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Python Programming Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 344</li> </ul> <button id="colour-scheme-cycler" onClick="setColourScheme(nextColourScheme())"> <svg aria-hidden="true" class="colour-scheme-icon-when-auto"><use href="#svg-sun-half"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-dark"><use href="#svg-moon"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-light"><use href="#svg-sun"></use></svg> Toggle light / dark / auto colour theme </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 344 – Exception Chaining and Embedded Tracebacks</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author:</dt> <dd class="field-odd">Ka-Ping Yee</dd> <dt class="field-even">Status:</dt> <dd class="field-even"><abbr title="Replaced by another succeeding PEP">Superseded</abbr></dd> <dt class="field-odd">Type:</dt> <dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd> <dt class="field-even">Created:</dt> <dd class="field-even">12-May-2005</dd> <dt class="field-odd">Python-Version:</dt> <dd class="field-odd">2.5</dd> <dt class="field-even">Post-History:</dt> <dd class="field-even"></dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#numbering-note">Numbering Note</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#motivation">Motivation</a></li> <li><a class="reference internal" href="#history">History</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#implicit-exception-chaining">Implicit Exception Chaining</a></li> <li><a class="reference internal" href="#explicit-exception-chaining">Explicit Exception Chaining</a></li> <li><a class="reference internal" href="#traceback-attribute">Traceback Attribute</a></li> <li><a class="reference internal" href="#enhanced-reporting">Enhanced Reporting</a></li> <li><a class="reference internal" href="#c-api">C API</a></li> <li><a class="reference internal" href="#compatibility">Compatibility</a></li> <li><a class="reference internal" href="#open-issue-extra-information">Open Issue: Extra Information</a></li> <li><a class="reference internal" href="#open-issue-suppressing-context">Open Issue: Suppressing Context</a></li> <li><a class="reference internal" href="#open-issue-limiting-exception-types">Open Issue: Limiting Exception Types</a></li> <li><a class="reference internal" href="#open-issue-yield">Open Issue: yield</a></li> <li><a class="reference internal" href="#open-issue-garbage-collection">Open Issue: Garbage Collection</a></li> <li><a class="reference internal" href="#possible-future-compatible-changes">Possible Future Compatible Changes</a></li> <li><a class="reference internal" href="#possible-future-incompatible-changes">Possible Future Incompatible Changes</a></li> <li><a class="reference internal" href="#acknowledgements">Acknowledgements</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="numbering-note"> <h2><a class="toc-backref" href="#numbering-note" role="doc-backlink">Numbering Note</a></h2> This PEP has been renumbered to <a class="pep reference internal" href="../pep-3134/" title="PEP 3134 – Exception Chaining and Embedded Tracebacks">PEP 3134</a>. The text below is the last version submitted under the old number. </section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> This PEP proposes three standard attributes on exception instances: the <code class="docutils literal notranslate">__context__</code> attribute for implicitly chained exceptions, the <code class="docutils literal notranslate">__cause__</code> attribute for explicitly chained exceptions, and the <code class="docutils literal notranslate">__traceback__</code> attribute for the traceback. A new <code class="docutils literal notranslate">raise ... from</code> statement sets the <code class="docutils literal notranslate">__cause__</code> attribute. </section> <section id="motivation"> <h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2> During the handling of one exception (exception A), it is possible that another exception (exception B) may occur. In today’s Python (version 2.4), if this happens, exception B is propagated outward and exception A is lost. In order to debug the problem, it is useful to know about both exceptions. The <code class="docutils literal notranslate">__context__</code> attribute retains this information automatically. Sometimes it can be useful for an exception handler to intentionally re-raise an exception, either to provide extra information or to translate an exception to another type. The <code class="docutils literal notranslate">__cause__</code> attribute provides an explicit way to record the direct cause of an exception. In today’s Python implementation, exceptions are composed of three parts: the type, the value, and the traceback. The <code class="docutils literal notranslate">sys</code> module, exposes the current exception in three parallel variables, <code class="docutils literal notranslate">exc_type</code>, <code class="docutils literal notranslate">exc_value</code>, and <code