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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?