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Date: Thu, 28 Jun 2012 17:12:40 +0100
From: MRAB <python@mrabarnett.plus.com>
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To: python-list@python.org
Subject: Re: retry many times decorator
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On 28/06/2012 16:28, andrea crotti wrote:
> Hi everyone, I'm replacing a perl system that has to work a lot with
> databases and perforce (and a few other things).
>
> This script have to run completely unsupervisioned, so it's important
> that it doesn't just quit at the first attempt waiting for human
> intervent..
>
> They say that the network sometimes has problems so all over the code
> there are things like:
> until ($dbh = DBI->connect('...'))
> {
>      sleep( 5 * 60 );
> )
>
>
> Since I really don't want to do that I tried to do a decorator:
>
> class retry_n_times:
>      def __init__(self, ntimes=3, timeout=3, fatal=True):
>          self.ntimes = ntimes
>          self.timeout = timeout
>          self.fatal = fatal
>
>      def __call__(self, func):
>          def _retry_n_times(*args, **kwargs):
>              attempts = 0
>              while True:
>                  logger.debug("Attempt number %s of %s" % (attempts,
> func.__name__))
>                  ret = func(*args, **kwargs)
>                  if ret:
>                      return ret
>                  else:
>                      sleep(self.timeout)
>
>                  attempts += 1
>                  if attempts == self.ntimes:
>                      logger.error("Giving up the attempts while running
> %s" % func.__name__)
>                      if self.fatal:
>                          exit(100)
>
>          return _retry_n_times
>
> which can be used as
>
> @retry_n_times(ntimes=10)
> def connect():
>      try:
>          conn = mysql_connection()
>      except Exception:
>          return False
>      else:
>          return True
>
>
> So the function to be decorated has to return a boolean..  The problem
> is that I would like to keep the exception message to report a bit
> better what could be the problem, in case the retry fails.
>
> Any idea about how to improve it?
>
Returning a boolean isn't very Pythonic. It would be better, IMHO, if
it could swallow a specified exception (or specified exceptions?)
raised when an attempt failed, up to the maximum permitted number of
attempts. If the final attempt fails, propagate the exception.