class="docutils literal notranslate">exc_traceback</code>, the <code class="docutils literal notranslate">sys.exc_info()</code> function returns a tuple of these three parts, and the <code class="docutils literal notranslate">raise</code> statement has a three-argument form accepting these three parts. Manipulating exceptions often requires passing these three things in parallel, which can be tedious and error-prone. Additionally, the <code class="docutils literal notranslate">except</code> statement can only provide access to the value, not the traceback. Adding the <code class="docutils literal notranslate">__traceback__</code> attribute to exception values makes all the exception information accessible from a single place. </section> <section id="history"> <h2><a class="toc-backref" href="#history" role="doc-backlink">History</a></h2> Raymond Hettinger <a class="footnote-reference brackets" href="#id16" id="id1">[1]</a> raised the issue of masked exceptions on Python-Dev in January 2003 and proposed a <code class="docutils literal notranslate">PyErr_FormatAppend()</code> function that C modules could use to augment the currently active exception with more information. Brett Cannon <a class="footnote-reference brackets" href="#id17" id="id2">[2]</a> brought up chained exceptions again in June 2003, prompting a long discussion. Greg Ewing <a class="footnote-reference brackets" href="#id18" id="id3">[3]</a> identified the case of an exception occurring in a <code class="docutils literal notranslate">finally</code> block during unwinding triggered by an original exception, as distinct from the case of an exception occurring in an <code class="docutils literal notranslate">except</code> block that is handling the original exception. Greg Ewing <a class="footnote-reference brackets" href="#id19" id="id4">[4]</a> and Guido van Rossum <a class="footnote-reference brackets" href="#id20" id="id5">[5]</a>, and probably others, have previously mentioned adding a traceback attribute to <code class="docutils literal notranslate">Exception</code> instances. This is noted in <a class="pep reference internal" href="../pep-3000/" title="PEP 3000 – Python 3000">PEP 3000</a>. This PEP was motivated by yet another recent Python-Dev reposting of the same ideas <a class="footnote-reference brackets" href="#id21" id="id6">[6]</a> <a class="footnote-reference brackets" href="#id22" id="id7">[7]</a>. </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> The Python-Dev discussions revealed interest in exception chaining for two quite different purposes. To handle the unexpected raising of a secondary exception, the exception must be retained implicitly. To support intentional translation of an exception, there must be a way to chain exceptions explicitly. This PEP addresses both. Several attribute names for chained exceptions have been suggested on Python-Dev <a class="footnote-reference brackets" href="#id17" id="id8">[2]</a>, including <code class="docutils literal notranslate">cause</code>, <code class="docutils literal notranslate">antecedent</code>, <code class="docutils literal notranslate">reason</code>, <code class="docutils literal notranslate">original</code>, <code class="docutils literal notranslate">chain</code>, <code class="docutils literal notranslate">chainedexc</code>, <code class="docutils literal notranslate">xc_chain</code>, <code class="docutils literal notranslate">excprev</code>, <code class="docutils literal notranslate">previous</code> and <code class="docutils literal notranslate">precursor</code>. For an explicitly chained exception, this PEP suggests <code class="docutils literal notranslate">__cause__</code> because of its specific meaning. For an implicitly chained exception, this PEP proposes the name <code class="docutils literal notranslate">__context__</code> because the intended meaning is more specific than temporal precedence but less specific than causation: an exception occurs in the context of handling another exception. This PEP suggests names with leading and trailing double-underscores for these three attributes because they are set by the Python VM. Only in very special cases should they be set by normal assignment. This PEP handles exceptions that occur during <code class="docutils literal notranslate">except</code> blocks and <code class="docutils literal notranslate">finally</code> blocks in the same way. Reading the traceback makes it clear where the exceptions occurred, so additional mechanisms for distinguishing the two cases would only add unnecessary complexity. This PEP proposes that the outermost exception object (the one exposed for matching by <code class="docutils literal notranslate">except</code> clauses) be the most recently raised exception for compatibility with current behaviour. This PEP proposes that tracebacks display the outermost exception last, because this would be consistent with the chronological order of tracebacks (from oldest to most recent frame) and because the actual thrown exception is easier