How to retry after exception

Question

I have a loop starting with for i in range 0  100   Normally it runs correctly  but sometimes it fails due to network conditions  Currently I have it set so that on failure  it will continue in the except clause  continue on to the next number for i    Is it possible for me to reassign the same number to i and run through the failed iteration of the loop again

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increment your loop variable only when the try clause succeeds

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Alternatives to retrying  tenacity and backoff  2020 update  The retrying library was previously the way to go  but sadly it has some bugs and it hasn t got any updates since 2016  Other alternatives seem to be backoff and tenacity  During the time of writing this  the tenacity had more GItHub stars  2 3k vs 1 2k  and was updated more recently  hence I chose to use it  Here is an example  from functools import partial import random   producing random errors for this example  from tenacity import retry  stop after delay  wait fixed  retry if exception type    Custom error type for this example class CommunicationError Exception       pass    Define shorthand decorator for the used settings  retry on communication error   partial      retry      stop stop after delay 10      max  10 seconds wait      wait wait fixed 0 4      wait 400ms      retry retry if exception type CommunicationError          retry on communication error def do something unreliable i       if random randint 1  5     3          print  Run    i   Error occured  Retrying            raise CommunicationError    for i in range 100       do something unreliable i   The above code outputs something like  Run  3 Error occured  Retrying  Run  5 Error occured  Retrying  Run  6 Error occured  Retrying  Run  6 Error occured  Retrying  Run  10 Error occured  Retrying         More settings for the tenacity retry are listed on the tenacity GitHub page

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The clearest way would be to explicitly set i  For example   i   0 while i  lt  100      i    1     try            do stuff      except MyException          continue

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Do a while True inside your for loop  put your try code inside  and break from that while loop only when your code succeeds   for i in range 0 100       while True          try                do stuff         except SomeSpecificException              continue         break

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Here is a solution similar to others  but it will raise the exception if it doesn t succeed in the prescribed number or retries   tries   3 for i in range tries       try          do the thing       except KeyError as e          if i  lt  tries - 1    i is zero indexed             continue         else              raise     break

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A generic solution with a timeout   import time  def onerror retry exception  callback  timeout 2  timedelta  1       end time   time time     timeout     while True          try              yield callback               break         except exception              if time time    gt  end time                  raise             elif timedelta  gt  0                  time sleep timedelta    Usage   for retry in onerror retry SomeSpecificException  do stuff       retry

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Using while and a counter   count   1 while count  lt   3     try 3 times     try            do the logic           break     except SomeSpecificException as e            If trying 3rd time and still error              Just throw the error- we don t have anything to hide            if count    3              raise         count    1

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x000D   x000D  attempts   3 x000D  while attempts  x000D    try  x000D           x000D           x000D        lt status ok gt  x000D       break x000D    except  x000D      attempts - 1 x000D  else    executed only break was not  raised x000D      lt status failed gt  x000D   x000D   x000D

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for   in range 5       try            replace this with something that may fail         raise ValueError  foo          replace Exception with a more specific exception     except Exception as e          err   e         continue        no exception  continue remainder of code     else          break    did not break the for loop  therefore all attempts   raised an exception else      raise err   My version is similar to several of the above  but doesn t use a separate while loop  and re-raises the latest exception if all retries fail  Could explicitly set err   None at the top  but not strictly necessary as it should only execute the final else block if there was an error and therefore err is set

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i recently worked with my python on a solution to this problem and i am happy to share it with stackoverflow visitors please give feedback if it is needed   print   nmonthly salary per day and year converter  title    print        25    def income counter day  salary  month       global result2  result  is ready  result3     result   salary   month     result2   result   day     result3   salary   12     is ready   True     return result  result2  result3  is ready   i   0 for i in range 5       try          month   int input   ntotal days of the current month              salary   int input  total salary per month              day   int input  Total Days to calculate gt              income counter day day  salary salary  month month          if is ready              print f Your Salary per one day is   round result                 print f your income in  day  days will be   round result2                 print f your total income in one year will be   round result3                 break         else              continue     except ZeroDivisionError          is ready   False         i    1         print  a month does nt have 0 days  please try again           print f total chances left   5 - i        except ValueError          is ready   False         i    1         print  Invalid value  please type a number           print f total chances left   5 - i

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Here s my idea on how to fix this   j   19 def calc y       global j     try          j   j   8 - y         x   int y j      this will eventually raise DIV 0 when j 0         print  i      str y     j      str j     x      str x       except          j   j   1     when the exception happens  increment  j  and retry         calc y  for i in range 50       calc i

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The retrying package is a nice way to retry a block of code on failure   For example    retry wait random min 1000  wait random max 2000  def wait random 1 to 2 s        print  Randomly wait 1 to 2 seconds between retries

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You can use Python retrying package  Retrying  It is written in Python to simplify the task of adding retry behavior to just about anything

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If you want a solution without nested loops and invoking break on success you could developer a quick wrap retriable for any iterable  Here s an example of a networking issue that I run into often - saved authentication expires  The use of it would read like this    client   get client   smart loop   retriable list of values    for value in smart loop      try          client do something with value      except ClientAuthExpired          client   get client           smart loop retry           continue     except NetworkTimeout          smart loop retry           continue

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The more  functional  approach without using those ugly while loops   def tryAgain retries 0       if retries  gt  10  return     try            Do stuff     except          retries  1         tryAgain retries   tryAgain

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I prefer to limit the number of retries  so that if there s a problem with that specific item you will eventually continue onto the next one  thus   for i in range 100     for attempt in range 10       try          do thing     except          perhaps reconnect  etc      else        break   else        we failed all the attempts - deal with the consequences

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Here is my take on this issue  The following retry function supports the following features    Returns the value of the invoked function when it succeeds Raises the exception of the invoked function if attempts exhausted Limit for the number of attempts  0 for unlimited  Wait  linear or exponential  between attempts Retry only if the exception is an instance of a specific exception type  Optional logging of attempts   import time  def retry func  ex type Exception  limit 0  wait ms 100  wait increase ratio 2  logger None       attempt   1     while True          try              return func           except Exception as ex              if not isinstance ex  ex type                   raise ex             if 0  lt  limit  lt   attempt                  if logger                      logger warning  no more attempts                   raise ex              if logger                  logger error  failed execution attempt   d   attempt  exc info ex               attempt    1             if logger                  logger info  waiting  d ms before attempt   d   wait ms  attempt              time sleep wait ms   1000              wait ms    wait increase ratio   Usage    def fail randomly        y   random randint 0  10      if y  lt  10          y   0     return x   y   logger   logging getLogger   logger setLevel logging INFO  logger addHandler logging StreamHandler stream sys stdout    logger info  starting   result   retry retry fail randomly  ex type ZeroDivisionError  limit 20  logger logger  logger info  result is   s   result    See my post for more info

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Using recursion  for i in range 100       def do            try                 Network related scripts         except SpecificException as ex              do       do      invoke do   whenever required inside this loop

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There is something similar in the Python Decorator Library   Please bear in mind that it does not test for exceptions  but the return value  It retries until the decorated function returns True   A slightly modified version should do the trick

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I use following in my codes      for i in range 0  10       try           things I need to do     except ValueError          print  Try     failed with ValueError  Sleeping for 2 secs before next try   format i           time sleep 2          continue     break

[python] How to retry after exception?

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