[PATCH V2] error: make HintException a mix-in class not derived from BaseException (API)

[Augie Fackler]

On Sat, Jul 09, 2016 at 03:20:36PM +0900, Yuya Nishihara wrote:
> # HG changeset patch
> # User Yuya Nishihara <yuya at tcha.org>
> # Date 1468042110 -32400
> #      Sat Jul 09 14:28:30 2016 +0900
> # Node ID bf279db10eb66c70322b062ef1818ddfb06e6daa
> # Parent  b4d117cee636be8a566f56e84d4b351a736a1299
> error: make HintException a mix-in class not derived from BaseException (API)
>
> HintException is unrelated to the hierarchy of errors. It is an implementation
> detail whether a class inherits from HintException or not, a sort of "private
> inheritance" in C++.
>
> New Hint isn't an exception class, which prevents catching error by its type:
>
>     try:
>         dosomething()
>     except error.Hint:
>         pass
>
> Unfortunately, this passes on PyPy 5.3.1, but not on Python 2, and raises more
> detailed TypeError on Python 3.
>
> diff --git a/mercurial/error.py b/mercurial/error.py
> --- a/mercurial/error.py
> +++ b/mercurial/error.py
> @@ -15,12 +15,17 @@ from __future__ import absolute_import
>
>  # Do not import anything here, please
>
> -class HintException(Exception):
> +class Hint(object):
> +    """Mix-in to provide a hint of an error
> +
> +    This should come first in the inheritance list to consume **kw and pass
> +    only *args to the exception class.
> +    """
>      def __init__(self, *args, **kw):
> -        Exception.__init__(self, *args)
> +        super(Hint, self).__init__(*args)
>          self.hint = kw.get('hint')

should we do kw.pop('hint', None) and then forward **kw?

>
> -class RevlogError(HintException):
> +class RevlogError(Hint, Exception):
>      pass
>
>  class FilteredIndexError(IndexError):
> @@ -50,10 +55,10 @@ class ManifestLookupError(LookupError):
>  class CommandError(Exception):
>      """Exception raised on errors in parsing the command line."""
>
> -class InterventionRequired(HintException):
> +class InterventionRequired(Hint, Exception):
>      """Exception raised when a command requires human intervention."""
>
> -class Abort(HintException):
> +class Abort(Hint, Exception):
>      """Raised if a command needs to print an error and exit."""
>
>  class HookLoadError(Abort):
> @@ -87,10 +92,10 @@ class ResponseExpected(Abort):
>          from .i18n import _
>          Abort.__init__(self, _('response expected'))
>
> -class OutOfBandError(HintException):
> +class OutOfBandError(Hint, Exception):
>      """Exception raised when a remote repo reports failure"""
>
> -class ParseError(HintException):
> +class ParseError(Hint, Exception):
>      """Raised when parsing config files and {rev,file}sets (msg[, pos])"""
>
>  class UnknownIdentifier(ParseError):
> @@ -102,7 +107,7 @@ class UnknownIdentifier(ParseError):
>          self.function = function
>          self.symbols = symbols
>
> -class RepoError(HintException):
> +class RepoError(Hint, Exception):
>      pass
>
>  class RepoLookupError(RepoError):
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