Mailman 3 August 2008 - Python-Dev

[optparse] I wonder if optparse is dead ? because...
by KLEIN Stéphane 30 Aug '08

30 Aug '08

Hello, I wonder if optparse is dead ? because : * svn access is down (svn co svn://starship.python.net/optik/trunk optik) ( http://sourceforge.net/mailarchive/forum.php? thread_name=466F541C.6010804%40users.sourceforge.net&forum_name=optik- users ) * last release is out 2 years ago ( http://sourceforge.net/mailarchive/forum.php? thread_name=20060723161651.GA2076%40cthulhu.gerg.ca&forum_name=optik- users ) * Greg Ward mail ( gward at python.net ) and is home page (http:// www.gerg.ca/) are away * I'm interested by this feature http://sourceforge.net/tracker/index.php? func=detail&aid=1819510&group_id=38019&atid=421097 but it hasn't comment since one year ago. Otherwise optparse is a great module ! Thanks for your information, Stephane

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Documentation Error for __hash__
by Michael Foord 30 Aug '08

30 Aug '08

Hello all, The documentation for __hash__ seems to be outdated. I'm happy to submit a patch, so long as I am not misunderstanding something. http://docs.python.org/dev/reference/datamodel.html#object.__hash__ The documentation states: If a class does not define a __cmp__() or __eq__() method it should not define a __hash__() operation either; if it defines __cmp__() or __eq__() but not __hash__(), its instances will not be usable as dictionary keys. If a class defines mutable objects and implements a __cmp__() or __eq__() method, it should not implement __hash__(), since the dictionary implementation requires that a key’s hash value is immutable (if the object’s hash value changes, it will be in the wrong hash bucket). This may have been true for old style classes, but as new style classes inherit a default __hash__ from object - mutable objects *will* be usable as dictionary keys (hashed on identity) *unless* they implement a __hash__ method that raises a type error. Shouldn't the advice be that classes that implement comparison methods should always implement __hash__ (wasn't this nearly enforced?), and that mutable objects should raise a TypeError in __hash__. Additionally the following documentation states that __reversed__ is new in Python 2.6 and I think it was actually new in Python 2.4 (it certainly works for Python 2.5 and IronPython 1 which targets 2.4...). http://docs.python.org/dev/reference/datamodel.html#object.__reversed__ Michael Foord -- http://www.ironpythoninaction.com/ http://www.voidspace.org.uk/ http://www.trypython.org/ http://www.ironpython.info/ http://www.theotherdelia.co.uk/ http://www.resolverhacks.net/

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The recursion checking problem
by Antoine Pitrou 30 Aug '08