to find on the last line. To keep things simpler, the C API calls for setting an exception will not automatically set the exception’s <code class="docutils literal notranslate">__context__</code>. Guido van Rossum has expressed concerns with making such changes <a class="footnote-reference brackets" href="#id23" id="id9">[8]</a>. As for other languages, Java and Ruby both discard the original exception when another exception occurs in a <code class="docutils literal notranslate">catch/rescue</code> or <code class="docutils literal notranslate">finally/ensure</code> clause. Perl 5 lacks built-in structured exception handling. For Perl 6, RFC number 88 <a class="footnote-reference brackets" href="#id24" id="id10">[9]</a> proposes an exception mechanism that implicitly retains chained exceptions in an array named <code class="docutils literal notranslate">@@</code>. In that RFC, the most recently raised exception is exposed for matching, as in this PEP; also, arbitrary expressions (possibly involving <code class="docutils literal notranslate">@@</code>) can be evaluated for exception matching. Exceptions in C# contain a read-only <code class="docutils literal notranslate">InnerException</code> property that may point to another exception. Its documentation <a class="footnote-reference brackets" href="#id25" id="id11">[10]</a> says that “When an exception X is thrown as a direct result of a previous exception Y, the <code class="docutils literal notranslate">InnerException</code> property of X should contain a reference to Y.” This property is not set by the VM automatically; rather, all exception constructors take an optional <code class="docutils literal notranslate">innerException</code> argument to set it explicitly. The <code class="docutils literal notranslate">__cause__</code> attribute fulfills the same purpose as <code class="docutils literal notranslate">InnerException</code>, but this PEP proposes a new form of <code class="docutils literal notranslate">raise</code> rather than extending the constructors of all exceptions. C# also provides a <code class="docutils literal notranslate">GetBaseException</code> method that jumps directly to the end of the <code class="docutils literal notranslate">InnerException</code> chain; this PEP proposes no analog. The reason all three of these attributes are presented together in one proposal is that the <code class="docutils literal notranslate">__traceback__</code> attribute provides convenient access to the traceback on chained exceptions. </section> <section id="implicit-exception-chaining"> <h2><a class="toc-backref" href="#implicit-exception-chaining" role="doc-backlink">Implicit Exception Chaining</a></h2> Here is an example to illustrate the <code class="docutils literal notranslate">__context__</code> attribute: <div class="highlight-default notranslate"><div class="highlight"><pre>def compute(a, b): try: a/b except Exception, exc: log(exc) def log(exc): file = open('logfile.txt') # oops, forgot the 'w' print >>file, exc file.close() </pre></div> </div> Calling <code class="docutils literal notranslate">compute(0, 0)</code> causes a <code class="docutils literal notranslate">ZeroDivisionError</code>. The <code class="docutils literal notranslate">compute()</code> function catches this exception and calls <code class="docutils literal notranslate">log(exc)</code>, but the <code class="docutils literal notranslate">log()</code> function also raises an exception when it tries to write to a file that wasn’t opened for writing. In today’s Python, the caller of <code class="docutils literal notranslate">compute()</code> gets thrown an <code class="docutils literal notranslate">IOError</code>. The <code class="docutils literal notranslate">ZeroDivisionError</code> is lost. With the proposed change, the instance of <code class="docutils literal notranslate">IOError</code> has an additional <code class="docutils literal notranslate">__context__</code> attribute that retains the <code class="docutils literal notranslate">ZeroDivisionError</code>. The following more elaborate example demonstrates the handling of a mixture of <code class="docutils literal notranslate">finally</code> and <code class="docutils literal notranslate">except</code> clauses: <div class="highlight-default notranslate"><div class="highlight"><pre>def main(filename): file = open(filename) # oops, forgot the 'w' try: try: compute() except Exception, exc: log(file, exc) finally: file.clos() # oops, misspelled 'close' def compute(): 1/0 def log(file, exc): try: print >>file, exc # oops, file is not writable except: display(exc) def display(exc): print ex # oops, misspelled 'exc' </pre></div> </div> Calling <code class="docutils literal notranslate">main()</code> with the name of an existing file will trigger four exceptions. The ultimate result will be an <code class="docutils literal notranslate">AttributeError</code> due to the misspelling of <code class="docutils literal notranslate">clos</code>, whose <code class="docutils