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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.