30 Aug '08

Hi, I was working on a recursion overflow checking bug (http://bugs.python.org/issue2548) and, while I've managed to produce a working patch, I've also become uncomfortable with the very idea of trying to plug all those holes just for the sake of plugging them. I'll try to explain why, by describing the conflicting factors I've identified: - more and more, we are adding calls to Py_EnterRecursiveCall() and Py_LeaveRecursiveCall() all over the interpreter, to avoid special/obscure cases of undetected infinite recursion; this can probably be considered a good thing, but: - after a recursion error has been raised (technically a RuntimeError), usually some code has to do cleanup after noticing the exception; this cleanup now can very easily bump into the recursion limit again, due to the point mentioned above (the funniest example of this is PyErr_ExceptionMatches, which makes a call to PyObject_IsSubclass which itself increases the recursion count because __subclasscheck__ can be recursively invoked...). - to counter the latter problem, py3k has introduced a somewhat smarter mechanism (which I've tracked down to a commit in the defunct p3yk branch by Martin): when the recursion limit is exceeded, a special flag named "overflowed" is set in the thread state structure which disables the primary recursion check, so that cleanup code has a bit of room to increase the recursion count a bit. A secondary recursion check exists (equal to the primary one /plus/ 50) and, if it is reached, the interpreter aborts with a fatal error. The "overflowed" flag is cleared when the recursion count drops below the primary recursion limit /minus/ 50. Now it looks rather smart but: - unfortunately, some functions inside the interpreter discard every exception by design. The primary example is PyDict_GetItem(), which is certainly used quite a lot :-)... When PyDict_GetItem() returns NULL, the caller can only assume that the key isn't in the dict, it has no way to know that there was a critical problem due to a recursion overflow. I encountered the latter problem when trying to backport the py3k recursion overflow algorithm to trunk. A fatal error suddenly appeared in test_cpickle, and it turned out that the recursion count was exceeded in PyObject_RichCompare(), the error was then cleared in PyDict_GetItem(), but the "overflowed" flag was still set so that a subsequent recursion overflow would trigger the secondary check and lead to the fatal error. I guess that, if it doesn't happen in py3k, it's just by chance: the recursion overflow is probably happening at another point where errors don't get discarded. Indeed, the failure I got on trunk was manifesting itself when running "regrtest.py test_cpickle" but not directly "test_cpickle.py"... which shows how delicate the recursion mechanism has become. My attempt to solve the latter problem while still backporting the py3k scheme involves clearing the "overflowed" flag in PyErr_Clear(). This makes all tests pass ok, but also means the "overflowed" flag loses a lot of its meaning... since PyErr_Clear() is called in a lot of places (and, especially, in PyDict_GetItem()). Also, at this point I fear that the solution to the problem is becoming, because of its complexity, perhaps worse than the problem itself. That's why I'm bringing it here, to have your opinion. (I also suggest that we stop trying to fix recursion checking bugs until the stable release, so as to give us some time to do the Right Thing later - if there is such a thing) Regards Antoine.

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script containing all opcs
by Michal Revucky 30 Aug '08

30 Aug '08

hello everyone, i would like to get a python script which executes all interpreter's opcodes, or how am i supposed to create such script... i just need to make sure that all opcodes (as defined in Include/opcode.h) are executed by this scrip i need this script for testing purposes while rewritting the python's ceval.c code... bye michal

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Summary of Python tracker Issues
by Python tracker 29 Aug '08

