Abstract

In Python 2.4 and before, any (classic) class can be raised as an exception. The plan for 2.5 was to allow new-style classes, but this makes the problem worse – it would mean any class (or instance) can be raised! This is a problem as it prevents any guarantees from being made about the interface of exceptions. This PEP proposes introducing a new superclass that all raised objects must inherit from. Imposing the restriction will allow a standard interface for exceptions to exist that can be relied upon. It also leads to a known hierarchy for all exceptions to adhere to.

One might counter that requiring a specific base class for a particular interface is unPythonic. However, in the specific case of exceptions there’s a good reason (which has generally been agreed to on python-dev): requiring hierarchy helps code that wants to catch exceptions by making it possible to catch all exceptions explicitly by writing except BaseException: instead of except *:. [1]

Introducing a new superclass for exceptions also gives us the chance to rearrange the exception hierarchy slightly for the better. As it currently stands, all exceptions in the built-in namespace inherit from Exception. This is a problem since this includes two exceptions (KeyboardInterrupt and SystemExit) that often need to be excepted from the application’s exception handling: the default behavior of shutting the interpreter down without a traceback is usually more desirable than whatever the application might do (with the possible exception of applications that emulate Python’s interactive command loop with >>> prompt). Changing it so that these two exceptions inherit from the common superclass instead of Exception will make it easy for people to write except clauses that are not overreaching and not catch exceptions that should propagate up.

This PEP is based on previous work done for PEP 348.

Requiring a Common Superclass

This PEP proposes introducing a new exception named BaseException that is a new-style class and has a single attribute, args. Below is the code as the exception will work in Python 3.0 (how it will work in Python 2.x is covered in the Transition Plan section):

class BaseException(object):

    """Superclass representing the base of the exception hierarchy.

    Provides an 'args' attribute that contains all arguments passed
    to the constructor.  Suggested practice, though, is that only a
    single string argument be passed to the constructor.

    """

    def __init__(self, *args):
        self.args = args

    def __str__(self):
        if len(self.args) == 1:
            return str(self.args[0])
        else:
            return str(self.args)

    def __repr__(self):
        return "%s(*%s)" % (self.__class__.__name__, repr(self.args))

No restriction is placed upon what may be passed in for args for backwards-compatibility reasons. In practice, though, only a single string argument should be used. This keeps the string representation of the exception to be a useful message about the exception that is human-readable; this is why the __str__ method special-cases on length-1 args value. Including programmatic information (e.g., an error code number) should be stored as a separate attribute in a subclass.

The raise statement will be changed to require that any object passed to it must inherit from BaseException. This will make sure that all exceptions fall within a single hierarchy that is anchored at BaseException [1]. This also guarantees a basic interface that is inherited from BaseException. The change to raise will be enforced starting in Python 3.0 (see the Transition Plan below).

With BaseException being the root of the exception hierarchy, Exception will now inherit from it.

Exception Hierarchy Changes

With the exception hierarchy now even more important since it has a basic root, a change to the existing hierarchy is called for. As it stands now, if one wants to catch all exceptions that signal an error and do not mean the interpreter should be allowed to exit, you must specify all but two exceptions specifically in an except clause or catch the two exceptions separately and then re-raise them and have all other exceptions fall through to a bare except clause:

except (KeyboardInterrupt, SystemExit):
    raise
except:
    ...

That is needlessly explicit. This PEP proposes moving KeyboardInterrupt and SystemExit to inherit directly from BaseException.

- BaseException
  |- KeyboardInterrupt
  |- SystemExit
  |- Exception
     |- (all other current built-in exceptions)

Doing this makes catching Exception more reasonable. It would catch only exceptions that signify errors. Exceptions that signal that the interpreter should exit will not be caught and thus be allowed to propagate up and allow the interpreter to terminate.

KeyboardInterrupt has been moved since users typically expect an application to exit when they press the interrupt key (usually Ctrl-C). If people have overly broad except clauses the expected behaviour does not occur.

