Python try-else

Question

What is the intended use of the optional else clause of the try statement

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Perhaps a use might be    debug       def debuglog text  obj None         Simple little logger        try          debug     does global exist      except NameError          pass      if not  don t even bother displaying     except          print  Unknown cause  Debug debuglog          else            debug does exist            Now test if you want to log this debug message           from caller  obj          try              if obj in debug                  print text        stdout         except TypeError              print  The global  debug  flag should be an iterable            except              print  Unknown cause  Debug debuglog       def myfunc        debuglog  Made it to myfunc     myfunc   debug    myfunc   myfunc     Maybe this will lead you too a use

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try      statements   statements that can raise exceptions except      statements   statements that will be executed to handle exceptions else      statements   statements that will be executed if there is no exception  Example   try      age int input  Enter your age      except      print   You have entered an invalid value    else      if age  lt   21          print  You are not allowed to enter  you are too young        else          print  Welcome  you are old enough     The Output    gt  gt  gt   Enter your age  a You have entered an invalid value   gt  gt  gt  RESTART  gt  gt  gt   Enter your age  25 Welcome  you are old enough   gt  gt  gt RESTART  gt  gt  gt   Enter your age  13 You are not allowed to enter  you are too young   gt  gt  gt   Copied from   https   geek-university com python the-try-except-else-statements

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The statements in the else block are executed if execution falls off the bottom of the try - if there was no exception  Honestly  I ve never found a need   However  Handling Exceptions notes      The use of the else clause is better   than adding additional code to the try   clause because it avoids accidentally   catching an exception that wasn   t   raised by the code being protected by   the try     except statement    So  if you have a method that could  for example  throw an IOError  and you want to catch exceptions it raises  but there s something else you want to do if the first operation succeeds  and you don t want to catch an IOError from that operation  you might write something like this   try      operation that can throw ioerror   except IOError      handle the exception somehow   else        we don t want to catch the IOError if it s raised     another operation that can throw ioerror   finally      something we always need to do     If you just put another operation that can throw ioerror   after operation that can throw ioerror  the except would catch the second call s errors  And if you put it after the whole try block  it ll always be run  and not until after the finally  The else lets you make sure   the second operation s only run if there s no exception  it s run before the finally block  and any IOErrors it raises aren t caught here

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Looking at Python reference it seems that else is executed after try when there s no exception  The optional else clause is executed if and when control flows off the end of the try clause  2 Exceptions in the else clause are not handled by the preceding except clauses   Dive into python has an example where  if I understand correctly  in try block they try to import a module  when that fails you get exception and bind default but when it works you have an option to go into else block and bind what is required  see link for the example and explanation    If you tried to do work in catch block it might throw another exception - I guess that s where the else block comes handy

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The else  block is confusing and  nearly  useless   It s also part of the for and while statements   Actually  even on an if-statement  the else  can be abused in truly terrible ways creating bugs that are very hard to find   Consider this      if a  lt  10           condition stated explicitly    elif a  gt  10 and b  lt  10           condition confusing but at least explicit    else           Exactly what is true here           Can be hard to reason out what condition is true   Think twice about else    It is generally a problem   Avoid it except in an if-statement and even then consider documenting the else- condition to make it explicit

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I have found the try      else  construct useful in the situation where you are running database queries and logging the results of those queries to a separate database of the same flavour type  Let s say I have lots of worker threads all handling database queries submitted to a queue    in a long running loop try      query   queue get       conn   connect to db  lt main db gt       curs   conn cursor       try          curs execute   lt some query on user input that may fail even if sanitized  gt       except DBError          logconn   connect to db  lt logging db gt           logcurs   logconn cursor           logcurs execute   lt update in DB log with record of failed query           logcurs close           logconn close       else            we can t put this in main try block because an error connecting          to the logging DB would be indistinguishable from an error in           the mainquery            We can t put this after the whole try  except  finally  block          because then we don t know if the query was successful or not          logconn   connect to db  lt logging db gt           logcurs   logconn cursor           logcurs execute   lt update in DB log with record of successful query           logcurs close           logconn close            do something in response to successful query except DBError       This DBError is because of a problem with the logging database  but       we can t let that crash the whole thread over what might be a      temporary network glitch finally      curs close       conn close        other cleanup if necessary like telling the queue the task is finished   Of course if you can distinguish between the possible exceptions that might be thrown  you don t have to use this  but if code reacting to a successful piece of code might throw the same exception as the successful piece  and you can t just let the second possible exception go  or return immediately on success  which would kill the thread in my case   then this does come in handy

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Even though you can t think of a use of it right now  you can bet there has to be a use for it  Here is an unimaginative sample   With else   a    1 2 3  try      something   a 2  except      print  out of bounds  else      print something   Without else   try      something   a 2  except      print  out of bounds   if  something  in locals        print something   Here you have the variable something defined if no error is thrown  You can remove this outside the try block  but then it requires some messy detection if a variable is defined

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There is one big reason to use else - style and readability   It s generally a good idea to keep code that can cause exceptions near the code that deals with them   For example  compare these   try      from EasyDialogs import AskPassword       20 other lines     getpass   AskPassword except ImportError      getpass   default getpass   and  try      from EasyDialogs import AskPassword except ImportError      getpass   default getpass else        20 other lines     getpass   AskPassword   The second one is good when the except can t return early  or re-throw the exception   If possible  I would have written   try      from EasyDialogs import AskPassword except ImportError      getpass   default getpass     return False    or throw Exception  something more descriptive      20 other lines getpass   AskPassword   Note  Answer copied from recently-posted duplicate here  hence all this  AskPassword  stuff