29 Aug '08

ACTIVITY SUMMARY (08/22/08 - 08/29/08) Python tracker at http://bugs.python.org/ To view or respond to any of the issues listed below, click on the issue number. Do NOT respond to this message. 2029 open (+66) / 13524 closed (+16) / 15553 total (+82) Open issues with patches: 655 Average duration of open issues: 704 days. Median duration of open issues: 1760 days. Open Issues Breakdown open 2015 (+65) pending 14 ( +1) Issues Created Or Reopened (84) _______________________________ __eq__ / __hash__ check doesn't take inheritance into account 08/28/08 http://bugs.python.org/issue2235 reopened ncoghlan patch Objects/obmalloc.c:529: warning: comparison is always false due 08/24/08 http://bugs.python.org/issue3642 reopened christian.heimes 64bit IA5 Encoding should be in the default encodings 08/22/08 CLOSED http://bugs.python.org/issue3649 created pascal.bach Memory leak in bytes.split() 08/22/08 CLOSED http://bugs.python.org/issue3650 created amaury.forgeotdarc patch, needs review eval() leaks 1 reference every time 08/22/08 CLOSED http://bugs.python.org/issue3651 created amaury.forgeotdarc patch, needs review Remove DeprecationWarning in _warnings about 'line' 08/23/08 http://bugs.python.org/issue3652 created brett.cannon segfault calling sys.excepthook with non-Exception argument 08/23/08 CLOSED http://bugs.python.org/issue3653 created ajaksu2 patch, needs review Duplicated test name in regex test script 08/23/08 CLOSED http://bugs.python.org/issue3654 created mrabarnett latexwriter: \footnotemark can only take numbers as arguments 08/23/08 http://bugs.python.org/issue3655 created pv unicode encoding has lots of leaks of bytes 08/24/08 CLOSED http://bugs.python.org/issue3656 created nnorwitz pickle can pickle the wrong function 08/24/08 http://bugs.python.org/issue3657 created nnorwitz fix for pychecker property complaints 08/24/08 http://bugs.python.org/issue3658 created skip.montanaro patch, easy, needs review sqlite: enumeration value 'TYPE_STRING' not handled in switch 08/24/08 http://bugs.python.org/issue3659 created christian.heimes reference leaks in 3.0 08/24/08 http://bugs.python.org/issue3660 created nnorwitz patch sys.call_tracing segfaults 08/24/08 http://bugs.python.org/issue3661 created ajaksu2 _fileio._FileIO segfaults 08/24/08 CLOSED http://bugs.python.org/issue3662 created ajaksu2 needs review Extra DECREF on syntax errors 08/24/08 CLOSED http://bugs.python.org/issue3663 created amaury.forgeotdarc patch, needs review Pickler.dump from a badly initialized Pickler segfaults 08/24/08 http://bugs.python.org/issue3664 created ajaksu2 Support \u and \U escapes in regexes 08/24/08 http://bugs.python.org/issue3665 created georg.brandl patch atexit.register with bad input segfaults on exit 08/24/08 http://bugs.python.org/issue3666 created ajaksu2 Reloading an extension module always leaks 08/24/08 http://bugs.python.org/issue3667 created amaury.forgeotdarc patch, needs review "s*" argument parser marker leaks memory 08/24/08 http://bugs.python.org/issue3668 created amaury.forgeotdarc patch, needs review sqlite3.Connection.iterdump docs pythonicity 08/24/08 http://bugs.python.org/issue3669 created lehmannro patch Reporting bugs - no such sections 08/24/08 CLOSED http://bugs.python.org/issue3670 created kjohnson What's New in 2.6 - corrections 08/24/08 http://bugs.python.org/issue3671 created kjohnson Ill-formed surrogates not treated as errors during encoding/deco 08/24/08 http://bugs.python.org/issue3672 created Rhamphoryncus bsddb module leaks memory 08/25/08 http://bugs.python.org/issue3673 created nnorwitz patch, patch test_dbm_ndbm skip is unexpected on win32? 