literal notranslate">__context__</code> points to a <code class="docutils literal notranslate">NameError</code> due to the misspelling of <code class="docutils literal notranslate">ex</code>, whose <code class="docutils literal notranslate">__context__</code> points to an <code class="docutils literal notranslate">IOError</code> due to the file being read-only, whose <code class="docutils literal notranslate">__context__</code> points to a <code class="docutils literal notranslate">ZeroDivisionError</code>, whose <code class="docutils literal notranslate">__context__</code> attribute is <code class="docutils literal notranslate">None</code>. The proposed semantics are as follows: <ol class="arabic simple"> <li>Each thread has an exception context initially set to <code class="docutils literal notranslate">None</code>.</li> <li>Whenever an exception is raised, if the exception instance does not already have a <code class="docutils literal notranslate">__context__</code> attribute, the interpreter sets it equal to the thread’s exception context.</li> <li>Immediately after an exception is raised, the thread’s exception context is set to the exception.</li> <li>Whenever the interpreter exits an <code class="docutils literal notranslate">except</code> block by reaching the end or executing a <code class="docutils literal notranslate">return</code>, <code class="docutils literal notranslate">yield</code>, <code class="docutils literal notranslate">continue</code>, or <code class="docutils literal notranslate">break</code> statement, the thread’s exception context is set to <code class="docutils literal notranslate">None</code>.</li> </ol> </section> <section id="explicit-exception-chaining"> <h2><a class="toc-backref" href="#explicit-exception-chaining" role="doc-backlink">Explicit Exception Chaining</a></h2> The <code class="docutils literal notranslate">__cause__</code> attribute on exception objects is always initialized to <code class="docutils literal notranslate">None</code>. It is set by a new form of the <code class="docutils literal notranslate">raise</code> statement: <div class="highlight-default notranslate"><div class="highlight"><pre>raise EXCEPTION from CAUSE </pre></div> </div> which is equivalent to: <div class="highlight-default notranslate"><div class="highlight"><pre>exc = EXCEPTION exc.__cause__ = CAUSE raise exc </pre></div> </div> In the following example, a database provides implementations for a few different kinds of storage, with file storage as one kind. The database designer wants errors to propagate as <code class="docutils literal notranslate">DatabaseError</code> objects so that the client doesn’t have to be aware of the storage-specific details, but doesn’t want to lose the underlying error information: <div class="highlight-default notranslate"><div class="highlight"><pre>class DatabaseError(StandardError): pass class FileDatabase(Database): def __init__(self, filename): try: self.file = open(filename) except IOError, exc: raise DatabaseError('failed to open') from exc </pre></div> </div> If the call to <code class="docutils literal notranslate">open()</code> raises an exception, the problem will be reported as a <code class="docutils literal notranslate">DatabaseError</code>, with a <code class="docutils literal notranslate">__cause__</code> attribute that reveals the <code class="docutils literal notranslate">IOError</code> as the original cause. </section> <section id="traceback-attribute"> <h2><a class="toc-backref" href="#traceback-attribute" role="doc-backlink">Traceback Attribute</a></h2> The following example illustrates the <code class="docutils literal notranslate">__traceback__</code> attribute: <div class="highlight-default notranslate"><div class="highlight"><pre>def do_logged(file, work): try: work() except Exception, exc: write_exception(file, exc) raise exc from traceback import format_tb def write_exception(file, exc): ... type = exc.__class__ message = str(exc) lines = format_tb(exc.__traceback__) file.write(... type ... message ... lines ...) ... </pre></div> </div> In today’s Python, the <code class="docutils literal notranslate">do_logged()</code> function would have to extract the traceback from <code class="docutils literal notranslate">sys.exc_traceback</code> or <code class="docutils literal notranslate">sys.exc_info()</code> <a class="footnote-reference brackets" href="#id17" id="id12">[2]</a> and pass both the value and the traceback to <code class="docutils literal notranslate">write_exception()</code>. With the proposed change, <code class="docutils literal notranslate">write_exception()</code> simply gets one argument and obtains the exception using the <code class="docutils literal notranslate">__traceback__</code> attribute. The proposed semantics are as follows: <ol class="arabic simple"> <li>Whenever an exception is caught, if the exception instance does not already have a <code class="docutils literal notranslate">__traceback__</code> attribute, the interpreter sets it to the newly caught traceback.