Catching an exception while using a Python with statement

Question

To my shame  I can t figure out how to handle exception for python  with  statement  If I have a code   with open  a txt   as f      print f readlines     I really want to handle  file not found exception  in order to do somehing  But I can t write  with open  a txt   as f      print f readlines   except      print  oops    and can t write  with open  a txt   as f      print f readlines   else      print  oops    enclosing  with  in a try except statement doesn t work else  exception is not raised  What can I do in order to process failure inside  with  statement in a Pythonic way

User · Answer

The best  Pythonic  way to do this  exploiting the with statement  is listed as Example  6 in PEP 343  which gives the background of the statement    contextmanager def opened w error filename  mode  r        try          f   open filename  mode      except IOError  err          yield None  err     else          try              yield f  None         finally              f close     Used as follows   with opened w error   etc passwd    a   as  f  err       if err          print  IOError    err     else          f write  guido  0 0     bin sh n

User · Answer

Differentiating between the possible origins of exceptions raised from a compound with statement  Differentiating between exceptions that occur in a with statement is tricky because they can originate in different places  Exceptions can be raised from either of the following places  or functions called therein     ContextManager   init   ContextManager   enter   the body of the with ContextManager   exit     For more details see the documentation about Context Manager Types   If we want to distinguish between these different cases  just wrapping the with into a try    except is not sufficient  Consider the following example  using ValueError as an example but of course it could be substituted with any other exception type    try      with ContextManager            BLOCK except ValueError as err      print err    Here the except will catch exceptions originating in all of the four different places and thus does not allow to distinguish between them  If we move the instantiation of the context manager object outside the with  we can distinguish between   init   and BLOCK     enter       exit     try      mgr   ContextManager   except ValueError as err      print    init   raised    err  else      try          with mgr              try                  BLOCK             except TypeError     catching another type  which we want to handle here                  pass     except ValueError as err            At this point we still cannot distinguish between exceptions raised from             enter    BLOCK    exit    also BLOCK since we didn t catch ValueError in the body          pass   Effectively this just helped with the   init   part but we can add an extra sentinel variable to check whether the body of the with started to execute  i e  differentiating between   enter   and the others    try      mgr   ContextManager        init   could raise except ValueError as err      print    init   raised    err  else      try          entered body   False         with mgr              entered body   True      enter   did not raise at this point             try                  BLOCK             except TypeError     catching another type  which we want to handle here                  pass     except ValueError as err          if not entered body              print    enter   raised    err          else                At this point we know the exception came either from BLOCK or from   exit               pass   The tricky part is to differentiate between exceptions originating from BLOCK and   exit   because an exception that escapes the body of the with will be passed to   exit   which can decide how to handle it  see the docs   If however   exit   raises itself  the original exception will be replaced by the new one  To deal with these cases we can add a general except clause in the body of the with to store any potential exception that would have otherwise escaped unnoticed and compare it with the one caught in the outermost except later on - if they are the same this means the origin was BLOCK or otherwise it was   exit    in case   exit   suppresses the exception by returning a true value the outermost except will simply not be executed    try      mgr   ContextManager        init   could raise except ValueError as err      print    init   raised    err  else      entered body   exc escaped from body   False     try          with mgr              entered body   True      enter   did not raise at this point             try                  BLOCK             except TypeError     catching another type  which we want to handle here                  pass             except Exception as err     this exception would normally escape without notice                   we store this exception to check in the outer  except  clause                   whether it is the same  otherwise it comes from   exit                    exc escaped from body   err                 raise    re-raise since we didn t intend to handle it  just needed to store it     except ValueError as err          if not entered body              print    enter   raised    err          elif err is exc escaped from body              print  BLOCK raised    err          else              print    exit   raised    err    Alternative approach using the equivalent form mentioned in PEP 343  PEP 343 -- The  with  Statement specifies an equivalent  non-with  version of the with statement  Here we can readily wrap the various parts with try     except and thus differentiate between the different potential error sources   import sys  try      mgr   ContextManager   except ValueError as err      print    init   raised    err  else      try          value   type mgr    enter   mgr      except ValueError as err          print    enter   raised    err      else          exit   type mgr    exit           exc   True         try              try                  BLOCK             except TypeError                  pass             except                  exc   False                 try                      exit val   exit mgr   sys exc info                    except ValueError as err                      print    exit   raised    err                  else                      if not exit val                          raise         except ValueError as err              print  BLOCK raised    err          finally              if exc                  try                      exit mgr  None  None  None                  except ValueError as err                      print    exit   raised    err    Usually a simpler approach will do just fine  The need for such special exception handling should be quite rare and normally wrapping the whole with in a try     except block will be sufficient  Especially if the various error sources are indicated by different  custom  exception types  the context managers need to be designed accordingly  we can readily distinguish between them  For example   try      with ContextManager            BLOCK except InitError     raised from   init           except AcquireResourceError     raised from   enter           except ValueError     raised from BLOCK         except ReleaseResourceError     raised from   exit

User · Answer

Catching an exception while using a Python  with  statement   The with statement has been available without the   future   import since Python 2 6  You can get it as early as Python 2 5  but at this point it s time to upgrade   with   from   future   import with statement   Here s the closest thing to correct that you have  You re almost there  but with doesn t have an except clause    with open  a txt   as f       print f readlines    except                        lt - with doesn t have an except clause      print  oops      A context manager s   exit   method  if it returns False will reraise the error when it finishes  If it returns True  it will suppress it  The open builtin s   exit   doesn t return True  so you just need to nest it in a try  except block   try      with open  a txt   as f          print f readlines    except Exception as error       print  oops     And standard boilerplate  don t use a bare except  which catches BaseException and every other possible exception and warning  Be at least as specific as Exception  and for this error  perhaps catch IOError  Only catch errors you re prepared to handle   So in this case  you d do    gt  gt  gt  try          with open  a txt   as f              print f readlines        except IOError as error           print  oops        oops

User · Answer

from   future   import with statement  try      with open   a txt    as f           print f readlines   except EnvironmentError    parent of IOError  OSError  and  WindowsError where available     print  oops    If you want different handling for errors from the open call vs the working code you could do   try      f   open  foo txt   except IOError      print  error   else      with f          print f readlines

[python] Catching an exception while using a Python 'with' statement

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