Is it a good practice to use try-except-else in Python

Question

From time to time in Python  I see the block   try     try this whatever  except SomeException as exception      Handle exception else     return something   What is the reason for the try-except-else to exist   I do not like that kind of programming  as it is using exceptions to perform flow control  However  if it is included in the language  there must be a good reason for it  isn t it   It is my understanding that exceptions are not errors  and that they should only be used for exceptional conditions  e g  I try to write a file into disk and there is no more space  or maybe I do not have permission   and not for flow control    Normally I handle exceptions as   something   some default value try      something   try this whatever  except SomeException as exception       Handle exception finally      return something   Or if I really do not want to return anything if an exception happens  then   try      something   try this whatever      return something except SomeException as exception       Handle exception

User · Answer

Whenever you see this   try      y   1   x except ZeroDivisionError      pass else      return y   Or even this   try      return 1   x except ZeroDivisionError      return None   Consider this instead   import contextlib with contextlib suppress ZeroDivisionError       return 1   x

User · Answer

See the following example which illustrate everything about try-except-else-finally   for i in range 3       try          y   1   i     except ZeroDivisionError          print f  ti    i            print   tError report  ZeroDivisionError       else          print f  ti    i            print f  tNo error report and y equals  y        finally          print  Try block is run      Implement it and come by       i   0     Error report  ZeroDivisionError Try block is run      i   1     No error report and y equals 1 0 Try block is run      i   2     No error report and y equals 0 5 Try block is run

User · Answer

What is the reason for the try-except-else to exist    A try block allows you to handle an expected error  The except block should only catch exceptions you are prepared to handle  If you handle an unexpected error  your code may do the wrong thing and hide bugs   An else clause will execute if there were no errors  and by not executing that code in the try block  you avoid catching an unexpected error  Again  catching an unexpected error can hide bugs   Example  For example   try      try this whatever  except SomeException as the exception      handle the exception  else      return something   The  try  except  suite has two optional clauses  else and finally  So it s actually try-except-else-finally   else will evaluate only if there is no exception from the try block  It allows us to simplify the more complicated code below   no error   None try      try this whatever      no error   True except SomeException as the exception      handle the exception  if no error      return something   so if we compare an else to the alternative  which might create bugs  we see that it reduces the lines of code and we can have a more readable  maintainable  and less buggy code-base   finally  finally will execute no matter what  even if another line is being evaluated with a return statement   Broken down with pseudo-code  It might help to break this down  in the smallest possible form that demonstrates all features  with comments  Assume this syntactically correct  but not runnable unless the names are defined  pseudo-code is in a function   For example   try      try this whatever  except SomeException as the exception      handle SomeException the exception        Handle a instance of SomeException or a subclass of it  except Exception as the exception      generic handle the exception        Handle any other exception that inherits from Exception       - doesn t include GeneratorExit  KeyboardInterrupt  SystemExit       Avoid bare  except   else    there was no exception whatsoever     return something         if no exception  the  something    gets evaluated        but the return will not be executed due to the return in the       finally block below  finally        this block will execute no matter what  even if no exception        after  something  is eval d but before that value is returned       but even if there is an exception        a return here will hijack the return functionality  e g       return True   hijacks the return in the else clause above   It is true that we could include the code in the else block in the try block instead  where it would run if there were no exceptions  but what if that code itself raises an exception of the kind we re catching  Leaving it in the try block would hide that bug   We want to minimize lines of code in the try block to avoid catching exceptions we did not expect  under the principle that if our code fails  we want it to fail loudly  This is a best practice      It is my understanding that exceptions are not errors   In Python  most exceptions are errors   We can view the exception hierarchy by using pydoc  For example  in Python 2     python -m pydoc exceptions   or Python 3     python -m pydoc builtins   Will give us the hierarchy  We can see that most kinds of Exception are errors  although Python uses some of them for things like ending for loops  StopIteration   This is Python 3 s hierarchy   BaseException     Exception         ArithmeticError             FloatingPointError             OverflowError             ZeroDivisionError         AssertionError         AttributeError         BufferError         EOFError         ImportError             ModuleNotFoundError         LookupError             IndexError             KeyError         MemoryError         NameError             UnboundLocalError         OSError             BlockingIOError             ChildProcessError             ConnectionError                 BrokenPipeError                 ConnectionAbortedError                 ConnectionRefusedError                 ConnectionResetError             FileExistsError             FileNotFoundError             InterruptedError             IsADirectoryError             NotADirectoryError             PermissionError             ProcessLookupError             TimeoutError         ReferenceError         RuntimeError             NotImplementedError             RecursionError         StopAsyncIteration         StopIteration         SyntaxError             IndentationError                 TabError         SystemError         TypeError         ValueError             UnicodeError                 UnicodeDecodeError                 UnicodeEncodeError                 UnicodeTranslateError         Warning             BytesWarning             DeprecationWarning             FutureWarning             ImportWarning             PendingDeprecationWarning             ResourceWarning             RuntimeWarning             SyntaxWarning             UnicodeWarning             UserWarning     GeneratorExit     KeyboardInterrupt     SystemExit   A commenter asked      Say you have a method which pings an external API and you want to handle the exception at a class outside the API wrapper  do you simply return e from the method under the except clause where e is the exception object    No  you don t return the exception  just reraise it with a bare raise to preserve the stacktrace   try      try this whatever  except SomeException as the exception      handle the exception      raise   Or  in Python 3  you can raise a new exception and preserve the backtrace with exception chaining   try      try this whatever  except SomeException as the exception      handle the exception      raise DifferentException from the exception   I elaborate in my answer here