</li> </ol> </section> <section id="enhanced-reporting"> <h2><a class="toc-backref" href="#enhanced-reporting" role="doc-backlink">Enhanced Reporting</a></h2> The default exception handler will be modified to report chained exceptions. The chain of exceptions is traversed by following the <code class="docutils literal notranslate">__cause__</code> and <code class="docutils literal notranslate">__context__</code> attributes, with <code class="docutils literal notranslate">__cause__</code> taking priority. In keeping with the chronological order of tracebacks, the most recently raised exception is displayed last; that is, the display begins with the description of the innermost exception and backs up the chain to the outermost exception. The tracebacks are formatted as usual, with one of the lines: <div class="highlight-default notranslate"><div class="highlight"><pre>The above exception was the direct cause of the following exception: </pre></div> </div> or <div class="highlight-default notranslate"><div class="highlight"><pre>During handling of the above exception, another exception occurred: </pre></div> </div> between tracebacks, depending whether they are linked by <code class="docutils literal notranslate">__cause__</code> or <code class="docutils literal notranslate">__context__</code> respectively. Here is a sketch of the procedure: <div class="highlight-default notranslate"><div class="highlight"><pre>def print_chain(exc): if exc.__cause__: print_chain(exc.__cause__) print '\nThe above exception was the direct cause...' elif exc.__context__: print_chain(exc.__context__) print '\nDuring handling of the above exception, ...' print_exc(exc) </pre></div> </div> In the <code class="docutils literal notranslate">traceback</code> module, the <code class="docutils literal notranslate">format_exception</code>, <code class="docutils literal notranslate">print_exception</code>, <code class="docutils literal notranslate">print_exc</code>, and <code class="docutils literal notranslate">print_last functions</code> will be updated to accept an optional <code class="docutils literal notranslate">chain</code> argument, <code class="docutils literal notranslate">True</code> by default. When this argument is <code class="docutils literal notranslate">True</code>, these functions will format or display the entire chain of exceptions as just described. When it is <code class="docutils literal notranslate">False</code>, these functions will format or display only the outermost exception. The <code class="docutils literal notranslate">cgitb</code> module should also be updated to display the entire chain of exceptions. </section> <section id="c-api"> <h2><a class="toc-backref" href="#c-api" role="doc-backlink">C API</a></h2> The <code class="docutils literal notranslate">PyErr_Set*</code> calls for setting exceptions will not set the <code class="docutils literal notranslate">__context__</code> attribute on exceptions. <code class="docutils literal notranslate">PyErr_NormalizeException</code> will always set the <code class="docutils literal notranslate">traceback</code> attribute to its <code class="docutils literal notranslate">tb</code> argument and the <code class="docutils literal notranslate">__context__</code> and <code class="docutils literal notranslate">__cause__</code> attributes to <code class="docutils literal notranslate">None</code>. A new API function, <code class="docutils literal notranslate">PyErr_SetContext(context)</code>, will help C programmers provide chained exception information. This function will first normalize the current exception so it is an instance, then set its <code class="docutils literal notranslate">__context__</code> attribute. A similar API function, <code class="docutils literal notranslate">PyErr_SetCause(cause)</code>, will set the <code class="docutils literal notranslate">__cause__</code> attribute. </section> <section id="compatibility"> <h2><a class="toc-backref" href="#compatibility" role="doc-backlink">Compatibility</a></h2> Chained exceptions expose the type of the most recent exception, so they will still match the same <code class="docutils literal notranslate">except</code> clauses as they do now. The proposed changes should not break any code unless it sets or uses attributes named <code class="docutils literal notranslate">__context__</code>, <code class="docutils literal notranslate">__cause__</code>, or <code class="docutils literal notranslate">__traceback__</code> on exception instances. As of 2005-05-12, the Python standard library contains no mention of such attributes. </section> <section id="open-issue-extra-information"> <h2><a class="toc-backref" href="#open-issue-extra-information" role="doc-backlink">Open Issue: Extra Information</a></h2> Walter Dörwald <a class="footnote-reference brackets" href="#id26" id="id13">[11]</a> expressed a desire to attach extra information to an exception during its upward propagation without changing its type. This could be a useful feature, but it is not addressed by this PEP. It could conceivably be addressed by a separate PEP establishing conventions for other informational attributes on exceptions. </section> <section id="open-issue-suppressing-context"> <h2><a class="toc-backref" href="#open-issue-suppressing-context" role="doc-backlink">Open Issue: Suppressing Context</a></h2> As written, this PEP makes it impossible to suppress <code class="docutils literal notranslate">__context__</code>, since setting <code class="docutils literal notranslate">exc.