Which exception should I raise on bad illegal argument combinations in Python

Question

I was wondering about the best practices for indicating invalid argument combinations in Python  I ve come across a few situations where you have a function like so   def import to orm name  save False  recurse False                param name  Name of some external entity to import       param save  Save the ORM object before returning       param recurse  Attempt to import associated objects as well  Because you         need the original object to have a key to relate to  save must be          True  for recurse to be  True        raise BadValueError  If  recurse and not save        return  The ORM object              pass   The only annoyance with this is that every package has its own  usually slightly differing BadValueError  I know that in Java there exists java lang IllegalArgumentException -- is it well understood that everybody will be creating their own BadValueErrors in Python or is there another  preferred method

User · Answer

It depends on what the problem with the arguments is.

If the argument has the wrong type, raise a TypeError. For example, when you get a string instead of one of those Booleans.

if not isinstance(save, bool):
    raise TypeError(f"Argument save must be of type bool, not {type(save)}")

Note, however, that in Python we rarely make any checks like this. If the argument really is invalid, some deeper function will probably do the complaining for us. And if we only check the boolean value, perhaps some code user will later just feed it a string knowing that non-empty strings are always True. It might save him a cast.

If the arguments have invalid values, raise ValueError. This seems more appropriate in your case:

if recurse and not save:
    raise ValueError("If recurse is True, save should be True too")

Or in this specific case, have a True value of recurse imply a True value of save. Since I would consider this a recovery from an error, you might also want to complain in the log.

if recurse and not save:
    logging.warning("Bad arguments in import_to_orm() - if recurse is True, so should save be")
    save = True

User · Answer

I ve mostly just seen the builtin ValueError used in this situation

User · Answer

I ve mostly just seen the builtin ValueError used in this situation

User · Answer

I would inherit from ValueError  class IllegalArgumentError ValueError       pass   It is sometimes better to create your own exceptions  but inherit from a built-in one  which is as close to what you want as possible   If you need to catch that specific error  it is helpful to have a name

User · Answer

I m not sure I agree with inheritance from ValueError -- my interpretation of the documentation is that ValueError is only supposed to be raised by builtins    inheriting from it or raising it yourself seems incorrect      Raised when a built-in operation or   function receives an argument that has   the right type but an inappropriate   value  and the situation is not   described by a more precise exception   such as IndexError    -- ValueError documentation

User · Answer

I would inherit from ValueError  class IllegalArgumentError ValueError       pass   It is sometimes better to create your own exceptions  but inherit from a built-in one  which is as close to what you want as possible   If you need to catch that specific error  it is helpful to have a name

User · Answer

I think the best way to handle this is the way python itself handles it  Python raises a TypeError  For example       python -c  print sum     Traceback  most recent call last   File   lt string gt    line 1  in  lt module gt  TypeError  sum expected at least 1 arguments  got 0   Our junior dev just found this page in a google search for  python exception wrong arguments  and I m surprised that the obvious  to me  answer wasn t ever suggested in the decade since this question was asked

User · Answer

I would inherit from ValueError  class IllegalArgumentError ValueError       pass   It is sometimes better to create your own exceptions  but inherit from a built-in one  which is as close to what you want as possible   If you need to catch that specific error  it is helpful to have a name

User · Answer

I would inherit from ValueError  class IllegalArgumentError ValueError       pass   It is sometimes better to create your own exceptions  but inherit from a built-in one  which is as close to what you want as possible   If you need to catch that specific error  it is helpful to have a name

User · Answer

I m not sure I agree with inheritance from ValueError -- my interpretation of the documentation is that ValueError is only supposed to be raised by builtins    inheriting from it or raising it yourself seems incorrect      Raised when a built-in operation or   function receives an argument that has   the right type but an inappropriate   value  and the situation is not   described by a more precise exception   such as IndexError    -- ValueError documentation

User · Answer

I ve mostly just seen the builtin ValueError used in this situation

User · Answer

Agree with Markus  suggestion to roll your own exception  but the text of the exception should clarify that the problem is in the argument list  not the individual argument values   I d propose   class BadCallError ValueError       pass   Used when keyword arguments are missing that were required for the specific call  or argument values are individually valid but inconsistent with each other   ValueError would still be right when a specific argument is right type but out of range   Shouldn t this be a standard exception in Python     In general  I d like Python style to be a bit sharper in distinguishing bad inputs to a function  caller s fault  from bad results within the function  my fault    So there might also be a BadArgumentError to distinguish value errors in arguments from value errors in locals

