python How do I know what type of exception occurred

Question

I have a function called by the main program   try      someFunction   except      print  exception happened     but in the middle of the execution of the function it raises exception  so it jumps to the except part   How can I see exactly what happened in the someFunction   that caused the exception to happen

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To add to Lauritz s answer  I created a decorator wrapper for exception handling and the wrapper logs which type of exception occurred   class general function handler object       def   init   self  func           self func   func     def   get   self  obj  type None           return self   class   self func   get   obj  type       def   call   self   args    kwargs           try              retval   self func  args    kwargs          except Exception  e               logging warning  Exception in  s    self func              template    An exception of type  0  occured  Arguments  n 1 r               message   template format type e    name    e args              logging exception message              sys exit 1    exit on all exceptions for now         return retval   This can be called on a class method or a standalone function with the decorator    general function handler  See my blog about for the full example  http   ryaneirwin wordpress com 2014 05 31 python-decorators-and-exception-handling

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Get the name of the class that exception object belongs   e   class     name     and using print exc   function will also print stack trace which is essential info for any error message   Like this   from traceback import print exc  class CustomException Exception   pass  try      raise CustomException  hi   except Exception  e      print  type is    e   class     name       print exc         print  exception happened     You will get output like this   type is  CustomException Traceback  most recent call last     File  exc py   line 7  in  lt module gt      raise CustomException  hi   CustomException  hi   And after print and analysis  the code can decide not to handle exception and just execute raise   from traceback import print exc  class CustomException Exception   pass  def calculate        raise CustomException  hi    try      calculate   except Exception  e      if e   class      CustomException          print  special case of   e   class     name     not interfering          raise     print  handling exception    Output   special case of CustomException not interfering   And interpreter prints exception   Traceback  most recent call last     File  test py   line 9  in  lt module gt      calculate     File  test py   line 6  in calculate     raise CustomException  hi     main   CustomException  hi   After raise original exception continues to propagate further up the call stack   Beware of possible pitfall  If you raise new exception it caries new  shorter  stack trace   from traceback import print exc  class CustomException Exception   pass  def calculate        raise CustomException  hi    try      calculate   except Exception  e      if e   class      CustomException          print  special case of   e   class     name     not interfering           raise CustomException e message          raise e     print  handling exception    Output   special case of CustomException not interfering Traceback  most recent call last     File  test py   line 13  in  lt module gt      raise CustomException e message    main   CustomException  hi       Notice how traceback does not include calculate   function from line 9 which is the origin of original exception e

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Most answers point to except       as        syntax  rightly so  but at the same time nobody wants to talk about an elephant in the room  where the elephant is sys exc info   function  From the documentation of sys module  emphasis mine       This function returns a tuple of three values that give information   about the exception that is currently being handled            If no exception is being handled anywhere on the stack  a tuple   containing three None values is returned  Otherwise  the values   returned are  type  value  traceback   Their meaning is  type gets the   type of the exception being handled  a subclass of BaseException     value gets the exception instance  an instance of the exception type     traceback gets a traceback object  see the Reference Manual  which   encapsulates the call stack at the point where the exception   originally occurred    I think the sys exc info   could be treated as the most direct answer to the original question of How do I know what type of exception occurred

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Use the below for both Exception type and Exception text import sys print str sys exc info   0   split      1  strip   gt    strip  quot   quot    quot - quot   str sys exc info   1      if you want only exception type  Use -- gt  import sys print str sys exc info   0   split      1  strip   gt    strip  quot   quot       Thanks Rajeshwar

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The other answers all point out that you should not catch generic exceptions  but no one seems to want to tell you why  which is essential to understanding when you can break the  rule   Here is an explanation  Basically  it s so that you don t hide    the fact that an error occurred  the specifics of the error that occurred  error hiding antipattern    So as long as you take care to do none of those things  it s OK to catch the generic exception  For instance  you could provide information about the exception to the user another way  like    Present exceptions as dialogs in a GUI Transfer exceptions from a worker thread or process to the controlling thread or process in a multithreading or multiprocessing application   So how to catch the generic exception  There are several ways  If you just want the exception object  do it like this   try      someFunction   except Exception as ex      template    An exception of type  0  occurred  Arguments  n 1 r       message   template format type ex    name    ex args      print message   Make sure message is brought to the attention of the user in a hard-to-miss way  Printing it  as shown above  may not be enough if the message is buried in lots of other messages  Failing to get the users attention is tantamount to swallowing all exceptions  and if there s one impression you should have come away with after reading the answers on this page  it s that this is not a good thing  Ending the except block with a raise statement will remedy the problem by transparently reraising the exception that was caught   The difference between the above and using just except  without any argument is twofold    A bare except  doesn t give you the exception object to inspect The exceptions SystemExit  KeyboardInterrupt and GeneratorExit aren t caught by the above code  which is generally what you want  See the exception hierarchy    If you also want the same stacktrace you get if you do not catch the exception  you can get that like this  still inside the except clause    import traceback print traceback format exc     If you use the logging module  you can print the exception to the log  along with a message  like this   import logging log   logging getLogger   log exception  Message for you  sir      If you want to dig deeper and examine the stack  look at variables etc   use the post mortem function of the pdb module inside the except block   import pdb pdb post mortem     I ve found this last method to be invaluable when hunting down bugs