__context__</code> to <code class="docutils literal notranslate">None</code> in an <code class="docutils literal notranslate">except</code> or <code class="docutils literal notranslate">finally</code> clause will only result in it being set again when <code class="docutils literal notranslate">exc</code> is raised. </section> <section id="open-issue-limiting-exception-types"> <h2><a class="toc-backref" href="#open-issue-limiting-exception-types" role="doc-backlink">Open Issue: Limiting Exception Types</a></h2> To improve encapsulation, library implementors may want to wrap all implementation-level exceptions with an application-level exception. One could try to wrap exceptions by writing this: <div class="highlight-default notranslate"><div class="highlight"><pre>try: ... implementation may raise an exception ... except: import sys raise ApplicationError from sys.exc_value </pre></div> </div> or this <div class="highlight-default notranslate"><div class="highlight"><pre>try: ... implementation may raise an exception ... except Exception, exc: raise ApplicationError from exc </pre></div> </div> but both are somewhat flawed. It would be nice to be able to name the current exception in a catch-all <code class="docutils literal notranslate">except</code> clause, but that isn’t addressed here. Such a feature would allow something like this: <div class="highlight-default notranslate"><div class="highlight"><pre>try: ... implementation may raise an exception ... except *, exc: raise ApplicationError from exc </pre></div> </div> </section> <section id="open-issue-yield"> <h2><a class="toc-backref" href="#open-issue-yield" role="doc-backlink">Open Issue: yield</a></h2> The exception context is lost when a <code class="docutils literal notranslate">yield</code> statement is executed; resuming the frame after the <code class="docutils literal notranslate">yield</code> does not restore the context. Addressing this problem is out of the scope of this PEP; it is not a new problem, as demonstrated by the following example: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> def gen(): ... try: ... 1/0 ... except: ... yield 3 ... raise ... >>> g = gen() >>> g.next() 3 >>> g.next() TypeError: exceptions must be classes, instances, or strings (deprecated), not NoneType </pre></div> </div> </section> <section id="open-issue-garbage-collection"> <h2><a class="toc-backref" href="#open-issue-garbage-collection" role="doc-backlink">Open Issue: Garbage Collection</a></h2> The strongest objection to this proposal has been that it creates cycles between exceptions and stack frames <a class="footnote-reference brackets" href="#id27" id="id14">[12]</a>. Collection of cyclic garbage (and therefore resource release) can be greatly delayed: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> try: >>> 1/0 >>> except Exception, err: >>> pass </pre></div> </div> will introduce a cycle from err -> traceback -> stack frame -> err, keeping all locals in the same scope alive until the next GC happens. Today, these locals would go out of scope. There is lots of code which assumes that “local” resources – particularly open files – will be closed quickly. If closure has to wait for the next GC, a program (which runs fine today) may run out of file handles. Making the <code class="docutils literal notranslate">__traceback__</code> attribute a weak reference would avoid the problems with cyclic garbage. Unfortunately, it would make saving the <code class="docutils literal notranslate">Exception</code> for later (as <code class="docutils literal notranslate">unittest</code> does) more awkward, and it would not allow as much cleanup of the <code class="docutils literal notranslate">sys</code> module. A possible alternate solution, suggested by Adam Olsen, would be to instead turn the reference from the stack frame to the <code class="docutils literal notranslate">err</code> variable into a weak reference when the variable goes out of scope <a class="footnote-reference brackets" href="#id28" id="id15">[13]</a>. </section> <section id="possible-future-compatible-changes"> <h2><a class="toc-backref" href="#possible-future-compatible-changes" role="doc-backlink">Possible Future Compatible Changes</a></h2> These changes are consistent with the appearance of exceptions as a single object rather than a triple at the interpreter level. <ul class="simple"> <li>If <a class="pep reference internal" href="../pep-0340/" title="PEP 340 – Anonymous Block Statements">PEP 340</a> or <a class="pep reference internal" href="../pep-0343/" title="PEP 343 – The “with” Statement">PEP 343</a> is accepted, replace the three (<code class="docutils literal notranslate">type</code>, <code class="docutils literal notranslate">value</code>, <code class="docutils literal notranslate">traceback</code>) arguments to <code class="docutils literal notranslate">__exit__</code> with a single exception argument.