08/25/08 http://bugs.python.org/issue3674 created ocean-city patch Python 2.6 can't read sets pickled with Python 3.0 08/25/08 http://bugs.python.org/issue3675 created hagen patch Obsolete references to PEP 291 in py3k lib 08/25/08 http://bugs.python.org/issue3676 created orent importing from UNC roots doesn't work 08/25/08 http://bugs.python.org/issue3677 created krisvale Ignored LDFLAGS during linking libpython$(VERSION).so 08/25/08 http://bugs.python.org/issue3678 created Arfrever patch, easy, needs review pressing HOME key in IDLE editor ends IDLE 08/25/08 http://bugs.python.org/issue3679 created tjd Cycles with some iterator are leaking. 08/25/08 http://bugs.python.org/issue3680 created schuppenies patch Cannot read saved csv file in a single run 08/25/08 http://bugs.python.org/issue3681 created paul test_math: math.log(-ninf) fails to raise exception on OpenBSD 08/26/08 http://bugs.python.org/issue3682 created djmdjm compilation --without-threads fails 08/26/08 http://bugs.python.org/issue3683 created djmdjm needs review traceback.format_exception_only() misplaces the caret for certai 08/26/08 http://bugs.python.org/issue3684 created brodierao patch Crash while compiling Python 3000 in OpenBSD 4.4 08/26/08 http://bugs.python.org/issue3685 created henry.precheur patch PKG-INFO file should differentiate between authors and maintaine 08/26/08 http://bugs.python.org/issue3686 created lemburg Popen() object stdout attribute reassignment behaviour 08/26/08 http://bugs.python.org/issue3687 created vincent.legoll open() rejects bytes as filename 08/26/08 CLOSED http://bugs.python.org/issue3688 created dlitz listreverseiterator has a decreasing len() 08/26/08 http://bugs.python.org/issue3689 created laxrulz777 sys.getsizeof wrong for Py3k bool objects 08/26/08 http://bugs.python.org/issue3690 created schuppenies patch Incorrect variable reference 08/26/08 CLOSED http://bugs.python.org/issue3691 created timh improper scope in list comprehension, when used in class declara 08/26/08 CLOSED http://bugs.python.org/issue3692 created kaizhu Obscure array.array error message 08/27/08 http://bugs.python.org/issue3693 created tjreedy Undetected error in _struct.pack_into 08/27/08 http://bugs.python.org/issue3694 created ajaksu2 patch Source tarball for Sphinx 0.4.3 missing 08/27/08 http://bugs.python.org/issue3695 created hircus Error parsing arguments on OpenBSD <= 4.4 08/27/08 http://bugs.python.org/issue3696 created henry.precheur patch, needs review "Fatal Python error: Cannot recover from stack overflow" on Wind 08/27/08 http://bugs.python.org/issue3697 created pitrou patch, needs review incompatible arguments in warning formatting for idle 08/27/08 http://bugs.python.org/issue3698 created gcoffin test_bigaddrspace broken 08/27/08 http://bugs.python.org/issue3699 created pitrou test_bigmem broken 08/27/08 http://bugs.python.org/issue3700 created pitrou test_ntpath.test_relpath fails when launched from a different Wi 08/27/08 http://bugs.python.org/issue3701 created pitrou test_urllib2.test_trivial fails when run from another Windows dr 08/27/08 http://bugs.python.org/issue3702 created pitrou open() on directory raises IOError with unhelpful message 08/27/08 http://bugs.python.org/issue3703 created hagen patch, needs review cookielib doesn't handle URLs with / in parameters 08/27/08 http://bugs.python.org/issue3704 created andyk patch, needs review py3k aborts if "-c" or "-m" is given a non-ascii value 08/27/08 http://bugs.python.org/issue3705 created pitrou patch, needs review Fix error message for wrong exec() argument type 08/27/08 CLOSED http://bugs.python.org/issue3706 created georg.brandl