PEP 352 – Required Superclass for Exceptions
- Brett Cannon, Guido van Rossum
- Standards Track
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
except BaseException: instead of
except *:. 
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,
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) 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
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
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.
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 . 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
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
or catch the two exceptions separately and then re-raise them and
have all other exceptions fall through to a bare
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
sys.exit() is called the interpreter should normally
be allowed to terminate. Unfortunately overly broad
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.
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 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 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]
messageattribute (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 everything that was deprecated above:
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
Unfortunately, while implementing the removal of the
attribute in Python 3.0 at the PyCon 2007 sprint
, 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.
This document has been placed in the public domain.
Last modified: 2022-08-24 22:40:18+00:00 GMT