How do I raise the same Exception with a custom message in Python

Question

I have this try block in my code   try      do something that might raise an exception   except ValueError as err      errmsg    My custom error message       raise ValueError errmsg    Strictly speaking  I am actually raising another ValueError  not the ValueError thrown by do something       which is referred to as err in this case  How do I attach a custom message to err  I try the following code but fails due to err  a ValueError instance  not being callable   try      do something that might raise an exception   except ValueError as err      errmsg    My custom error message       raise err errmsg

User · Answer

This code template should allow you to raise an exception with a custom message   try       raise ValueError except ValueError as err      raise type err   my message

User · Answer

Python 3 built-in exceptions have the strerror field   except ValueError as err    err strerror    New error message    raise err

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Update  For Python 3  check Ben s answer    To attach a message to the current exception and re-raise it   the outer try except is just to show the effect   For python 2 x where x  6   try      try        raise ValueError    something bad        except ValueError as err        err message err message   hello        raise                re-raise current exception except ValueError as e      print   got error of type    str type e     with message    e message    This will also do the right thing if err is derived from ValueError  For example UnicodeDecodeError    Note that you can add whatever you like to err  For example err problematic array  1 2 3      Edit   Ducan points in a comment the above does not work with python 3 since  message is not a member of ValueError  Instead you could use this  valid python 2 6 or later or 3 x    try      try        raise ValueError     except ValueError as err         if not err args              err args              err args   err args     hello           raise  except ValueError as e      print   error was    str type e   str e args     Edit2   Depending on what the purpose is  you can also opt for adding the extra information under your own variable name  For both python2 and python3   try      try        raise ValueError     except ValueError as err         err extra info    hello         raise  except ValueError as e      print   error was    str type e   str e       if  extra info  in dir e          print e extra info

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This is the function I use to modify the exception message in Python 2 7 and 3 x while preserving the original traceback  It requires six  def reraise modify caught exc  append msg  prepend False          Append message to exception while preserving attributes       Preserves exception class  and exception traceback       Note          This function needs to be called inside an except because          sys exc info    requires the exception context       Args          caught exc Exception   The caught exception object         append msg str   The message to append to the caught exception         prepend bool   If True prepend the message to args instead of appending      Returns          None      Side Effects          Re-raises the exception with the preserved data   trace but         modified message             ExceptClass   type caught exc        Keep old traceback     traceback   sys exc info   2      if not caught exc args            If no args  create our own tuple         arg list    append msg      else            Take the last arg           If it is a string           append your message            Otherwise append it to the           arg list Not as pretty          arg list   list caught exc args  -1           last arg   caught exc args -1          if isinstance last arg  str               if prepend                  arg list append append msg   last arg              else                  arg list append last arg   append msg          else              arg list     last arg  append msg      caught exc args   tuple arg list      six reraise ExceptClass                  caught exc                  traceback

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Try below  try      raise ValueError  quot Original message   quot   except Exception as err      message    My custom error message          Change the order below to  quot  message   str err    quot  if custom message is needed first       err args    str err    message       raise   Output  --------------------------------------------------------------------------- ValueError                                Traceback  most recent call last        1 try  ---- gt  2     raise ValueError  quot Original message quot         3 except Exception as err        4     message    My custom error message         5     err args    str err     quot    quot    message    ValueError  Original message  My custom error message

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Either raise the new exception with your error message using  raise Exception  your error message     or  raise ValueError  your error message     within the place where you want to raise it OR attach  replace  error message into current exception using  from   Python 3 x supported only    except ValueError as e    raise ValueError  your message   from e