User · Answer

I would just raise ValueError  unless you need a more specific exception    def import to orm name  save False  recurse False       if recurse and not save          raise ValueError  save must be True if recurse is True     There s really no point in doing class BadValueError ValueError  pass - your custom class is identical in use to ValueError  so why not use that

User · Answer

It depends on what the problem with the arguments is.

If the argument has the wrong type, raise a TypeError. For example, when you get a string instead of one of those Booleans.

if not isinstance(save, bool):
    raise TypeError(f"Argument save must be of type bool, not {type(save)}")

Note, however, that in Python we rarely make any checks like this. If the argument really is invalid, some deeper function will probably do the complaining for us. And if we only check the boolean value, perhaps some code user will later just feed it a string knowing that non-empty strings are always True. It might save him a cast.

If the arguments have invalid values, raise ValueError. This seems more appropriate in your case:

if recurse and not save:
    raise ValueError("If recurse is True, save should be True too")

Or in this specific case, have a True value of recurse imply a True value of save. Since I would consider this a recovery from an error, you might also want to complain in the log.

if recurse and not save:
    logging.warning("Bad arguments in import_to_orm() - if recurse is True, so should save be")
    save = True

User · Answer

I m not sure I agree with inheritance from ValueError -- my interpretation of the documentation is that ValueError is only supposed to be raised by builtins    inheriting from it or raising it yourself seems incorrect      Raised when a built-in operation or   function receives an argument that has   the right type but an inappropriate   value  and the situation is not   described by a more precise exception   such as IndexError    -- ValueError documentation

User · Answer

I m not sure I agree with inheritance from ValueError -- my interpretation of the documentation is that ValueError is only supposed to be raised by builtins    inheriting from it or raising it yourself seems incorrect      Raised when a built-in operation or   function receives an argument that has   the right type but an inappropriate   value  and the situation is not   described by a more precise exception   such as IndexError    -- ValueError documentation

User · Answer

I ve mostly just seen the builtin ValueError used in this situation

User · Answer

I think the best way to handle this is the way python itself handles it  Python raises a TypeError  For example       python -c  print sum     Traceback  most recent call last   File   lt string gt    line 1  in  lt module gt  TypeError  sum expected at least 1 arguments  got 0   Our junior dev just found this page in a google search for  python exception wrong arguments  and I m surprised that the obvious  to me  answer wasn t ever suggested in the decade since this question was asked

User · Answer

I would just raise ValueError  unless you need a more specific exception    def import to orm name  save False  recurse False       if recurse and not save          raise ValueError  save must be True if recurse is True     There s really no point in doing class BadValueError ValueError  pass - your custom class is identical in use to ValueError  so why not use that

User · Answer

I would just raise ValueError  unless you need a more specific exception    def import to orm name  save False  recurse False       if recurse and not save          raise ValueError  save must be True if recurse is True     There s really no point in doing class BadValueError ValueError  pass - your custom class is identical in use to ValueError  so why not use that

User · Answer

I would just raise ValueError  unless you need a more specific exception    def import to orm name  save False  recurse False       if recurse and not save          raise ValueError  save must be True if recurse is True     There s really no point in doing class BadValueError ValueError  pass - your custom class is identical in use to ValueError  so why not use that

User · Answer

Agree with Markus  suggestion to roll your own exception  but the text of the exception should clarify that the problem is in the argument list  not the individual argument values   I d propose   class BadCallError ValueError       pass   Used when keyword arguments are missing that were required for the specific call  or argument values are individually valid but inconsistent with each other   ValueError would still be right when a specific argument is right type but out of range   Shouldn t this be a standard exception in Python     In general  I d like Python style to be a bit sharper in distinguishing bad inputs to a function  caller s fault  from bad results within the function  my fault    So there might also be a BadArgumentError to distinguish value errors in arguments from value errors in locals

[python] Which exception should I raise on bad/illegal argument combinations in Python?

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