patch, patch, needs review help('finally') behaves bizarrely 08/27/08 http://bugs.python.org/issue3707 created ms patch, easy, needs review os.urandom(1.1): infinite loop 08/27/08 http://bugs.python.org/issue3708 created ajaksu2 patch, easy, needs review BaseHTTPRequestHandler innefficient when sending HTTP header 08/27/08 http://bugs.python.org/issue3709 created yangman Reference leak in thread._local 08/28/08 http://bugs.python.org/issue3710 created tamino patch docs still say to use .dll for compiled extensions 08/28/08 CLOSED http://bugs.python.org/issue3711 created gagenellina patch memoryview leaks references 08/28/08 http://bugs.python.org/issue3712 created amaury.forgeotdarc patch, needs review Compile warning for Objects/stringlib 08/28/08 CLOSED http://bugs.python.org/issue3713 created kirkshorts patch nntplib module broken by str to unicode conversion 08/28/08 http://bugs.python.org/issue3714 created hdima patch hashlib's docstring throws exception in pydoc 08/28/08 http://bugs.python.org/issue3715 created erickt mistake in 3.4.2 Customizing attribute access 08/28/08 http://bugs.python.org/issue3716 created Alex7564 Py_InitModule* is still referenced in docs 08/28/08 http://bugs.python.org/issue3717 created erickt environment variable MACHDEP and python build system 08/28/08 http://bugs.python.org/issue3718 created rpetrov patch platform.py: _syscmd_file() can't handle target path with space 08/29/08 http://bugs.python.org/issue3719 created jfdp segfault in for loop with evil iterator 08/29/08 http://bugs.python.org/issue3720 created gideon patch, needs review invalid literal for int() with base 16: '' 08/29/08 http://bugs.python.org/issue3721 created xhomol11 print followed by exception eats print with doctest 08/29/08 http://bugs.python.org/issue3722 created cjw296 Py_NewInterpreter does not work 08/29/08 http://bugs.python.org/issue3723 created amaury.forgeotdarc math.log(x, 10) gives different result than math.log10(x) 08/29/08 http://bugs.python.org/issue3724 created segfaulthunter patch telnetlib module broken by str to unicode conversion 08/29/08 http://bugs.python.org/issue3725 created hdima Allow ',<whitespace>' delimiters in logging.config.fileConfig() 08/29/08 http://bugs.python.org/issue3726 created wcmaier patch poplib module broken by str to unicode conversion 08/29/08 http://bugs.python.org/issue3727 created hdima imaplib module broken by str to unicode conversion 08/29/08 CLOSED http://bugs.python.org/issue3728 created hdima SystemError on calling len() if __len__() doesn't return an int 08/29/08 http://bugs.python.org/issue3729 created hagen patch Update BaseHTTPServer docs 08/29/08 http://bugs.python.org/issue3730 created catlee Issues Now Closed (35) ______________________ LookupError: unknown encoding: X-MAC-JAPANESE 317 days http://bugs.python.org/issue1276 lemburg patch Error using >>> from OpenGL.GLUT import * 301 days http://bugs.python.org/issue1346 igorcamp Using __(get|set|del)slice__ needs a Py3K warning 160 days http://bugs.python.org/issue2351 benjamin.peterson patch, needs review Fixer to handle new places where parentheses are needed 158 days http://bugs.python.org/issue2367 benjamin.peterson patch, 26backport bytes() should respect __bytes__ 161 days http://bugs.python.org/issue2415 benjamin.peterson patch, needs review urllib2 can't handle http://www.wikispaces.com 157 days http://bugs.python.org/issue2464 orsenthil patch Restore isinstance and issubclass speed in 2.6 147 days http://bugs.python.org/issue2534 pitrou patch, patch, needs review Locking should be removed from the new buffer protocol 80 days http://bugs.python.org/issue3046 teoliphant