</li> <li>Deprecate <code class="docutils literal notranslate">sys.exc_type</code>, <code class="docutils literal notranslate">sys.exc_value</code>, <code class="docutils literal notranslate">sys.exc_traceback</code>, and <code class="docutils literal notranslate">sys.exc_info()</code> in favour of a single member, <code class="docutils literal notranslate">sys.exception</code>.</li> <li>Deprecate <code class="docutils literal notranslate">sys.last_type</code>, <code class="docutils literal notranslate">sys.last_value</code>, and <code class="docutils literal notranslate">sys.last_traceback</code> in favour of a single member, <code class="docutils literal notranslate">sys.last_exception</code>.</li> <li>Deprecate the three-argument form of the <code class="docutils literal notranslate">raise</code> statement in favour of the one-argument form.</li> <li>Upgrade <code class="docutils literal notranslate">cgitb.html()</code> to accept a single value as its first argument as an alternative to a <code class="docutils literal notranslate">(type, value, traceback)</code> tuple.</li> </ul> </section> <section id="possible-future-incompatible-changes"> <h2><a class="toc-backref" href="#possible-future-incompatible-changes" role="doc-backlink">Possible Future Incompatible Changes</a></h2> These changes might be worth considering for Python 3000. <ul class="simple"> <li>Remove <code class="docutils literal notranslate">sys.exc_type</code>, <code class="docutils literal notranslate">sys.exc_value</code>, <code class="docutils literal notranslate">sys.exc_traceback</code>, and <code class="docutils literal notranslate">sys.exc_info()</code>.</li> <li>Remove <code class="docutils literal notranslate">sys.last_type</code>, <code class="docutils literal notranslate">sys.last_value</code>, and <code class="docutils literal notranslate">sys.last_traceback</code>.</li> <li>Replace the three-argument <code class="docutils literal notranslate">sys.excepthook</code> with a one-argument API, and changing the <code class="docutils literal notranslate">cgitb</code> module to match.</li> <li>Remove the three-argument form of the <code class="docutils literal notranslate">raise</code> statement.</li> <li>Upgrade <code class="docutils literal notranslate">traceback.print_exception</code> to accept an <code class="docutils literal notranslate">exception</code> argument instead of the <code class="docutils literal notranslate">type</code>, <code class="docutils literal notranslate">value</code>, and <code class="docutils literal notranslate">traceback</code> arguments.</li> </ul> </section> <section id="acknowledgements"> <h2><a class="toc-backref" href="#acknowledgements" role="doc-backlink">Acknowledgements</a></h2> Brett Cannon, Greg Ewing, Guido van Rossum, Jeremy Hylton, Phillip J. Eby, Raymond Hettinger, Walter Dörwald, and others. </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="id16" role="doc-footnote"> <dt class="label" id="id16">[<a href="#id1">1</a>]</dt> <dd>Raymond Hettinger, “Idea for avoiding exception masking” <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2003-January/032492.html">https://mail.python.org/pipermail/python-dev/2003-January/032492.html</a></aside> <aside class="footnote brackets" id="id17" role="doc-footnote"> <dt class="label" id="id17">[2] (<a href='#id2'>1</a>, <a href='#id8'>2</a>, <a href='#id12'>3</a>) </dt> <dd>Brett Cannon explains chained exceptions <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2003-June/036063.html">https://mail.python.org/pipermail/python-dev/2003-June/036063.html</a></aside> <aside class="footnote brackets" id="id18" role="doc-footnote"> <dt class="label" id="id18">[<a href="#id3">3</a>]</dt> <dd>Greg Ewing points out masking caused by exceptions during finally <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2003-June/036290.html">https://mail.python.org/pipermail/python-dev/2003-June/036290.html</a></aside> <aside class="footnote brackets" id="id19" role="doc-footnote"> <dt class="label" id="id19">[<a href="#id4">4</a>]</dt> <dd>Greg Ewing suggests storing the traceback in the exception object <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2003-June/036092.html">https://mail.python.org/pipermail/python-dev/2003-June/036092.html</a></aside> <aside class="footnote brackets" id="id20" role="doc-footnote"> <dt class="label" id="id20">[<a href="#id5">5</a>]</dt> <dd>Guido van Rossum mentions exceptions having a traceback attribute <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2005-April/053060.html">https://mail.python.org/pipermail/python-dev/2005-April/053060.html</a></aside> <aside