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[Python-Dev] unsubscriptable vs object does not support indexing

Dino Viehland dinov at microsoft.com
Wed Sep 23 02:23:56 CEST 2009 
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
...
>>> c()[2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'c' object does not support indexing

But compare this to:

>>> [].append[42]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'builtin_function_or_method' object is unsubscriptable
>>> (lambda: 42)[42]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'function' object is unsubscriptable
>>> property()[42]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
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?
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