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If you re lucky enough to only support python 3 x  this really becomes a thing of beauty     raise from  We can chain the exceptions using raise from   try      1   0 except ZeroDivisionError as e      raise Exception  Smelly socks   from e   In this case  the exception your caller would catch has the line number of the place where we raise our exception   Traceback  most recent call last     File  test py   line 2  in  lt module gt      1   0 ZeroDivisionError  division by zero  The above exception was the direct cause of the following exception   Traceback  most recent call last     File  test py   line 4  in  lt module gt      raise Exception  Smelly socks   from e Exception  Smelly socks   Notice the bottom exception only has the stacktrace from where we raised our exception  Your caller could still get the original exception by accessing the   cause   attribute of the exception they catch   with traceback  Or you can use with traceback   try      1   0 except ZeroDivisionError as e      raise Exception  Smelly socks   with traceback e   traceback      Using this form  the exception your caller would catch has the traceback from where the original error occurred   Traceback  most recent call last     File  test py   line 2  in  lt module gt      1   0 ZeroDivisionError  division by zero  During handling of the above exception  another exception occurred   Traceback  most recent call last     File  test py   line 4  in  lt module gt      raise Exception  Smelly socks   with traceback e   traceback      File  test py   line 2  in  lt module gt      1   0 Exception  Smelly socks   Notice the bottom exception has the line where we performed the invalid division as well as the line where we reraise the exception

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None of the above solutions did exactly what I wanted  which was to add some information to the first part of the error message i e  I wanted my users to see my custom message first   This worked for me   exception raised   False try      do something that might raise an exception   except ValueError as e      message   str e      exception raised   True  if exception raised      message to prepend    Custom text      raise ValueError message to prepend   message

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try      try          int  a       except ValueError as e          raise ValueError  There is a problem   0   format e   except ValueError as err      print err   prints   There is a problem  invalid literal for int   with base 10   a

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if you want to custom the error type  a simple thing you can do is to define an error class based on ValueError

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This only works with Python 3   You can modify the exception s original arguments and add your own arguments  An exception remembers the args it was created with   I presume this is so that you can modify the exception  In the function reraise we prepend the exception s original arguments with any new arguments that we want  like a message    Finally we re-raise the exception while preserving the trace-back history  def reraise e   args        re-raise an exception with extra arguments    param e  The exception to reraise    param args  Extra args to add to the exception            e args is a tuple of arguments that the exception with instantiated with        e args   args   e args      Recreate the expection and preserve the traceback info so thta we can see      where this exception originated        raise e with traceback e   traceback         def bad      raise ValueError  bad    def very      try      bad     except Exception as e      reraise e   very    def very very      try      very     except Exception as e      reraise e   very    very very    output Traceback  most recent call last     File  quot main py quot   line 35  in  lt module gt      very very     File  quot main py quot   line 30  in very very     reraise e   very     File  quot main py quot   line 15  in reraise     raise e with traceback e   traceback      File  quot main py quot   line 28  in very very     very     File  quot main py quot   line 24  in very     reraise e   very     File  quot main py quot   line 15  in reraise     raise e with traceback e   traceback      File  quot main py quot   line 22  in very     bad     File  quot main py quot   line 18  in bad     raise ValueError  bad   ValueError    very    very    bad

User · Answer

The current answer did not work good for me  if the exception is not re-caught the appended message is not shown   But doing like below both keeps the trace and shows the appended message regardless if the exception is re-caught or not   try    raise ValueError  Original message   except ValueError as err    t  v  tb   sys exc info     raise t  ValueError err message     Appended Info    tb     I used Python 2 7  have not tried it in Python 3

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It seems all the answers are adding info to e args 0   thereby altering the existing error message   Is there a downside to extending the args tuple instead   I think the possible upside is  you can leave the original error message alone for cases where parsing that string is needed  and you could add multiple elements to the tuple if your custom error handling produced several messages or error codes  for cases where the traceback would be parsed programmatically  like via a system monitoring tool       Approach  1  if the exception may not be derived from Exception and well-behaved   def to int x       try          return int x      except Exception as e          e args    e args if e args else tuple        Custom message            raise   gt  gt  gt  to int  12   12   gt  gt  gt  to int  12 monkeys   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt stdin gt    line 3  in to int ValueError    invalid literal for int   with base 10   12 monkeys     Custom message     or     Approach  2  if the exception is always derived from Exception and well-behaved   def to int x       try          return int x      except Exception as e          e args      Custom message            raise   gt  gt  gt  to int  12   12   gt  gt  gt  to int  12 monkeys   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt stdin gt    line 3  in to int ValueError    invalid literal for int   with base 10   12 monkeys     Custom message     Can you see a downside to this approach

[python] How do I raise the same Exception with a custom message in Python?

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