loop.add_reader and <<EOF #73903

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1. pylover commented on Mar 3, 2017

   

   pylovermannequin

   on Mar 3, 2017

   MannequinAuthor

   More actions

   Assume this simple script for reading from stdin:

   #! /usr/bin/env python3
   
   import sys
   import os
   import asyncio
   
   
   async def main(loop):
       done = False
       fileno = sys.stdin.fileno()
   
       def _reader():
           nonlocal done
           chunk = os.read(fileno, 1024)
           if not chunk:
               loop.remove_reader(fileno)
               done = True
               return
           print(chunk.decode(), end='')
   
       loop.add_reader(fileno, _reader)
       while not done:
           await asyncio.sleep(1)
   
   
   if __name__ == '__main__':
       main_loop = asyncio.get_event_loop()
       main_loop.run_until_complete(main(main_loop))

   When I run it by:

   $ ./stdin_issue.py <<EOF
   > hello
   > EOF

   I get:

   Traceback (most recent call last):
     File "/usr/lib/python3.5/asyncio/selector_events.py", line 234, in add_reader
       key = self._selector.get_key(fd)
     File "/usr/lib/python3.5/selectors.py", line 191, in get_key
       raise KeyError("{!r} is not registered".format(fileobj)) from None
   KeyError: '0 is not registered'
   
   During handling of the above exception, another exception occurred:
   
   Traceback (most recent call last):
     File "./stdin_issue.py", line 41, in <module>
       main_loop.run_until_complete(main(main_loop))
     File "/usr/lib/python3.5/asyncio/base_events.py", line 387, in run_until_complete
       return future.result()
     File "/usr/lib/python3.5/asyncio/futures.py", line 274, in result
       raise self._exception
     File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
       result = coro.send(None)
     File "./stdin_issue.py", line 34, in main
       loop.add_reader(fileno, _reader)
     File "/usr/lib/python3.5/asyncio/selector_events.py", line 237, in add_reader
       (handle, None))
     File "/usr/lib/python3.5/selectors.py", line 411, in register
       self._epoll.register(key.fd, epoll_events)
   PermissionError: [Errno 1] Operation not permitted
   

   But the:

   echo "Hello" | ./stdin_issue.py 

   Is working well.

   I was already tried this with the select.select directly, and it working:

   def using_select():
       files = [sys.stdin.fileno()]
   
       while True:
           readables, _, errors = select(files, [], files)
           if errors:
               print('ERROR:', errors)
               return
           if readables:
               for f in readables:
                   chunk = os.read(f, 1024)
                   if not chunk:
                       return
                   print(chunk.decode(), end='')
   
2. pylover

   added

   topic-asyncio

   on Mar 3, 2017
3. csabella

   added

   3.8 (EOL)end of lifeend of life

   on Mar 19, 2019
4. defbk commented on Jun 10, 2019

   

   defbkmannequin

   on Jun 10, 2019

   Mannequin

   More actions

   Checked latest master (c879ff2), 3.8 - 3.5 branches - can not reproduce.
   OS - macOS
5. asvetlov commented on Jun 10, 2019

   

   asvetlov

   on Jun 10, 2019

   Contributor

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   If fd is a regular file it is not supported well by all select/poll/epoll.
   Registration may succeed but the data is marked as "already ready for reading".

   Not sure if we should do anything.
   The situation may be changed after eventual providing asyncio API for files (there is no actual work for it yet).
6. ezio-melotti

   transferred this issue fromon Apr 10, 2022
7. kumaraditya303 commented on May 20, 2022

   

   kumaraditya303

   on May 20, 2022

   Contributor

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   As Andrew explained, there isn't something to be fixed here as using a blocking fd is not supported hence closing.
8. kumaraditya303

   closed this as completedon May 20, 2022
9. erlend-aasland

   closed this as not plannedon May 20, 2022
10. allisonkarlitskaya commented on Oct 30, 2022

    

    allisonkarlitskaya

    on Oct 30, 2022

    Contributor

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    Also happens for /dev/null.
11. gvanrossum commented on Oct 30, 2022

    

    gvanrossum

    on Oct 30, 2022

    Member

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    Yeah, /dev/null is more like a regular file.