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That s it  The  else  block of a try-except clause exists for code that runs when  and only when  the tried operation succeeds  It can be used  and it can be abused   try      fp  open  configuration file    rb   except EnvironmentError      confdata       it s ok if the file can t be opened else      confdata  fp read       fp close      your code continues here   working with  possibly empty  confdata   Personally  I like it and use it when appropriate  It semantically groups statements

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There s a nice example of try-else in PEP 380  Basically  it comes down to doing different exception handling in different parts of the algorithm   It s something like this   try      do init stuff   except      handle init suff execption   else      try          do middle stuff       except          handle middle stuff exception     This allows you to write the exception handling code nearer to where the exception occurs

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Try-except-else is great for combining the EAFP pattern with duck-typing   try    cs   x cleanupSet except AttributeError    pass else    for v in cs      v cleanup     You might thing this na  ve code is fine   try    for v in x cleanupSet      v clenaup   except AttributeError    pass   This is a great way of accidentally hiding severe bugs in your code  I typo-ed cleanup there  but the AttributeError that would let me know is being swallowed  Worse  what if I d written it correctly  but the cleanup method was occasionally being passed a user type that had a misnamed attribute  causing it to silently fail half-way through and leave a file unclosed  Good luck debugging that one

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I have found else useful for dealing with a possibly incorrect config file   try      value  unit   cfg  lock   split   except ValueError      msg    lock monitoring config must consist of two words separated by white space      self log  warn   msg  else         get on with lock monitoring if config is ok   An exception reading the lock config disables lock monitoring and ValueErrors log a helpful warning message

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I find it really useful when you ve got cleanup to do that has to be done even if there s an exception   try      data   something that can go wrong   except Exception as e    yes  I know that s a bad way to do it        handle exception e  else      do stuff data  finally      clean up

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An else block can often exist to complement functionality that occurs in every except block   try      test consistency valuable data  except Except1      inconsistency type   1 except Except2      inconsistency type   2 except        Something else is wrong     raise else      inconsistency type   0      Process each individual inconsistency down here instead of inside the except blocks  Use 0 to mean no inconsistency        In this case  inconsistency type is set in each except block  so that behaviour is complemented in the no-error case in else   Of course  I m describing this as a pattern that may turn up in your own code someday  In this specific case  you just set inconsistency type to 0 before the try block anyway

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From Errors and Exceptions   Handling exceptions - docs python org     The try     except statement has an optional else clause  which    when present  must follow all except clauses  It is useful for code   that must be executed if the try clause does not raise an exception    For example   for arg in sys argv 1        try          f   open arg   r       except IOError          print  cannot open   arg     else          print arg   has   len f readlines      lines          f close         The use of the else clause is better than adding additional code to   the try clause because it avoids accidentally catching an exception   that wasn   t raised by the code being protected by the try     except   statement

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Suppose your programming logic depends on whether a dictionary has an entry with a given key  You can test the result of dict get key  using if    else    construct  or you can do   try      val   dic key  except KeyError      do some stuff   else      do some stuff with val val

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Python try-else      What is the intended use of the optional else clause of the try statement    The intended use is to have a context for more code to run if there were no exceptions where it was expected to be handled   This context avoids accidentally handling errors you did not expect   But it s important to understand the precise conditions that cause the else clause to run  because return  continue  and break can interrupt the control flow to else   In Summary  The else statement runs if there are no exceptions and if not interrupted by a return  continue  or break statement    The other answers miss that last part   From the docs      The optional else clause is executed if and when control flows off the   end of the try clause      Bolding added   And the footnote reads       Currently  control    flows off the end    except in the case of an   exception or the execution of a return  continue  or break statement    It does require at least one preceding except clause  see the grammar   So it really isn t  try-else   it s  try-except-else -finally    with the else  and finally  being optional    The Python Tutorial elaborates on the intended usage      The try     except statement has an optional else clause  which  when   present  must follow all except clauses  It is useful for code that   must be executed if the try clause does not raise an exception  For   example   for arg in sys argv 1        try          f   open arg   r       except IOError          print  cannot open   arg     else          print arg   has   len f readlines      lines          f close         The use of the else clause is better than adding additional code to   the try clause because it avoids accidentally catching an exception   that wasn   t raised by the code being protected by the try     except   statement    Example differentiating else versus code following the try block  If you handle an error  the else block will not run  For example   def handle error        try          raise RuntimeError  oops        except RuntimeError as error          print  handled a RuntimeError  no big deal        else          print  if this prints  we had no error      won t print      print  And now we have left the try block       will print    And now    gt  gt  gt  handle error   handled a RuntimeError  no big deal  And now we have left the try block

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I would add another use case that seems straight forward when handling DB sessions         getting a DB connection      conn   db engine connect          and binding to a DB session     session   db get session bind conn       try            we build the query to DB         q   session query MyTable  filter MyTable col1     query val              i e retrieve one row         data set   q one or none              return results         return    col1   data set col1   col2   data set col2             except            here we make sure to rollback the transaction             handy when we update stuff into DB         session rollback           raise      else            when no errors then we can commit DB changes         session commit        finally            and finally we can close the session         session close

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One use  test some code that should raise an exception   try      this should raise TypeError   except TypeError      pass except      assert False   Raised the wrong exception type  else      assert False   Didn t raise any exception     This code should be abstracted into a more generic test in practice

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Here is another place where I like to use this pattern    while data in items       try         data   json loads data       except ValueError as e          log error      else            work on the  data

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One of the use scenarios I can think of is unpredictable exceptions  which can be circumvented if you try again  For instance  when the operations in try block involves random numbers   while True      try          r   random random           some operation that fails for specific r r      except Exception          continue     else          break   But if the exception can be predicted  you should always choose validation beforehand over an exception  However  not everything can be predicted  so this code pattern has its place

[python] Python try-else

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