Bug: Async JS Promise cannot find running Python event loop on Python 3.13 (Windows) #507

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1. github-project-automation

   added this to PythonMonkey Kanban Boardon May 28, 2026
2. github-project-automation

   moved this to Backlog in PythonMonkey Kanban Boardon May 28, 2026
3. wiwichips commented on May 28, 2026

   

   wiwichips

   on May 28, 2026

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   I assume changing it to the following would make it work?

   await pm.eval("""
         () => {
           new Promise((resolve) => setTimeout((...args) => {
               console.log(args);
               resolve();
           }, 1000, 42, "abc")
           )
         }
       """)()

   lmk if I'm wrong, I didn't test it...
4. LetuGitHub commented on May 29, 2026

   

   LetuGitHub

   on May 29, 2026

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   我猜改成以下内容会让它正常工作？

   await pm.eval("""
   () => {
   new Promise((resolve) => setTimeout((...args) => {
   console.log(args);
   resolve();
   }, 1000, 42, "abc")
   )
   }
   """)()
   如果我错了请告诉我，我没测试过......

   Thanks. I tested the snippet you provided, but I still get the same error.

       await pm.eval("""
             ~~~~~~~~~~~
       ...<6 lines>...
         }
         ~
       """)()
       ~~~~^^
   RuntimeError: PythonMonkey cannot find a running Python event-loop to make asynchronous calls.
5. wiwichips commented on May 29, 2026

   

   wiwichips

   on May 29, 2026

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   FWIW - I can't reproduce this issue with python 3.12.8 and pm 1.3.1 on macos

   wkp@m2~$ cat ex.py 
   import pythonmonkey as pm
   import asyncio
   
   async def async_fn():
       print(asyncio.get_running_loop())
   
       # initial bug report repro
       await pm.eval("""
           new Promise((resolve) => setTimeout((...args) => {
               console.log(args);
               resolve();
           }, 1000, 42, "abc")
           )
       """)
       await pm.eval("async (x) => await x")(asyncio.sleep(0.5))
   
   
       # other way
       await pm.eval("""
         () => {
           return new Promise((resolve) => setTimeout((...args) => {
               console.log(args);
               resolve();
           }, 1000, 42, "abc")
           )
         }
       """)()
   
   asyncio.run(async_fn())
   
   
   wkp@m2~$ python3 ex.py 
   <_UnixSelectorEventLoop running=True closed=False debug=False>
   [ 42, 'abc' ]
   [ 42, 'abc' ]
   
   
   wkp@m2~$ python3 --version 
   Python 3.12.8
   
   
   wkp@m2~$ pip show pythonmonkey
   Name: pythonmonkey
   Version: 1.3.1
   Summary: Seamless interop between Python and JavaScript.
   Home-page: https://pythonmonkey.io/
   Author: Distributive Corp.
   Author-email: pm@distributive.network
   License: MIT
   Location: /Users/wkp/.local/share/mise/installs/python/3.12.8/lib/python3.12/site-packages
   Requires: aiohttp, pminit
   Required-by:

   Will try 3.13 on windows later...

   👍React with 👍1Reacted by LetuGitHub
6. LetuGitHub commented on May 30, 2026

   

   LetuGitHub

   on May 30, 2026

   Author

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   FWIW - I can't reproduce this issue with python 3.12.8 and pm 1.3.1 on macos

   wkp@m2~$ cat ex.py
   import pythonmonkey as pm
   import asyncio

   async def async_fn():
   print(asyncio.get_running_loop())

   # initial bug report repro
   await pm.eval("""
       new Promise((resolve) => setTimeout((...args) => {
           console.log(args);
           resolve();
       }, 1000, 42, "abc")
       )
   """)
   await pm.eval("async (x) => await x")(asyncio.sleep(0.5))
   
   
   # other way
   await pm.eval("""
     () => {
       return new Promise((resolve) => setTimeout((...args) => {
           console.log(args);
           resolve();
       }, 1000, 42, "abc")
       )
     }
   """)()
   

   asyncio.run(async_fn())

   wkp@m2~$ python3 ex.py
   <_UnixSelectorEventLoop running=True closed=False debug=False>
   [ 42, 'abc' ]
   [ 42, 'abc' ]

   wkp@m2~$ python3 --version
   Python 3.12.8

   wkp@m2~$ pip show pythonmonkey
   Name: pythonmonkey
   Version: 1.3.1
   Summary: Seamless interop between Python and JavaScript.
   Home-page: https://pythonmonkey.io/
   Author: Distributive Corp.
   Author-email: pm@distributive.network
   License: MIT
   Location: /Users/wkp/.local/share/mise/installs/python/3.12.8/lib/python3.12/site-packages
   Requires: aiohttp, pminit
   Required-by:
   Will try 3.13 on windows later...

   Thanks for looking into this. I'm currently waiting for the Windows test results. Please let me know what you find.
7. LetuGitHub commented on May 30, 2026

   

   LetuGitHub

   on May 30, 2026

   Author

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   After switching to Python 3.13.13, the issue disappeared and it now returns the results correctly.

   import asyncio
   import pythonmonkey as pm
   
   async def async_fn():
       await pm.eval("""
           new Promise((resolve) => setTimeout((...args) => {
               console.log(args);
               resolve();
           }, 1000, 42, "abc"))
       """)
   
   asyncio.run(async_fn())

   $ python --version
   Python 3.13.13
   
   $ python .\demo.py
   [ 42, 'abc' ]
   
   $ pip show pythonmonkey
   
   Name: pythonmonkey
   Version: 1.3.1
   Summary: Seamless interop between Python and JavaScript.
   Home-page: https://pythonmonkey.io/
   Author: Distributive Corp.
   Author-email: pm@distributive.network
   License: MIT
   Location: c:\Users\13944\Desktop\program\python\Lecourt\.venv\Lib\site-packages
   Requires: aiohttp, pminit, pyreadline3
   Required-by: