466 lines
14 KiB
Plaintext
466 lines
14 KiB
Plaintext
PEP: 317
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Title: Eliminate Implicit Exception Instantiation
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Version: $Revision$
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Last-Modified: $Date$
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Author: Steven Taschuk <staschuk@telusplanet.net>
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Status: Rejected
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 06-May-2003
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Python-Version: 2.4
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Post-History: 09-Jun-2003
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Abstract
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========
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"For clarity in new code, the form ``raise class(argument, ...)``
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is recommended (i.e. make an explicit call to the constructor)."
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-- Guido van Rossum, in 1997 [1]_
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This PEP proposes the formal deprecation and eventual elimination of
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forms of the ``raise`` statement which implicitly instantiate an
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exception. For example, statements such as ::
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raise HullBreachError
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raise KitchenError, 'all out of baked beans'
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must under this proposal be replaced with their synonyms ::
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raise HullBreachError()
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raise KitchenError('all out of baked beans')
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Note that these latter statements are already legal, and that this PEP
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does not change their meaning.
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Eliminating these forms of ``raise`` makes it impossible to use string
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exceptions; accordingly, this PEP also proposes the formal deprecation
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and eventual elimination of string exceptions.
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Adoption of this proposal breaks backwards compatibility. Under the
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proposed implementation schedule, Python 2.4 will introduce warnings
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about uses of ``raise`` which will eventually become incorrect, and
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Python 3.0 will eliminate them entirely. (It is assumed that this
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transition period -- 2.4 to 3.0 -- will be at least one year long, to
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comply with the guidelines of PEP 5 [2]_.)
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Motivation
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==========
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String Exceptions
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-----------------
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It is assumed that removing string exceptions will be uncontroversial,
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since it has been intended since at least Python 1.5, when the
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standard exception types were changed to classes [1]_.
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For the record: string exceptions should be removed because the
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presence of two kinds of exception complicates the language without
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any compensation. Instance exceptions are superior because, for
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example,
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* the class-instance relationship more naturally expresses the
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relationship between the exception type and value,
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* they can be organized naturally using superclass-subclass
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relationships, and
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* they can encapsulate error-reporting behaviour (for example).
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Implicit Instantiation
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----------------------
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Guido's 1997 essay [1]_ on changing the standard exceptions into
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classes makes clear why ``raise`` can instantiate implicitly:
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"The raise statement has been extended to allow raising a class
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exception without explicit instantiation. The following forms,
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called the "compatibility forms" of the raise statement [...] The
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motivation for introducing the compatibility forms was to allow
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backward compatibility with old code that raised a standard
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exception."
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For example, it was desired that pre-1.5 code which used string
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exception syntax such as ::
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raise TypeError, 'not an int'
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would work both on versions of Python in which ``TypeError`` was a
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string, and on versions in which it was a class.
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When no such consideration obtains -- that is, when the desired
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exception type is not a string in any version of the software which
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the code must support -- there is no good reason to instantiate
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implicitly, and it is clearer not to. For example:
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1. In the code ::
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try:
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raise MyError, raised
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except MyError, caught:
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pass
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the syntactic parallel between the ``raise`` and ``except``
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statements strongly suggests that ``raised`` and ``caught`` refer
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to the same object. For string exceptions this actually is the
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case, but for instance exceptions it is not.
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2. When instantiation is implicit, it is not obvious when it occurs,
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for example, whether it occurs when the exception is raised or when
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it is caught. Since it actually happens at the ``raise``, the code
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should say so.
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(Note that at the level of the C API, an exception can be "raised"
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and "caught" without being instantiated; this is used as an
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optimization by, for example, ``PyIter_Next``. But in Python, no
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such optimization is or should be available.)
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3. An implicitly instantiating ``raise`` statement with no arguments,
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such as ::
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raise MyError
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simply does not do what it says: it does not raise the named
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object.
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4. The equivalence of ::
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raise MyError
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raise MyError()
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conflates classes and instances, creating a possible source of
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confusion for beginners. (Moreover, it is not clear that the
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interpreter could distinguish between a new-style class and an
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instance of such a class, so implicit instantiation may be an
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obstacle to any future plan to let exceptions be new-style
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objects.)
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In short, implicit instantiation has no advantages other than
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backwards compatibility, and so should be phased out along with what
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it exists to ensure compatibility with, namely, string exceptions.
