Created on 2009-04-15 06:32 by cvrebert, last changed 2009-04-18 08:31 by georg.brandl. This issue is now closed.
|msg85983 - (view)||Author: Chris Rebert (cvrebert) *||Date: 2009-04-15 06:32|
Prompted by http://mail.python.org/pipermail/python-ideas/2009-April/004048.html The current error message issued when trying to use the get item () operator on an object that does not define __getitem__ can be hard to understand: >>> class A(object): pass ... >>> A()['a'] Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'A' object is unsubscriptable Problems observed: - "unsubscriptable" is easily misread in haste as "unscriptable", which can be quite confusing, especially to newbies - "subscripting" is not frequently used to describe the  operator, making the message more difficult to decipher (again, esp. for newbies) - the underlying lack of a __getitem__ method is not mentioned, thus not making it obvious how to remedy the error Suggestion: Use exception chaining and rephrase the error message to get something like: AttributeError: class 'A' has no attribute '__getitem__' The above exception was the direct cause of the following exception: TypeError: 'A' object does not support the 'get item' operator Similar changes should be made to __setitem__ & __delitem__.
|msg86105 - (view)||Author: Terry J. Reedy (terry.reedy) *||Date: 2009-04-18 07:29|
As I said on the python-ideas discussion, which definitely did *not* come to consensus, I disagree with this suggestion. To repeat and expand on what I said there: 1. 'unsubscriptable' could instead be changed to 'not subscriptable'. 2. 'subscription' *is* the way Python describes the use of . """A subscription selects an item of a sequence (string, tuple or list) or mapping (dictionary) object: subscription ::= primary "[" expression_list "]" """ 'array subscript' (from C, for example) is a standard term used for decades in computer languages. It is appropriate since a[i] is one standard single-text-line replacement for a<typographic subscript>i when one cannot actually 'type' a subscript. It *is* a way of representing a subscript, just as i**j is an in-line way of representing a superscript exponent, which we still call an exponent in spite of it not being 'raised'. Both constitute visible markup that is part of the compromise in representing 2D typography with a 1D stream of characters. Using '/' for division (and fractions) is another. 3. Python error message generally do not and I think should not mention the special method implementation that underlies surface level errors. In this particular case, the remedy to mistakenly trying to subscript something is to not do that. I think mentioning __getitem__ would be a distraction. One generally *cannot* just go around adding it.
|msg86110 - (view)||Author: Georg Brandl (georg.brandl) *||Date: 2009-04-18 08:31|
I concur. I changed "unsubscriptable" to "not subscriptable" in r71696.
|2009-04-18 08:31:49||georg.brandl||set||status: open -> closed|
nosy: + georg.brandl
messages: + msg86110
messages: + msg86105