Proper way to declare custom exceptions in modern Python

Question

What s the proper way to declare custom exception classes in modern Python  My primary goal is to follow whatever standard other exception classes have  so that  for instance  any extra string I include in the exception is printed out by whatever tool caught the exception   By  modern Python  I mean something that will run in Python 2 5 but be  correct  for the Python 2 6 and Python 3   way of doing things  And by  custom  I mean an Exception object that can include extra data about the cause of the error  a string  maybe also some other arbitrary object relevant to the exception   I was tripped up by the following deprecation warning in Python 2 6 2    gt  gt  gt  class MyError Exception           def   init   self  message               self message   message       gt  gt  gt  MyError  foo    sandbox py 3  DeprecationWarning  BaseException message has been deprecated as of Python 2 6   It seems crazy that BaseException has a special meaning for attributes named message  I gather from PEP-352 that attribute did have a special meaning in 2 5 they re trying to deprecate away  so I guess that name  and that one alone  is now forbidden  Ugh   I m also fuzzily aware that Exception has some magic parameter args  but I ve never known how to use it  Nor am I sure it s the right way to do things going forward  a lot of the discussion I found online suggested they were trying to do away with args in Python 3   Update  two answers have suggested overriding   init    and   str     unicode     repr    That seems like a lot of typing  is it necessary

User · Answer

As of Python 3 8  2018  https   docs python org dev whatsnew 3 8 html   the recommended method is still   class CustomExceptionName Exception          Exception raised when very uncommon things happen        pass   Please don t forget to document  why a custom exception is neccessary   If you need to  this is the way to go for exceptions with more data   class CustomExceptionName Exception          Still an exception raised when uncommon things happen        def   init   self  message  payload None           self message   message         self payload   payload   you could add more args     def   str   self           return str self message      str     obviously expects a string to be returned  so make sure not to send any other data types   and fetch them like   try      raise CustomExceptionName  Very bad mistake     Forgot upgrading from Python 1   except CustomExceptionName as error      print str error     Very bad mistake     print  Detail      format error payload     Detail  Forgot upgrading from Python 1   payload None is important to make it pickle-able  Before dumping it  you have to call error   reduce      Loading will work as expected   You maybe should investigate in finding a solution using pythons return statement if you need much data to be transferred to some outer structure  This seems to be clearer more pythonic to me  Advanced exceptions are heavily used in Java  which can sometimes be annoying  when using a framework and having to catch all possible errors

User · Answer

With modern Python Exceptions  you don t need to abuse  message  or override    str     or    repr     or any of it  If all you want is an informative message when your exception is raised  do this   class MyException Exception       pass  raise MyException  My hovercraft is full of eels     That will give a traceback ending with MyException  My hovercraft is full of eels   If you want more flexibility from the exception  you could pass a dictionary as the argument   raise MyException   message   My hovercraft is full of animals    animal   eels      However  to get at those details in an except block is a bit more complicated  The details are stored in the args attribute  which is a list  You would need to do something like this   try      raise MyException   message   My hovercraft is full of animals    animal   eels    except MyException as e      details   e args 0      print details  animal      It is still possible to pass in multiple items to the exception and access them via tuple indexes  but this is highly discouraged  and was even intended for deprecation a while back   If you do need more than a single piece of information and the above method is not sufficient for you  then you should subclass Exception as described in the tutorial   class MyError Exception       def   init   self  message  animal           self message   message         self animal   animal     def   str   self           return self message

User · Answer

See a very good article  The definitive guide to Python exceptions   The basic principles are    Always inherit from  at least  Exception  Always call BaseException   init   with only one argument  When building a library  define a base class inheriting from Exception  Provide details about the error   Inherit from builtin exceptions types when it makes sense    There is also information on organizing  in modules  and wrapping exceptions  I recommend to read the  guide

User · Answer

Proper way to declare custom exceptions in modern Python     This is fine  unless your exception is really a type of a more specific exception   class MyException Exception       pass   Or better  maybe perfect   instead of pass give a docstring   class MyException Exception          Raise for my specific kind of exception      Subclassing Exception Subclasses  From the docs     Exception      All built-in  non-system-exiting exceptions are derived from this class     All user-defined exceptions should also be derived from this   class    That means that if your exception is a type of a more specific exception  subclass that exception instead of the generic Exception  and the result will be that you still derive from Exception as the docs recommend   Also  you can at least provide a docstring  and not be forced to use the pass keyword    class MyAppValueError ValueError          Raise when my specific value is wrong      Set attributes you create yourself with a custom   init    Avoid passing a dict as a positional argument  future users of your code will thank you  If you use the deprecated message attribute  assigning it yourself will avoid a DeprecationWarning   class MyAppValueError ValueError          Raise when a specific subset of values in context of app is wrong        def   init   self  message  foo   args           self message   message   without this you may get DeprecationWarning           Special attribute you desire with your Error             perhaps the value that caused the error           self foo   foo                    allow users initialize misc  arguments as any other builtin Error         super MyAppValueError  self    init   message  foo   args     There s really no need to write your own   str   or   repr    The builtin ones are very nice  and your cooperative inheritance ensures that you use it   Critique of the top answer     Maybe I missed the question  but why not    class MyException Exception       pass   Again  the problem with the above is that in order to catch it  you ll either have to name it specifically  importing it if created elsewhere  or catch Exception   but you re probably not prepared to handle all types of Exceptions  and you should only catch exceptions you are prepared to handle   Similar criticism to the below  but additionally that s not the way to initialize via super  and you ll get a DeprecationWarning if you access the message attribute      Edit  to override something  or pass extra args   do this    class ValidationError Exception       def   init   self  message  errors              Call the base class constructor with the parameters it needs         super ValidationError  self    init   message             Now for your custom code            self errors   errors      That way you could pass dict of error messages to the second param  and get to it later with e errors   It also requires exactly two arguments to be passed in  aside from the self   No more  no less  That s an interesting constraint that future users may not appreciate    To be direct - it violates Liskov substitutability   I ll demonstrate both errors    gt  gt  gt  ValidationError  foo    bar    baz   message  Traceback  most recent call last     File   lt pyshell 10 gt    line 1  in  lt module gt      ValidationError  foo    bar    baz   message TypeError    init     takes exactly 3 arguments  4 given    gt  gt  gt  ValidationError  foo    bar   message   main   1  DeprecationWarning  BaseException message has been deprecated as of Python 2 6  foo    Compared to     gt  gt  gt  MyAppValueError  foo    FOO    bar   message  foo