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Specification
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=============
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The syntax of ``raise_stmt`` [3]_ is to be changed from ::
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raise_stmt ::= "raise" [expression ["," expression ["," expression]]]
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to ::
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raise_stmt ::= "raise" [expression ["," expression]]
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If no expressions are present, the ``raise`` statement behaves as it
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does presently: it re-raises the last exception that was active in the
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current scope, and if no exception has been active in the current
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scope, a ``TypeError`` is raised indicating that this is the problem.
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Otherwise, the first expression is evaluated, producing the *raised
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object*. Then the second expression is evaluated, if present,
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producing the *substituted traceback*. If no second expression is
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present, the substituted traceback is ``None``.
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The raised object must be an instance. The class of the instance is
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the exception type, and the instance itself is the exception value.
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If the raised object is not an instance -- for example, if it is a
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class or string -- a ``TypeError`` is raised.
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If the substituted traceback is not ``None``, it must be a traceback
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object, and it is substituted instead of the current location as the
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place where the exception occurred. If it is neither a traceback
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object nor ``None``, a ``TypeError`` is raised.
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Backwards Compatibility
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=======================
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Migration Plan
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--------------
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Future Statement
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''''''''''''''''
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Under the future statement [4]_ ::
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from __future__ import raise_with_two_args
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the syntax and semantics of the ``raise`` statement will be as
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described above. This future feature is to appear in Python 2.4; its
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effect is to become standard in Python 3.0.
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As the examples below illustrate, this future statement is only needed
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for code which uses the substituted traceback argument to ``raise``;
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simple exception raising does not require it.
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Warnings
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''''''''
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Three new warnings [5]_, all of category ``DeprecationWarning``, are
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to be issued to point out uses of ``raise`` which will become
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incorrect under the proposed changes.
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The first warning is issued when a ``raise`` statement is executed in
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which the first expression evaluates to a string. The message for
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this warning is::
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raising strings will be impossible in the future
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The second warning is issued when a ``raise`` statement is executed in
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which the first expression evaluates to a class. The message for this
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warning is::
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raising classes will be impossible in the future
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The third warning is issued when a ``raise`` statement with three
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expressions is compiled. (Not, note, when it is executed; this is
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important because the ``SyntaxError`` which this warning presages will
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occur at compile-time.) The message for this warning is::
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raising with three arguments will be impossible in the future
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These warnings are to appear in Python 2.4, and disappear in Python
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3.0, when the conditions which cause them are simply errors.
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Examples
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--------
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Code Using Implicit Instantiation
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'''''''''''''''''''''''''''''''''
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Code such as ::
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class MyError(Exception):
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pass
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raise MyError, 'spam'
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will issue a warning when the ``raise`` statement is executed. The
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``raise`` statement should be changed to instantiate explicitly::
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raise MyError('spam')
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Code Using String Exceptions
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''''''''''''''''''''''''''''
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Code such as ::
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MyError = 'spam'
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raise MyError, 'eggs'
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will issue a warning when the ``raise`` statement is executed. The
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exception type should be changed to a class::
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class MyError(Exception):
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pass
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and, as in the previous example, the ``raise`` statement should be
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changed to instantiate explicitly ::
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raise MyError('eggs')
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Code Supplying a Traceback Object
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'''''''''''''''''''''''''''''''''
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Code such as ::
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raise MyError, 'spam', mytraceback
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will issue a warning when compiled. The statement should be changed
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to ::
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raise MyError('spam'), mytraceback
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and the future statement ::
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from __future__ import raise_with_two_args
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should be added at the top of the module. Note that adding this
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future statement also turns the other two warnings into errors, so the
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changes described in the previous examples must also be applied.
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The special case ::
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raise sys.exc_type, sys.exc_info, sys.exc_traceback
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(which is intended to re-raise a previous exception) should be changed
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simply to ::
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raise
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A Failure of the Plan
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'''''''''''''''''''''
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It may occur that a ``raise`` statement which raises a string or
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implicitly instantiates is not executed in production or testing
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during the phase-in period for this PEP. In that case, it will not
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issue any warnings, but will instead suddenly fail one day in Python
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3.0 or a subsequent version. (The failure is that the wrong exception
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gets raised, namely a ``TypeError`` complaining about the arguments to
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``raise``, instead of the exception intended.)
