Brett Cannon wrote: > On Tue, Sep 22, 2009 at 20:08, R. David Murray <rdmur...@bitdance.com> wrote: >> On Wed, 23 Sep 2009 at 02:01, MRAB wrote: >>> >>> Dino Viehland wrote: >>>> >>>> Is there a reason or a rule by which CPython reports different error >>>> message for different failures to subscript? >>>> >>>> For example: >>>> >>>>>>> set()[2] >>>> Traceback (most recent call last): >>>> File "<stdin>", line 1, in <module> >>>> TypeError: 'set' object does not support indexing >>>>>>> class c(object): pass >>>> ... >> >> [....] >>>> >>>>>>> [].append[42] >>>> Traceback (most recent call last): >>>> File "<stdin>", line 1, in <module> >>>> TypeError: 'builtin_function_or_method' object is unsubscriptable >> >> [...] >>>> >>>> IronPython had a bug report that we were getting this wrong for set >>>> objects and that “does not support indexing” was also a clearer error >>>> message. I’m wondering if there’s some reason why the different error >>>> messages occur which I’m missing. Otherwise I could switch to using >>>> the >>>> more clear message or start marking types which should report the >>>> unsubscriptable error. Does anyone have any insights into this? >>>> >>> I thought that the difference might be between iterable and non-iterable >>> objects, but then I'd expect the class 'c' to behave like the other >>> non-iterables. In fact, the result is different again if it's an >>> old-style class: >>> >>>>>> class c: pass >>>>>> c()[2] >>> >>> Traceback (most recent call last): >>> File "<stdin>", line 1, in <module> >>> c()[2] >>> AttributeError: c instance has no attribute '__getitem__' >> >> Looking at the source, these messages are generated from abstract.c, and >> the difference appears to be whether or not the tp_as_sequence slot is >> filled in or not. If it is, but there is no sq_item method, then >> PySequence_GetItem gives the "does not support indexing" message. >> If it isn't filled in, you get the 'not subscriptable"(*) message from >> PyObject_GetItem. >> >> The logic appears to be, roughly, if an object does not have mapping >> methods, or has them but does not have mp_subscript, and does have >> sequence methods but does not have sq_item, then you get a 'does >> not support indexing'. Otherwise you get 'not subscriptable'. >> >> The question is, is this a useful distinction, or is it an >> implementation artifact? > > Let's ignore history, which I bet is the reason for the distinction, > and just look at the error messages; does the distinction make sense > to a newbie? I would say no and that the "does not support indexing" > error message is more useful. For expert programmers they could figure > out the problem with either error message. The only help is if you are > trying to debug a type, but I am willing to bet most of us didn't know > the distinction at the C level until David looked it up. > > So I am +1 on unified the message and +1 on using the "does not > support indexing" one.
+1 as well. Note that the wording of error message has never been something we guaranteed backwards compatibility for, so such changes are a lot easier to implement than API changes. (I just hope that doctest doesn't rely on the message texts.) -- Marc-Andre Lemburg eGenix.com Professional Python Services directly from the Source (#1, Sep 23 2009) >>> Python/Zope Consulting and Support ... http://www.egenix.com/ >>> mxODBC.Zope.Database.Adapter ... http://zope.egenix.com/ >>> mxODBC, mxDateTime, mxTextTools ... http://python.egenix.com/ ________________________________________________________________________ ::: Try our new mxODBC.Connect Python Database Interface for free ! :::: eGenix.com Software, Skills and Services GmbH Pastor-Loeh-Str.48 D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg Registered at Amtsgericht Duesseldorf: HRB 46611 http://www.egenix.com/company/contact/ _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
