#85 Threads and output

open
nobody
None
5
2003-10-04
2003-10-04
No

Hello,
If a thread writes output (into sys.stdin) while a
command should be executed, the command you tried to
execute never finishes executing, so you are stuck.
For example, type this:

>>> from threading import Thread
>>> def f():
for i in range(200):
print i,
>>> t = Thread(target=f)
>>> t.start()

Now, while the numbers are showing, type Enter. After
the numbers stopped showing, you'll notive that you
can't execute anything, until you restart the shell.

I noticed this because I wrote a function which
executes at a seperate thread and writes to sys.stderr.
If I try to execute something while the output is being
printed, I get stuck.

Bye,
Noam

Discussion

  • Noam Yorav-Raphael

    Patch against Python IDLE patched with syntax changes

     
  • Noam Yorav-Raphael

    Logged In: YES
    user_id=679426

    First, another problem:
    If you press Ctrl+C while an exception is being printed,
    IDLE gets stuck.
    You can test it easily using this:
    >>> def f(n):
    1/n
    f(n-1)
    >>> f(10)

    And here's the solution.
    The first problem (what I thought was threading) happens
    because when the subprocess prints output, the write()
    method of the Text is called, which calls its update()
    method, which causes Tkinter to handle pending events,
    including a request for execution. If this happens from a
    poll_subprocess call, the current pollresponse() call may
    receive the response for execution, and ignore it because it
    was called before the execution started.

    The second problem happens because the interrupt happens in
    the exception handling part of Executive's runcode(), and
    the main process doesn't know how to handle an exception
    which runcode() didn't handle.

    The solution for the first problem is to use the RPC's
    'responses' dict. RPC puts a response in the dict if it
    finds a Condition matching the sequence number in its
    'cvars' dict, so the solution puts a dummy Condition in
    cvars. This changes PyShell and RemoteDebugger, because they
    use PyShell.active_seq.
    Another change, to rpc.py, is that I added an optional
    argument for pollresponse(), which makes it quit after a
    certain period of time even if not all messages were handled
    - there's no need to wait for all messages to be handled in
    poll_subprocess.

    The solution for the second problem is to add a global
    variable to run.py, which states whether the subprocess
    should consider interrupt requests. It should consider them
    only when a user code is being executed.

    (The patch is against the current IDLE cvs, patched with my
    syntax improvements, but it works against unpatched IDLE
    too. I didn't check it against IDLEfork)

    Noam Raphael

     

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