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Such cases can be made rarer by prolonging the phase-in period; they
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cannot be made impossible short of issuing at compile-time a warning
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for every ``raise`` statement.
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Rejection
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=========
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If this PEP were accepted, nearly all existing Python code would need
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to be reviewed and probably revised; even if all the above arguments
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in favour of explicit instantiation are accepted, the improvement in
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clarity is too minor to justify the cost of doing the revision and the
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risk of new bugs introduced thereby.
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This proposal has therefore been rejected [6]_.
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Note that string exceptions are slated for removal independently of
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this proposal; what is rejected is the removal of implicit exception
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instantiation.
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Summary of Discussion
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=====================
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A small minority of respondents were in favour of the proposal, but
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the dominant response was that any such migration would be costly
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out of proportion to the putative benefit. As noted above, this
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point is sufficient in itself to reject the PEP.
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New-Style Exceptions
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--------------------
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Implicit instantiation might conflict with future plans to allow
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instances of new-style classes to be used as exceptions. In order to
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decide whether to instantiate implicitly, the ``raise`` machinery must
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determine whether the first argument is a class or an instance -- but
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with new-style classes there is no clear and strong distinction.
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Under this proposal, the problem would be avoided because the
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exception would already have been instantiated. However, there are
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two plausible alternative solutions:
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1. Require exception types to be subclasses of ``Exception``, and
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instantiate implicitly if and only if ::
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issubclass(firstarg, Exception)
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2. Instantiate implicitly if and only if ::
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isinstance(firstarg, type)
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Thus eliminating implicit instantiation entirely is not necessary to
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solve this problem.
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Ugliness of Explicit Instantiation
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----------------------------------
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Some respondents felt that the explicitly instantiating syntax is
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uglier, especially in cases when no arguments are supplied to the
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exception constructor::
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raise TypeError()
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The problem is particularly acute when the exception instance itself
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is not of interest, that is, when the only relevant point is the
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exception type::
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try:
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# ... deeply nested search loop ...
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raise Found
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except Found:
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# ...
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In such cases the symmetry between ``raise`` and ``except`` can be
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more expressive of the intent of the code.
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Guido opined that the implicitly instantiating syntax is "a tad
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prettier" even for cases with a single argument, since it has less
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punctuation.
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Performance Penalty of Warnings
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-------------------------------
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Experience with deprecating ``apply()`` shows that use of the warning
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framework can incur a significant performance penalty.
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Code which instantiates explicitly would not be affected, since the
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run-time checks necessary to determine whether to issue a warning are
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exactly those which are needed to determine whether to instantiate
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implicitly in the first place. That is, such statements are already
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incurring the cost of these checks.
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Code which instantiates implicitly would incur a large cost: timing
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trials indicate that issuing a warning (whether it is suppressed or
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not) takes about five times more time than simply instantiating,
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raising, and catching an exception.
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This penalty is mitigated by the fact that ``raise`` statements are
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rarely on performance-critical execution paths.
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Traceback Argument
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------------------
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As the proposal stands, it would be impossible to use the traceback
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argument to ``raise`` conveniently with all 2.x versions of Python.
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For compatibility with versions < 2.4, the three-argument form must be
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used; but this form would produce warnings with versions >= 2.4.
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Those warnings could be suppressed, but doing so is awkward because
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the relevant type of warning is issued at compile-time.
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If this PEP were still under consideration, this objection would be
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met by extending the phase-in period. For example, warnings could
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first be issued in 3.0, and become errors in some later release.
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References
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==========
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.. [1] "Standard Exception Classes in Python 1.5", Guido van Rossum.
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http://www.python.org/doc/essays/stdexceptions.html
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.. [2] "Guidelines for Language Evolution", Paul Prescod.
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http://www.python.org/peps/pep-0005.html
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.. [3] "Python Language Reference", Guido van Rossum.
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http://docs.python.org/reference/simple_stmts.html#raise
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.. [4] PEP 236 "Back to the __future__", Tim Peters.
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http://www.python.org/peps/pep-0236.html
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.. [5] PEP 230 "Warning Framework", Guido van Rossum.
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http://www.python.org/peps/pep-0230.html
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.. [6] Guido van Rossum, 11 June 2003 post to ``python-dev``.
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http://mail.python.org/pipermail/python-dev/2003-June/036176.html
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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indent-tabs-mode: nil
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End:
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