User · Answer

Just because no-one else has posted this opinion  I would say      avoid else clauses in try excepts because they re unfamiliar to most people   Unlike the keywords try  except  and finally  the meaning of the else clause isn t self-evident  it s less readable  Because it s not used very often  it ll cause people that read your code to want to double-check the docs to be sure they understand what s going on    I m writing this answer precisely because I found a try except else in my codebase and it caused a wtf moment and forced me to do some googling    So  wherever I see code like the OP example   try      try this whatever  except SomeException as the exception      handle the exception  else        do some more processing in non-exception case     return something   I would prefer to refactor to  try      try this whatever  except SomeException as the exception      handle the exception      return     lt 1 gt    do some more processing in non-exception case   lt 2 gt  return something     lt 1  explicit return  clearly shows that  in the exception case  we are finished working  lt 2  as a nice minor side-effect  the code that used to be in the else block is dedented by one level

User · Answer

This is my simple snippet on howto understand try-except-else-finally block in Python   def div a  b       try          a b     except ZeroDivisionError          print  Zero Division Error detected       else          print  No Zero Division Error       finally          print  Finally the division of  d  d is done     a  b     Let s try div 1 1   div 1  1  No Zero Division Error Finally the division of 1 1 is done   Let s try div 1 0  div 1  0  Zero Division Error detected Finally the division of 1 0 is done

User · Answer

OP  YOU ARE CORRECT   The else after try except in Python is ugly   it leads to another flow-control object where none is needed   try      x   blah   except      print  failed at blah    else      print  just succeeded with blah    A totally clear equivalent is   try      x   blah       print  just succeeded with blah  except      print  failed at blah      This is far clearer than an else clause   The else after try except is not frequently written  so it takes a moment to figure what the implications are   Just because you CAN do a thing  doesn t mean you SHOULD do a thing   Lots of features have been added to languages because someone thought it might come in handy   Trouble is  the more features  the less clear and obvious things are because people don t usually use those bells and whistles   Just my 5 cents here   I have to come along behind and clean up a lot of code written by 1st-year out of college developers who think they re smart and want to write code in some uber-tight  uber-efficient way when that just makes it a mess to try and read   modify later   I vote for readability every day and twice on Sundays

User · Answer

Python doesn t subscribe to the idea that exceptions should only be used for exceptional cases  in fact the idiom is  ask for forgiveness  not permission   This means that using exceptions as a routine part of your flow control is perfectly acceptable  and in fact  encouraged   This is generally a good thing  as working this way helps avoid some issues  as an obvious example  race conditions are often avoided   and it tends to make code a little more readable   Imagine you have a situation where you take some user input which needs to be processed  but have a default which is already processed  The try      except      else      structure makes for very readable code   try     raw value   int input    except ValueError     value   some processed value else    no error occured    value   process value raw value    Compare to how it might work in other languages   raw value   input   if valid number raw value       value   process value int raw value   else      value   some processed value   Note the advantages  There is no need to check the value is valid and parse it separately  they are done once  The code also follows a more logical progression  the main code path is first  followed by  if it doesn t work  do this    The example is naturally a little contrived  but it shows there are cases for this structure