patch bytearrays are not thread safe 66 days http://bugs.python.org/issue3139 pitrou patch multiprocessing build fails on Solaris 10 68 days http://bugs.python.org/issue3149 pitrou 2to3 Fix_imports optimization 59 days http://bugs.python.org/issue3218 benjamin.peterson patch invalid call to PyMem_Free() in fileio_init() 49 days http://bugs.python.org/issue3304 nnorwitz patch, easy making partial functions comparable 7 days http://bugs.python.org/issue3564 tjreedy SPAM 5 days http://bugs.python.org/issue3579 tjreedy invalid exception context 8 days http://bugs.python.org/issue3611 amaury.forgeotdarc patch, needs review shutil.rmtree() fails on invalid filename 8 days http://bugs.python.org/issue3616 pitrou test issues on 64bit windows 2 days http://bugs.python.org/issue3625 mhammond patch, patch, 64bit apple security patches need to be forward ported to py3k 3 days http://bugs.python.org/issue3627 nnorwitz Add more checks to testcapi 2 days http://bugs.python.org/issue3643 benjamin.peterson patch IA5 Encoding should be in the default encodings 3 days http://bugs.python.org/issue3649 pascal.bach Memory leak in bytes.split() 0 days http://bugs.python.org/issue3650 amaury.forgeotdarc patch, needs review eval() leaks 1 reference every time 4 days http://bugs.python.org/issue3651 benjamin.peterson patch, needs review segfault calling sys.excepthook with non-Exception argument 1 days http://bugs.python.org/issue3653 benjamin.peterson patch, needs review Duplicated test name in regex test script 1 days http://bugs.python.org/issue3654 georg.brandl unicode encoding has lots of leaks of bytes 0 days http://bugs.python.org/issue3656 amaury.forgeotdarc _fileio._FileIO segfaults 0 days http://bugs.python.org/issue3662 nnorwitz needs review Extra DECREF on syntax errors 2 days http://bugs.python.org/issue3663 pitrou patch, needs review Reporting bugs - no such sections 0 days http://bugs.python.org/issue3670 kjohnson open() rejects bytes as filename 0 days http://bugs.python.org/issue3688 amaury.forgeotdarc Incorrect variable reference 0 days http://bugs.python.org/issue3691 benjamin.peterson improper scope in list comprehension, when used in class declara 0 days http://bugs.python.org/issue3692 georg.brandl Fix error message for wrong exec() argument type 0 days http://bugs.python.org/issue3706 georg.brandl patch, patch, needs review docs still say to use .dll for compiled extensions 0 days http://bugs.python.org/issue3711 georg.brandl patch Compile warning for Objects/stringlib 0 days http://bugs.python.org/issue3713 christian.heimes patch imaplib module broken by str to unicode conversion 0 days http://bugs.python.org/issue3728 hdima Top Issues Most Discussed (10) ______________________________ 23 eval() leaks 1 reference every time 4 days closed http://bugs.python.org/issue3651 10 "s*" argument parser marker leaks memory 5 days open http://bugs.python.org/issue3668 9 import of site.py fails on startup 57 days open http://bugs.python.org/issue3279 8 Restore isinstance and issubclass speed in 2.6 147 days closed http://bugs.python.org/issue2534 7 Error parsing arguments on OpenBSD <= 4.4 2 days open http://bugs.python.org/issue3696 7 IA5 Encoding should be in the default encodings 3 days closed http://bugs.python.org/issue3649 6 os.urandom(1.1): infinite loop 2 days open http://bugs.python.org/issue3708 6 Objects/obmalloc.c:529: warning: comparison is always false due 5 days open http://bugs.python.org/issue3642 6 test_cpickle crash on AMD64 Windows build 8 days open http://bugs.python.org/issue3640 6 python3.0 interpreter on Cygwin ignores all arguments 8 days open http://bugs.python.org/issue3626

