python-peps/peps/pep-0317.rst

PEP: 317
Title: Eliminate Implicit Exception Instantiation
Version: $Revision$
Last-Modified: $Date$
Author: Steven Taschuk <staschuk@telusplanet.net>
Status: Rejected
Type: Standards Track
Content-Type: text/x-rst
Created: 06-May-2003
Python-Version: 2.4
Post-History: 09-Jun-2003


Abstract
========

    "For clarity in new code, the form ``raise class(argument, ...)``
    is recommended (i.e. make an explicit call to the constructor)."

    -- Guido van Rossum, in 1997 [1]_

This PEP proposes the formal deprecation and eventual elimination of
forms of the ``raise`` statement which implicitly instantiate an
exception.  For example, statements such as ::

    raise HullBreachError
    raise KitchenError, 'all out of baked beans'

must under this proposal be replaced with their synonyms ::

    raise HullBreachError()
    raise KitchenError('all out of baked beans')

Note that these latter statements are already legal, and that this PEP
does not change their meaning.

Eliminating these forms of ``raise`` makes it impossible to use string
exceptions; accordingly, this PEP also proposes the formal deprecation
and eventual elimination of string exceptions.

Adoption of this proposal breaks backwards compatibility.  Under the
proposed implementation schedule, Python 2.4 will introduce warnings
about uses of ``raise`` which will eventually become incorrect, and
Python 3.0 will eliminate them entirely.  (It is assumed that this
transition period -- 2.4 to 3.0 -- will be at least one year long, to
comply with the guidelines of :pep:`5`.)


Motivation
==========

String Exceptions
-----------------

It is assumed that removing string exceptions will be uncontroversial,
since it has been intended since at least Python 1.5, when the
standard exception types were changed to classes [1]_.

For the record: string exceptions should be removed because the
presence of two kinds of exception complicates the language without
any compensation.  Instance exceptions are superior because, for
example,

* the class-instance relationship more naturally expresses the
  relationship between the exception type and value,

* they can be organized naturally using superclass-subclass
  relationships, and

* they can encapsulate error-reporting behaviour (for example).


Implicit Instantiation
----------------------

Guido's 1997 essay [1]_ on changing the standard exceptions into
classes makes clear why ``raise`` can instantiate implicitly:

    "The raise statement has been extended to allow raising a class
    exception without explicit instantiation. The following forms,
    called the "compatibility forms" of the raise statement [...]  The
    motivation for introducing the compatibility forms was to allow
    backward compatibility with old code that raised a standard
    exception."

For example, it was desired that pre-1.5 code which used string
exception syntax such as ::

    raise TypeError, 'not an int'

would work both on versions of Python in which ``TypeError`` was a
string, and on versions in which it was a class.

When no such consideration obtains -- that is, when the desired
exception type is not a string in any version of the software which
the code must support -- there is no good reason to instantiate
implicitly, and it is clearer not to.  For example:

1. In the code ::

       try:
           raise MyError, raised
       except MyError, caught:
           pass

   the syntactic parallel between the ``raise`` and ``except``
   statements strongly suggests that ``raised`` and ``caught`` refer
   to the same object.  For string exceptions this actually is the
   case, but for instance exceptions it is not.

2. When instantiation is implicit, it is not obvious when it occurs,
   for example, whether it occurs when the exception is raised or when
   it is caught.  Since it actually happens at the ``raise``, the code
   should say so.

   (Note that at the level of the C API, an exception can be "raised"
   and "caught" without being instantiated; this is used as an
   optimization by, for example, ``PyIter_Next``.  But in Python, no
   such optimization is or should be available.)

3. An implicitly instantiating ``raise`` statement with no arguments,
   such as ::

       raise MyError

   simply does not do what it says: it does not raise the named
   object.

4. The equivalence of ::

       raise MyError
       raise MyError()

   conflates classes and instances, creating a possible source of
   confusion for beginners.  (Moreover, it is not clear that the
   interpreter could distinguish between a new-style class and an
   instance of such a class, so implicit instantiation may be an
   obstacle to any future plan to let exceptions be new-style
   objects.)

