Issue 36688: _dummy_thread lacks an RLock implementaiton

importing dummy_threading causes ImportError. It used to work on 3.6. There are tests at Lib/test/test_dummy_threading.py and rearranging the import so that "import dummy_threading as _threading" is the first line also causes error. This module was deprecated from 3.7 with respect to threading enabled always but I thought to add a report anyway. Looking at git log it seems a6a4dc816d68df04a7d592e0b6af8c7ecc4d4344 did some changes where catching the ImportError on Lib/functools.py was removed that could be causing this issue. Importing functools before dummy_threading works.

# master with functools imported before dummy_threading

➜  cpython git:(master) ✗ ./python.exe -c 'import functools; import dummy_threading; print("hello")'
hello

# Python 3.6

$ python3.6 -c 'import dummy_threading; print("hello")'
hello

# Python 3.7

$ python3.7 -c 'import dummy_threading'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/dummy_threading.py", line 45, in <module>
    import threading
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/threading.py", line 8, in <module>
    from traceback import format_exc as _format_exc
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/traceback.py", line 5, in <module>
    import linecache
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/linecache.py", line 8, in <module>
    import functools
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/functools.py", line 24, in <module>
    from _thread import RLock
ImportError: cannot import name 'RLock' from '_dummy_thread' (/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/_dummy_thread.py)

# master

$ cpython git:(master) ./python.exe -c 'import dummy_threading'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/dummy_threading.py", line 45, in <module>
    import threading
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/threading.py", line 8, in <module>
    from traceback import format_exc as _format_exc
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/traceback.py", line 5, in <module>
    import linecache
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/linecache.py", line 8, in <module>
    import functools
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/functools.py", line 20, in <module>
    from _thread import RLock
ImportError: cannot import name 'RLock' from '_dummy_thread' (/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/_dummy_thread.py)

# Patch to move dummy_threading import as first line

diff --git a/Lib/test/test_dummy_threading.py b/Lib/test/test_dummy_threading.py
index a0c2972a60..dc40abeda5 100644
--- a/Lib/test/test_dummy_threading.py
+++ b/Lib/test/test_dummy_threading.py
@@ -1,6 +1,6 @@
+import dummy_threading as _threading
 from test import support
 import unittest
-import dummy_threading as _threading
 import time

 class DummyThreadingTestCase(unittest.TestCase):


➜  cpython git:(master) ✗ ./python.exe Lib/test/test_dummy_threading.py
Traceback (most recent call last):
  File "Lib/test/test_dummy_threading.py", line 1, in <module>
    import dummy_threading as _threading
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/dummy_threading.py", line 45, in <module>
    import threading
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/threading.py", line 8, in <module>
    from traceback import format_exc as _format_exc
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/traceback.py", line 5, in <module>
    import linecache
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/linecache.py", line 8, in <module>
    import functools
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/functools.py", line 20, in <module>
    from _thread import RLock
ImportError: cannot import name 'RLock' from '_dummy_thread' (/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/_dummy_thread.py)

Basically _thread.RLock was added without an equivalent in _dummy_thread. Add that and it will fix the issue. It probably wouldn't be too hard since you have to implement it as if it's single-threaded and there's already a lock class in _dummy_thread to work off of. Heck, functools used to have its own dummy version in https://github.com/python/cpython/commit/a6a4dc816d68df04a7d592e0b6af8c7ecc4d4344 but it got ripped out, although it was extremely basic so I don't know if it would be the best version to put into _dummy_thread.

Ping.  This was marked as a 3.7regression and there is a PR waiting for review.  Do we have plans to remove _dummy_thread in 3.8 or 3.9?

I created bpo-37312: "Remove deprecated _dummy_thread and dummy_threading modules" (in Python 3.9).

New changeset c5905f39bcf4ef895d42eede41bb5a2f071a501d by Victor Stinner (Joost Lek) in branch 'master':
bpo-36688: Adding an implementation of RLock in _dummy_thread (GH-12943)
https://github.com/python/cpython/commit/c5905f39bcf4ef895d42eede41bb5a2f071a501d

> Ping.  This was marked as a 3.7regression and there is a PR waiting for review.

PR 12943 seems reasonable to me. I merged it.

> Do we have plans to remove _dummy_thread in 3.8 or 3.9?

I created bpo-37312 for Python 3.9. IMHO it's too late for 3.8.

New changeset 351b0e793e35510e8cbbcbb455a1b9544e808cdd by Miss Islington (bot) in branch '3.7':
bpo-36688: Adding an implementation of RLock in _dummy_thread (GH-12943)
https://github.com/python/cpython/commit/351b0e793e35510e8cbbcbb455a1b9544e808cdd

New changeset ad505918a1829e6fa2a48a7665234d60a9377e98 by Miss Islington (bot) in branch '3.8':
bpo-36688: Adding an implementation of RLock in _dummy_thread (GH-12943)
https://github.com/python/cpython/commit/ad505918a1829e6fa2a48a7665234d60a9377e98

Thanks Joost Lek for the fix and Karthikeyan Singaravelan for the bug report. It's now fixed in 3.7, 3.8 and master branches. I close the issue.

stage: patch review -> resolved

messages: + msg345804
versions: + Python 3.9

messages: + msg340681
title: import dummy_threading causes ImportError -> _dummy_thread lacks an RLock implementaiton