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Writing to same file from two threads

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Hi,

   I noticed in someone elses program that it writes single
lines to the same file from (what I call for loss of a better
name) the "main thread" of the program and from a thread sub-
sequentally started. This got me worried if it might result
in garbled output (i.e. having some output from A inside a
line written by B or vice versae) because the "main thread" or
the other thread could be interrupted during a call of write().
Is this a valid concern (and thus locking the file object is
required before writing to it) or am I guaranteed that this
can't happen? In the latter case I would be grateful for an
explanation what mechanism is responsible for this never to
happen.
                  Thanks and best regards, Jens

PS: I already have determined experimentally that a context
    switch definitely can happen between two calls of write()
    (and I expected nothing else), what I'm worried about are
    context switches somewhere within the very innards of what
    write() does.
-- 
  \   Jens Thoms Toerring  ___      jt@toerring.de
   \__________________________      http://toerring.de

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Thread

Writing to same file from two threads jt@toerring.de (Jens Thoms Toerring) - 2013-02-26 15:17 +0000
  Re: Writing to same file from two threads Paul Rubin <no.email@nospam.invalid> - 2013-02-26 09:32 -0800
    Re: Writing to same file from two threads jt@toerring.de (Jens Thoms Toerring) - 2013-02-26 20:08 +0000
      Re: Writing to same file from two threads Antoine Pitrou <solipsis@pitrou.net> - 2013-02-27 13:26 +0000
        Re: Writing to same file from two threads jt@toerring.de (Jens Thoms Toerring) - 2013-02-27 16:05 +0000
        Re: Writing to same file from two threads Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-28 01:05 +0000
          Re: Writing to same file from two threads Antoine Pitrou <solipsis@pitrou.net> - 2013-03-01 14:54 +0000

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