In short, implicit instantiation has no advantages other than
backwards compatibility, and so should be phased out along with what
it exists to ensure compatibility with, namely, string exceptions.


Specification
=============

The syntax of ``raise_stmt`` [3]_ is to be changed from ::

    raise_stmt ::= "raise" [expression ["," expression ["," expression]]]

to ::

    raise_stmt ::= "raise" [expression ["," expression]]

If no expressions are present, the ``raise`` statement behaves as it
does presently: it re-raises the last exception that was active in the
current scope, and if no exception has been active in the current
scope, a ``TypeError`` is raised indicating that this is the problem.

Otherwise, the first expression is evaluated, producing the *raised
object*.  Then the second expression is evaluated, if present,
producing the *substituted traceback*.  If no second expression is
present, the substituted traceback is ``None``.

The raised object must be an instance.  The class of the instance is
the exception type, and the instance itself is the exception value.
If the raised object is not an instance -- for example, if it is a
class or string -- a ``TypeError`` is raised.

If the substituted traceback is not ``None``, it must be a traceback
object, and it is substituted instead of the current location as the
place where the exception occurred.  If it is neither a traceback
object nor ``None``, a ``TypeError`` is raised.


Backwards Compatibility
=======================

Migration Plan
--------------

Future Statement
''''''''''''''''

Under the :pep:`236` future statement::

    from __future__ import raise_with_two_args

the syntax and semantics of the ``raise`` statement will be as
described above.  This future feature is to appear in Python 2.4; its
effect is to become standard in Python 3.0.

As the examples below illustrate, this future statement is only needed
for code which uses the substituted traceback argument to ``raise``;
simple exception raising does not require it.


Warnings
''''''''

Three new :pep:`warnings <230>`, all of category ``DeprecationWarning``, are
to be issued to point out uses of ``raise`` which will become
incorrect under the proposed changes.

The first warning is issued when a ``raise`` statement is executed in
which the first expression evaluates to a string.  The message for
this warning is::

    raising strings will be impossible in the future

The second warning is issued when a ``raise`` statement is executed in
which the first expression evaluates to a class.  The message for this
warning is::

    raising classes will be impossible in the future

The third warning is issued when a ``raise`` statement with three
expressions is compiled.  (Not, note, when it is executed; this is
important because the ``SyntaxError`` which this warning presages will
occur at compile-time.)  The message for this warning is::

    raising with three arguments will be impossible in the future

These warnings are to appear in Python 2.4, and disappear in Python
3.0, when the conditions which cause them are simply errors.


Examples
--------

Code Using Implicit Instantiation
'''''''''''''''''''''''''''''''''

Code such as ::

    class MyError(Exception):
        pass

    raise MyError, 'spam'

will issue a warning when the ``raise`` statement is executed.  The
``raise`` statement should be changed to instantiate explicitly::

    raise MyError('spam')


Code Using String Exceptions
''''''''''''''''''''''''''''

Code such as ::

    MyError = 'spam'
    raise MyError, 'eggs'

will issue a warning when the ``raise`` statement is executed.  The
exception type should be changed to a class::

    class MyError(Exception):
        pass

and, as in the previous example, the ``raise`` statement should be
changed to instantiate explicitly ::

    raise MyError('eggs')


Code Supplying a Traceback Object
'''''''''''''''''''''''''''''''''

Code such as ::

    raise MyError, 'spam', mytraceback

will issue a warning when compiled.  The statement should be changed
to ::

    raise MyError('spam'), mytraceback

and the future statement ::

    from __future__ import raise_with_two_args

should be added at the top of the module.  Note that adding this
future statement also turns the other two warnings into errors, so the
changes described in the previous examples must also be applied.

The special case ::

    raise sys.exc_type, sys.exc_info, sys.exc_traceback

(which is intended to re-raise a previous exception) should be changed
simply to ::

    raise


A Failure of the Plan
'''''''''''''''''''''

It may occur that a ``raise`` statement which raises a string or
implicitly instantiates is not executed in production or testing
during the phase-in period for this PEP.  In that case, it will not
issue any warnings, but will instead suddenly fail one day in Python
3.0 or a subsequent version.  (The failure is that the wrong exception
gets raised, namely a ``TypeError`` complaining about the arguments to
``raise``, instead of the exception intended.)

