[Python-Dev] unsubscriptable vs object does not support indexing (original) (raw)

R. David Murray rdmurray at bitdance.com
Wed Sep 23 05:08:07 CEST 2009

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 "", line 1, in TypeError: 'set' object does not support indexing > > > class c(object): pass ... [....] > > > [].append[42] Traceback (most recent call last): File "", line 1, in TypeError: 'builtinfunctionormethod' 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 "", line 1, in 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?

--David

(*) The error message changed format slightly in 2.7 due to http://bugs.python.org/issue5760, and the discussion there is worth reading in this context.

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