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what version of bsddb to use/accept for 2.6/3.0
by Neal Norwitz 29 Aug '08

29 Aug '08

Many buildbots are running bsddb 4.7, particularly the debian/ubuntu ones (4.7.25 which seems to be the latest). Some of them are crashing, others are not. The max version we support in both 2.6 and 3.0 is 4.7. Should we allow this version or should we use a lower maximum that is more likely to work (ie, not crash)? It looks like the WIndows buildbots use 4.4.20. Unfortunately, the Windows bots aren't in great shape either. Additionally, there are reference leaks in both 2.6 and 3.0: test_bsddb3 leaked [80, 80] references, sum=160 (2.6) test_bsddb3 leaked [63, 63] references, sum=126 (3.0) It would be nice to get as many of these things fixed up before release. Jesus, Greg, Trent, can you make some time over the next week to stabilize bsddb support? Thanks, n PS. To change the max version of bsddb supported in Unix, modify max_db_ver in setup.py.

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Re: [Python-Dev] Things to Know About Super
by Phillip J. Eby 29 Aug '08

29 Aug '08

At 05:50 PM 8/28/2008 +0200, Michele Simionato wrote: >On Aug 28, 5:30 pm, "Phillip J. Eby" <p...(a)telecommunity.com> wrote: > > How is that making things easier for application programmers? > >We have different definitions of "application programmer". For me a typical >application programmer is somebody who never fiddles with metaclasses, >which are the realm of framework builders. Application programmers use frameworks, and sometimes more than one. If they're subclassing from two different frameworks, each using a different metaclass, they will need to also multiple-inherit the metaclass. This is in fact so annoying that I created a "universal metaclass" in DecoratorTools whose sole function is to delegate metaclass __new__, __init__, and __call__ to class-level methods (e.g. __class_new__, __class_call__, etc.), thereby eliminating the need to have custom metaclasses for most use cases in the first place. Now, wherever possible, I use that single metaclass in my frameworks, so that there's no need to mix them. That, IMO, would be a more useful change than getting rid of super(); it would get rid of the explicit metaclass mixing. (It would still not remove the need for co-operative methods, as the class-delegated methods still need to be co-operative for MI to work.) There are, of course, other ways to create co-operative function calls besides super(), and I've certainly created more a few of them in my time. (E.g. generic function method combination, instancemethod() chains, and next-method-iterators, to name the ones that occur to me right off.) But these are more for cases where super() is wholly inadequate to the purpose, and none are anywhere near as convenient as super().

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confusing exec error message in 3.0
by Fredrik Lundh 29 Aug '08

29 Aug '08

(using 3.0a4) >>> exec(open("file.py")) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: exec() arg 1 must be a string, file, or code object, not TextIOWrapper so what's "file" referring to here? (the above works under 2.5, of course) </F>

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Re: [Python-Dev] Things to Know About Super
by Phillip J. Eby 28 Aug '08

28 Aug '08

At 06:35 AM 8/28/2008 +0200, Michele Simionato wrote: >Multiple inheritance of metaclasses is perhaps >the strongest use case for multiple inheritance, but is it strong >enough? I mean, in real code how many times did I need that? >I would not mind make life harder for gurus and simpler for >application programmers. Then you need to leave MI and co-operation the hell alone. Right now, an application programmer can mix metaclasses like this: class FooBar(Foo, Bar): class __metaclass__(Foo.__class__, Bar.__class__): pass ... Or, in 3.x: class FooBarClass(Foo.__class__, Bar.__class__): pass class FooBar(Foo, Bar, metaclass=FooBarClass): ... Either way, this is useful in cases where Foo and Bar come from different frameworks. That's the *only* way to get such things to co-operate, in fact. >I do not think removing cooperation >would be so bad in practice. In many practical cases, one could just write >the metaclass by hand, How is that making things easier for application programmers? >Maybe you would need to duplicate a couple of lines and/or to introduce >an helper function, ...which then has to have an agreed-upon protocol that all metaclass authors have to follow... which we already have... but which you're proposing to get rid of... so we can re-invent it lots of times... in mutually incompatible ways. :)

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Re: [Python-Dev] Things to Know About Super
by Phillip J. Eby 28 Aug '08

28 Aug '08

At 03:16 AM 8/27/2008 +0200, Michele Simionato wrote: >It is just a matter of how rare the use cases really are. Cooperative >methods has been introduced 6+ years ago. In all this time surely >they must have been used. How many compelling uses of cooperation >we can find in real life code? For instance in the standard library or >in some well known framework? This is a serious question I have been >wanting to ask for years. I am sure people here can find some example, >so just give me a pointer and we will see. ISTR pointing out on more than one occasion that a major use case for co-operative super() is in the implementation of metaclasses. The __init__ and __new__ signatures are fixed, multiple inheritance is possible, and co-operativeness is a must (as the base class methods *must* be called). I'm hard-pressed to think of a metaclass constructor or initializer that I've written in the last half-decade or more where I didn't use super() to make it co-operative. That, IMO, is a compelling use case even if there were not a single other example of the need for super. However, I'm pretty sure I've had other cases where it was necessary to co-operate in cases where multiple inheritance occurred later; ie. where it was possible for a subclass to add a new class between parents. Remember that subclasses of a new-style class do not always have the same MRO tail as the original class; i.e., a subclass of "class A(B, C):" is only constrained to have [A...B...C] in its MRO; semi-arbitrary classes may be inserted between e.g. A and B. So, a new-style class cannot, as a general rule, statically determine what base class implementation of a method should be invoked. I personally consider the rare case where I have to force such static knowledge to be an unfortunate wart in the design (of that code, not Python).

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