SystemExit has been moved for similar reasons. Since the exception is raised when sys.exit() is called the interpreter should normally be allowed to terminate. Unfortunately overly broad except clauses can prevent the explicitly requested exit from occurring.

To make sure that people catch Exception most of the time, various parts of the documentation and tutorials will need to be updated to strongly suggest that Exception be what programmers want to use. Bare except clauses or catching BaseException directly should be discouraged based on the fact that KeyboardInterrupt and SystemExit almost always should be allowed to propagate up.

Transition Plan

Since semantic changes to Python are being proposed, a transition plan is needed. The goal is to end up with the new semantics being used in Python 3.0 while providing a smooth transition for 2.x code. All deprecations mentioned in the plan will lead to the removal of the semantics starting in the version following the initial deprecation.

Here is BaseException as implemented in the 2.x series:

class BaseException(object):

    """Superclass representing the base of the exception hierarchy.

    The __getitem__ method is provided for backwards-compatibility
    and will be deprecated at some point.  The 'message' attribute
    is also deprecated.

    """

    def __init__(self, *args):
        self.args = args

    def __str__(self):
        return str(self.args[0]
                   if len(self.args) <= 1
                   else self.args)

    def __repr__(self):
        func_args = repr(self.args) if self.args else "()"
        return self.__class__.__name__ + func_args

    def __getitem__(self, index):
        """Index into arguments passed in during instantiation.

        Provided for backwards-compatibility and will be
        deprecated.

        """
        return self.args[index]

    def _get_message(self):
        """Method for 'message' property."""
        warnings.warn("the 'message' attribute has been deprecated "
                        "since Python 2.6")
        return self.args[0] if len(args) == 1 else ''

    message = property(_get_message,
                        doc="access the 'message' attribute; "
                            "deprecated and provided only for "
                            "backwards-compatibility")

Deprecation of features in Python 2.9 is optional. This is because it is not known at this time if Python 2.9 (which is slated to be the last version in the 2.x series) will actively deprecate features that will not be in 3.0. It is conceivable that no deprecation warnings will be used in 2.9 since there could be such a difference between 2.9 and 3.0 that it would make 2.9 too “noisy” in terms of warnings. Thus the proposed deprecation warnings for Python 2.9 will be revisited when development of that version begins, to determine if they are still desired.

• Python 2.5 [done]
  • all standard exceptions become new-style classes [done]
  • introduce BaseException [done]
  • Exception, KeyboardInterrupt, and SystemExit inherit from BaseException [done]
  • deprecate raising string exceptions [done]
• Python 2.6 [done]
  • deprecate catching string exceptions [done]
  • deprecate message attribute (see Retracted Ideas) [done]
• Python 2.7 [done]
  • deprecate raising exceptions that do not inherit from BaseException
• Python 3.0 [done]
  • drop everything that was deprecated above:
    • string exceptions (both raising and catching) [done]
    • all exceptions must inherit from BaseException [done]
    • drop __getitem__, message [done]

Retracted Ideas

A previous version of this PEP that was implemented in Python 2.5 included a ‘message’ attribute on BaseException. Its purpose was to begin a transition to BaseException accepting only a single argument. This was to tighten the interface and to force people to use attributes in subclasses to carry arbitrary information with an exception instead of cramming it all into args.

Unfortunately, while implementing the removal of the args attribute in Python 3.0 at the PyCon 2007 sprint [2], it was discovered that the transition was very painful, especially for C extension modules. It was decided that it would be better to deprecate the message attribute in Python 2.6 (and remove it in Python 2.7 and Python 3.0) and consider a more long-term transition strategy in Python 3.0 to remove multiple-argument support in BaseException in preference of accepting only a single argument. Thus the introduction of message and the original deprecation of args has been retracted.

References

• Issue for new-style exceptions: python/cpython#41459

Copyright

This document has been placed in the public domain.