User · Answer

I do not know if it is out of ignorance  but I do not like that   kind of programming  as it is using exceptions to perform flow control     In the Python world  using exceptions for flow control is common and normal   Even the Python core developers use exceptions for flow-control and that style is heavily baked into the language  i e   the iterator protocol uses StopIteration to signal loop termination    In addition  the try-except-style is used to prevent the race-conditions inherent in some of the  look-before-you-leap  constructs  For example  testing os path exists results in information that may be out-of-date by the time you use it   Likewise  Queue full returns information that may be stale   The try-except-else style will produce more reliable code in these cases       It my understanding that exceptions are not errors  they should only   be used for exceptional conditions    In some other languages  that rule reflects their cultural norms as reflected in their libraries  The  rule  is also based in-part on performance considerations for those languages   The Python cultural norm is somewhat different  In many cases  you must use exceptions for control-flow  Also  the use of exceptions in Python does not slow the surrounding code and calling code as it does in some compiled languages  i e  CPython already implements code for exception checking at every step  regardless of whether you actually use exceptions or not    In other words  your understanding that  exceptions are for the exceptional  is a rule that makes sense in some other languages  but not for Python       However  if it is included in the language itself  there must be a   good reason for it  isn t it     Besides helping to avoid race-conditions  exceptions are also very useful for pulling error-handling outside loops  This is a necessary optimization in interpreted languages which do not tend to have automatic loop invariant code motion   Also  exceptions can simplify code quite a bit in common situations where the ability to handle an issue is far removed from where the issue arose  For example  it is common to have top level user-interface code calling code for business logic which in turn calls low-level routines  Situations arising in the low-level routines  such as duplicate records for unique keys in database accesses  can only be handled in top-level code  such as asking the user for a new key that doesn t conflict with existing keys    The use of exceptions for this kind of control-flow allows the mid-level routines to completely ignore the issue and be nicely decoupled from that aspect of flow-control   There is a nice blog post on the indispensibility of exceptions here   Also  see this Stack nbsp Overflow answer  Are exceptions really for exceptional errors       What is the reason for the try-except-else to exist     The else-clause itself is interesting  It runs when there is no exception but before the finally-clause  That is its primary purpose   Without the else-clause  the only option to run additional code before finalization would be the clumsy practice of adding the code to the try-clause  That is clumsy because it risks raising exceptions in code that wasn t intended to be protected by the try-block   The use-case of running additional unprotected code prior to finalization doesn t arise very often  So  don t expect to see many examples in published code  It is somewhat rare   Another use-case for the else-clause is to perform actions that must occur when no exception occurs and that do not occur when exceptions are handled  For example   recip   float  Inf   try      recip   1   f x  except ZeroDivisionError      logging info  Infinite result   else      logging info  Finite result     Another example occurs in unittest runners   try      tests run    1     run testcase case  except Exception      tests failed    1     logging exception  Failing test case   r   case      print  F   end     else      logging info  Successful test case   r   case      print      end       Lastly  the most common use of an else-clause in a try-block is for a bit of beautification  aligning the exceptional outcomes and non-exceptional outcomes at the same level of indentation   This use is always optional and isn t strictly necessary

User · Answer

Is it a good practice to use try-except-else in python    The answer to this is that it is context dependent  If you do this   d   dict   try      item   d  item   except KeyError      item    default    It demonstrates that you don t know Python very well  This functionality is encapsulated in the dict get method   item   d get  item    default     The try except block is a much more visually cluttered and verbose way of writing what can be efficiently executing in a single line with an atomic method  There are other cases where this is true    However  that does not mean that we should avoid all exception handling  In some cases it is preferred to avoid race conditions  Don t check if a file exists  just attempt to open it  and catch the appropriate IOError  For the sake of simplicity and readability  try to encapsulate this or factor it out as apropos    Read the Zen of Python  understanding that there are principles that are in tension  and be wary of dogma that relies too heavily on any one of the statements in it

User · Answer

You should be careful about using the finally block  as it is not the same thing as using an else block in the try  except  The finally block will be run regardless of the outcome of the try except   In  10   dict      a   1   In  11   try               dict   b            except KeyError               pass          finally               print  something                something   As everyone has noted using the else block causes your code to be more readable  and only runs when an exception is not thrown  In  14   try               dict   b            except KeyError               pass          else               print  something

[python] Is it a good practice to use try-except-else in Python?

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