class="footnote brackets" id="id21" role="doc-footnote"> <dt class="label" id="id21">[<a href="#id6">6</a>]</dt> <dd>Ka-Ping Yee, “Tidier Exceptions” <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2005-May/053671.html">https://mail.python.org/pipermail/python-dev/2005-May/053671.html</a></aside> <aside class="footnote brackets" id="id22" role="doc-footnote"> <dt class="label" id="id22">[<a href="#id7">7</a>]</dt> <dd>Ka-Ping Yee, “Chained Exceptions” <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2005-May/053672.html">https://mail.python.org/pipermail/python-dev/2005-May/053672.html</a></aside> <aside class="footnote brackets" id="id23" role="doc-footnote"> <dt class="label" id="id23">[<a href="#id9">8</a>]</dt> <dd>Guido van Rossum discusses automatic chaining in <code class="docutils literal notranslate">PyErr_Set*</code> <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2003-June/036180.html">https://mail.python.org/pipermail/python-dev/2003-June/036180.html</a></aside> <aside class="footnote brackets" id="id24" role="doc-footnote"> <dt class="label" id="id24">[<a href="#id10">9</a>]</dt> <dd>Tony Olensky, “Omnibus Structured Exception/Error Handling Mechanism” <a class="reference external" href="http://dev.perl.org/perl6/rfc/88.html">http://dev.perl.org/perl6/rfc/88.html</a></aside> <aside class="footnote brackets" id="id25" role="doc-footnote"> <dt class="label" id="id25">[<a href="#id11">10</a>]</dt> <dd>MSDN .NET Framework Library, “Exception.InnerException Property” <a class="reference external" href="http://msdn.microsoft.com/library/en-us/cpref/html/frlrfsystemexceptionclassinnerexceptiontopic.asp">http://msdn.microsoft.com/library/en-us/cpref/html/frlrfsystemexceptionclassinnerexceptiontopic.asp</a></aside> <aside class="footnote brackets" id="id26" role="doc-footnote"> <dt class="label" id="id26">[<a href="#id13">11</a>]</dt> <dd>Walter Dörwald suggests wrapping exceptions to add details <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2003-June/036148.html">https://mail.python.org/pipermail/python-dev/2003-June/036148.html</a></aside> <aside class="footnote brackets" id="id27" role="doc-footnote"> <dt class="label" id="id27">[<a href="#id14">12</a>]</dt> <dd>Guido van Rossum restates the objection to cyclic trash <a class="reference external" href="https://mail.python.org/pipermail/python-3000/2007-January/005322.html">https://mail.python.org/pipermail/python-3000/2007-January/005322.html</a></aside> <aside class="footnote brackets" id="id28" role="doc-footnote"> <dt class="label" id="id28">[<a href="#id15">13</a>]</dt> <dd>Adam Olsen suggests using a weakref from stack frame to exception <a class="reference external" href="https://mail.python.org/pipermail/python-3000/2007-January/005363.html">https://mail.python.org/pipermail/python-3000/2007-January/005363.html</a></aside> </aside> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> This document has been placed in the public domain. </section> </section> <hr class="docutils" /> Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0344.rst">https://github.com/python/peps/blob/main/peps/pep-0344.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0344.rst">2025-02-01 08:59:27 GMT</a> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#numbering-note">Numbering Note</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#motivation">Motivation</a></li> <li><a class="reference internal" href="#history">History</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#implicit-exception-chaining">Implicit Exception Chaining</a></li> <li><a class="reference internal" href="#explicit-exception-chaining">Explicit Exception Chaining</a></li> <li><a class="reference internal" href="#traceback-attribute">Traceback Attribute</a></li> <li><a class="reference internal" href="#enhanced-reporting">Enhanced Reporting</a></li> <li><a class="reference internal" href="#c-api">C API</a></li> <li><a class="reference internal" href="#compatibility">Compatibility</a></li> <li><a class="reference internal" href="#open-issue-extra-information">Open Issue: Extra Information</a></li> <li><a class="reference internal" href="#open-issue-suppressing-context">Open Issue: Suppressing Context</a></li> <li><a class="reference internal" href="#open-issue-limiting-exception-types">Open Issue: Limiting Exception Types</a></li> <li><a class="reference internal" href="#open-issue-yield">Open Issue: yield</a></li> <li><a class="reference internal" href="#open-issue-garbage-collection">Open Issue: Garbage Collection</a></li> <li><a class="reference internal" href="#possible-future-compatible-changes">Possible Future Compatible Changes</a></li> <li><a class="reference internal" href="#possible-future-incompatible-changes">Possible Future Incompatible Changes</a></li> <li><a class="reference internal" href="#acknowledgements">Acknowledgements</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0344.rst">Page Source (GitHub)</a> 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