> The codec currently doesn't look at the base at all - and shouldn't
> need to:
> 
> It simply converts input characters that have a decimal digit value
> associated with them, to the usual ASCII digits in preparation
> for parsing them using the standard number parsing tools we have in
> Python.

Right. And as such, it shouldn't stop with digit 9, but continue into
digits a, b, c, and so on, as appropriate.

> This is to support number representations using non-ASCII code
> points for digits (e.g. Japanese or Sanskrit numbers)

Notice that it also supports bases other than 10:

80

So calling it "decimal" is a misnomer.

> Also note that we already have a hex codec in Python 2.x
> which converts between the hex representations of a string
> and its regular form. This was removed in 3.x for some reason
> I don't understand (probably just an oversight).

The hex codec doesn't have to do anything with number conversions;
nor does it have to do with character encodings. To introduce it was
a mistake in Python 2.x which has been fixed in 3.x (by removing
it and other similar "codecs", such as rot13).