closed")
...
>>> h = pywinusb.hid.HidDeviceFilter().get_devices()[0]
>>> h
HID device (vID=0x046d, pID=0xc534, v=0x2901); Logitech; USB Receiver, Path: \\?\hid#vid_046d&pid_c534&mi_01&col02#7&383a3a17&0&0001#{4d1e55b2-f16f-11cf-88cb-001111000030}
>>> h.open()
>>> c = C(h)
>>> c
<__main__.C object at 0x0000016ECD850F10>
>>> exit()
closing
closed

(py39venv) C:\work\python>

[mcuee]

Without using the interactive session:

(py39venv) C:\work\python> cat .\test_pywinusb.py
import pywinusb.hid

class C:
  def __init__(self,h):
    self.h = h
  def __del__(self):
    print("closing")
    self.h.close()
    print("closed")

h = pywinusb.hid.HidDeviceFilter().get_devices()[0]
h.open()
c = C(h)

(py39venv) C:\work\python> python .\test_pywinusb.py
closing
closed

[mcuee]

@Ariakenom Maybe you want to try again with more recent Python version to see if there is a real issue or not. Thanks.

[mcuee]

I can reproduce the issue with Python 3.6.8 64bit, Python 3.7.9 64bit and Python 3.8.10 64bit. Only Python 3.9 is okay (tested with Python 3.9.6 64bit).

(py36venv) C:\work\python> python
Python 3.6.8 (tags/v3.6.8:3c6b436a57, Dec 24 2018, 00:16:47) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.

(py36venv) C:\work\python> python .\test_pywinusb.py
closing
<hang>

C:\work\python> .\py37venv\Scripts\Activate.ps1
(py37venv) C:\work\python> python
Python 3.7.9 (tags/v3.7.9:13c94747c7, Aug 17 2020, 18:58:18) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
(py37venv) C:\work\python> python .\test_pywinusb.py
closing
<hang>

(py38venv) C:\work\python> python
Python 3.8.10 (tags/v3.8.10:3d8993a, May  3 2021, 11:48:03) [MSC v.1928 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.

(py38venv) C:\work\python> python .\test_pywinusb.py
closing
<hang>

(py39venv) C:\work\python> python
Python 3.9.6 (tags/v3.9.6:db3ff76, Jun 28 2021, 15:26:21) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
(py39venv) C:\work\python> python .\test_pywinusb.py
closing
closed