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

MRAB python at mrabarnett.plus.com
Wed Sep 23 03:01:15 CEST 2009

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 ... >>> c()[2] Traceback (most recent call last): File "", line 1, in TypeError: 'c' object does not support indexing But compare this to: >>> [].append[42] Traceback (most recent call last): File "", line 1, in TypeError: 'builtinfunctionormethod' object is unsubscriptable >>> (lambda: 42)[42] Traceback (most recent call last): File "", line 1, in TypeError: 'function' object is unsubscriptable >>> property()[42] Traceback (most recent call last): File "", line 1, in TypeError: 'property' 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'

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