User · Answer

Try this Example  class InvalidInputError Exception       def   init   self  msg           self msg   msg     def   str   self           return repr self msg   inp   int input  Enter a number between 1 to 10     try      if type inp     int or inp not in list range 1 11            raise InvalidInputError except InvalidInputError      print  Invalid input entered

User · Answer

No   message  is not forbidden  It s just deprecated  You application will work fine with using message  But you may want to get rid of the deprecation error  of course   When you create custom Exception classes for your application  many of them do not subclass just from Exception  but from others  like ValueError or similar  Then you have to adapt to their usage of variables   And if you have many exceptions in your application it s usually a good idea to have a common custom base class for all of them  so that users of your modules can do  try          except NelsonsExceptions            And in that case you can do the   init   and   str   needed there  so you don t have to repeat it for every exception  But simply calling the message variable something else than message does the trick   In any case  you only need the   init   or   str   if you do something different from what Exception itself does  And because if the deprecation  you then need both  or you get an error  That s not a whole lot of extra code you need per class

User · Answer

A really simple approach  class CustomError Exception       pass  raise CustomError  quot Hmm  seems like this was custom coded    quot    Or  have the error raise without printing   main    may look cleaner and neater   class CustomError Exception         module     Exception   module    raise CustomError  quot Improved CustomError  quot

User · Answer

To define your own exceptions correctly  there are a few best practices that you should follow   Define a base class inheriting from Exception  This will allow to easily catch any exceptions related to the project  class MyProjectError Exception        quot  quot  quot A base class for MyProject exceptions  quot  quot  quot   Organizing the exception classes in a separate module  e g  exceptions py  is generally a good idea   To create a specific exception  subclass the base exception class   To add support for extra argument s  to a custom exception  define a custom   init     method with a variable number of arguments  Call the base class s   init      passing any positional arguments to it  remember that BaseException Exception expect any number of positional arguments   class CustomError MyProjectError       def   init   self   args    kwargs           super     init    args          self foo   kwargs get  foo    To raise such exception with an extra argument you can use   raise CustomError  Something bad happened   foo  foo      This design adheres to the Liskov substitution principle  since you can replace an instance of a base exception class with an instance of a derived exception class  Also  it allows you to create an instance of a derived class with the same parameters as the parent

User · Answer

Maybe I missed the question  but why not   class MyException Exception       pass   Edit  to override something  or pass extra args   do this   class ValidationError Exception       def   init   self  message  errors              Call the base class constructor with the parameters it needs         super ValidationError  self    init   message             Now for your custom code            self errors   errors   That way you could pass dict of error messages to the second param  and get to it later with e errors    Python 3 Update  In Python 3   you can use this slightly more compact use of super     class ValidationError Exception       def   init   self  message  errors              Call the base class constructor with the parameters it needs         super     init   message             Now for your custom code            self errors   errors

User · Answer

You should override   repr   or   unicode   methods instead of using message  the args you provide when you construct the exception will be in the args attribute of the exception object

User · Answer

see how exceptions work by default if one vs more attributes are used  tracebacks omitted     gt  gt  gt  raise Exception  bad thing happened   Exception  bad thing happened   gt  gt  gt  raise Exception  bad thing happened    code is broken   Exception    bad thing happened    code is broken     so you might want to have a sort of  exception template   working as an exception itself  in a compatible way    gt  gt  gt  nastyerr   NastyError  bad thing happened    gt  gt  gt  raise nastyerr NastyError  bad thing happened   gt  gt  gt  raise nastyerr   NastyError  bad thing happened   gt  gt  gt  raise nastyerr  code is broken   NastyError    bad thing happened    code is broken     this can be done easily with this subclass  class ExceptionTemplate Exception       def   call   self   args           return self   class     self args   args         class NastyError ExceptionTemplate   pass   and if you don t like that default tuple-like representation  just add   str   method to the ExceptionTemplate class  like                 def   str   self           return      join self args    and you ll have   gt  gt  gt  raise nastyerr  code is broken   NastyError  bad thing happened  code is broken

[python] Proper way to declare custom exceptions in modern Python?

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