Such cases can be made rarer by prolonging the phase-in period; they
cannot be made impossible short of issuing at compile-time a warning
for every ``raise`` statement.


Rejection
=========

If this PEP were accepted, nearly all existing Python code would need
to be reviewed and probably revised; even if all the above arguments
in favour of explicit instantiation are accepted, the improvement in
clarity is too minor to justify the cost of doing the revision and the
risk of new bugs introduced thereby.

This proposal has therefore been rejected [6]_.

Note that string exceptions are slated for removal independently of
this proposal; what is rejected is the removal of implicit exception
instantiation.


Summary of Discussion
=====================

A small minority of respondents were in favour of the proposal, but
the dominant response was that any such migration would be costly
out of proportion to the putative benefit.  As noted above, this
point is sufficient in itself to reject the PEP.


New-Style Exceptions
--------------------

Implicit instantiation might conflict with future plans to allow
instances of new-style classes to be used as exceptions.  In order to
decide whether to instantiate implicitly, the ``raise`` machinery must
determine whether the first argument is a class or an instance -- but
with new-style classes there is no clear and strong distinction.

Under this proposal, the problem would be avoided because the
exception would already have been instantiated.  However, there are
two plausible alternative solutions:

1. Require exception types to be subclasses of ``Exception``, and
   instantiate implicitly if and only if ::

        issubclass(firstarg, Exception)

2. Instantiate implicitly if and only if ::

        isinstance(firstarg, type)

Thus eliminating implicit instantiation entirely is not necessary to
solve this problem.


Ugliness of Explicit Instantiation
----------------------------------

Some respondents felt that the explicitly instantiating syntax is
uglier, especially in cases when no arguments are supplied to the
exception constructor::

    raise TypeError()

The problem is particularly acute when the exception instance itself
is not of interest, that is, when the only relevant point is the
exception type::

    try:
        # ... deeply nested search loop ...
            raise Found
    except Found:
        # ...

In such cases the symmetry between ``raise`` and ``except`` can be
more expressive of the intent of the code.

Guido opined that the implicitly instantiating syntax is "a tad
prettier" even for cases with a single argument, since it has less
punctuation.


Performance Penalty of Warnings
-------------------------------

Experience with deprecating ``apply()`` shows that use of the warning
framework can incur a significant performance penalty.

Code which instantiates explicitly would not be affected, since the
run-time checks necessary to determine whether to issue a warning are
exactly those which are needed to determine whether to instantiate
implicitly in the first place.  That is, such statements are already
incurring the cost of these checks.

Code which instantiates implicitly would incur a large cost: timing
trials indicate that issuing a warning (whether it is suppressed or
not) takes about five times more time than simply instantiating,
raising, and catching an exception.

This penalty is mitigated by the fact that ``raise`` statements are
rarely on performance-critical execution paths.


Traceback Argument
------------------

As the proposal stands, it would be impossible to use the traceback
argument to ``raise`` conveniently with all 2.x versions of Python.

For compatibility with versions < 2.4, the three-argument form must be
used; but this form would produce warnings with versions >= 2.4.
Those warnings could be suppressed, but doing so is awkward because
the relevant type of warning is issued at compile-time.

If this PEP were still under consideration, this objection would be
met by extending the phase-in period.  For example, warnings could
first be issued in 3.0, and become errors in some later release.


References
==========

.. [1] "Standard Exception Classes in Python 1.5", Guido van Rossum.
       http://www.python.org/doc/essays/stdexceptions.html

.. [3] "Python Language Reference", Guido van Rossum.
       http://docs.python.org/reference/simple_stmts.html#raise

.. [6] Guido van Rossum, 11 June 2003 post to ``python-dev``.
       https://mail.python.org/pipermail/python-dev/2003-June/036176.html


Copyright
=========

This document has been placed in the public domain.



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