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The actual exception can be captured in the following way   try      i   1 0 except Exception as e      print e   You can learn more about exceptions from The Python Tutorial

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Your question is   How can I see exactly what happened in the someFunction   that caused the exception to happen    It seems to me that you are not asking about how to handle unforeseen exceptions in production code  as many answers assumed   but how to find out what is causing a particular exception during development   The easiest way is to use a debugger that can stop where the uncaught exception occurs  preferably not exiting  so that you can inspect the variables  For example  PyDev in the Eclipse open source IDE can do that  To enable that in Eclipse  open the Debug perspective  select Manage Python Exception Breakpoints in the Run menu  and check Suspend on uncaught exceptions

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You can start as Lauritz recommended  with   except Exception as ex    and then just to print ex like so   try       your try code here except Exception as ex      print ex

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These answers are fine for debugging  but for programmatically testing the exception  isinstance e  SomeException  can be handy  as it tests for subclasses of SomeException too  so you can create functionality that applies to hierarchies of exceptions

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Here s how I m handling my exceptions   The idea is to do try solving the issue if that s easy  and later add a more desirable solution if possible   Don t solve the issue in the code that generates the exception  or that code loses track of the original algorithm  which should be written to-the-point   However  pass what data is needed to solve the issue  and return a lambda just in case you can t solve the problem outside of the code that generates it   path    app p   def load        if os path exists path           try              with open path   rb   as file                  data   file read                   inst   pickle load data          except Exception as e              inst   solve e   load app data   easy lambda  App    path path        else          inst   App       inst loadWidgets      e g  A solver could search for app data if desc  load app data  def solve e  during  easy    kwargs       class name   e   class     name       print class name          str e       print   t during      during      return easy   For now  since I don t want to think tangentially to my app s purpose  I haven t added any complicated solutions   But in the future  when I know more about possible solutions  since the app is designed more   I could add in a dictionary of solutions indexed by during     In the example shown  one solution might be to look for app data stored somewhere else  say if the  app p  file got deleted by mistake   For now  since writing the exception handler is not a smart idea  we don t know the best ways to solve it yet  because the app design will evolve   we simply return the easy fix which is to act like we re running the app for the first time  in this case

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Hope this will help a little more import sys varExcepHandling  varExcepHandlingZer   2  0 try    print varExcepHandling varExcepHandlingZer  except Exception as ex     print sys exc info      sys exc info    will return a tuple  if you only want the exception class name use  sys exc info   0   Note - if you want to see all the exception classes just write dir   builtin

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Just refrain from catching the exception and the traceback that Python prints will tell you what exception occurred

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try      someFunction   except Exception  exc    this is how you get the type excType   exc   class     name     here we are printing out information about the Exception print  exception type   excType print  exception msg   str exc    It s easy to reraise an exception with more information added to it msg    there was a problem with someFunction  raise Exception msg    because of  s   s     excType  exc

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Unless somefunction is a very bad coded legacy function  you shouldn t need what you re asking   Use multiple except clause to handle in different ways different exceptions   try      someFunction   except ValueError        do something except ZeroDivision        do something else   The main point is that you shouldn t catch generic exception  but only the ones that you need to  I m sure that you don t want to shadow unexpected errors or bugs

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You usually should not catch all possible exceptions with try      except as this is overly broad  Just catch those that are expected to happen for whatever reason  If you really must  for example if you want to find out more about some problem while debugging  you should do  try          except Exception as ex      print ex   do whatever you want for debugging      raise      re-raise exception

[python] python: How do I know what type of exception occurred?

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