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What about cases where performance is unimportant, or where the key 
function is fast (e.g. an attribute access)? Then something like
bisect(a, x, key=attrgetter('size'))
is easy to write and read. Mightn't this be considered good design,
from some perspectives?
Another thought: if your list is a list of user-defined objects then a 
natural way to do the 'decorate' step of DSU might be to add a 'key' 
attribute to each object, rather than the usual method of constructing 
pairs. (This has the advantage that you might not have to bother with the 
'undecorate' step.) With a key argument, bisect could make use of this 
technique too.
Disclaimer: I haven't personally had any need for a key argument on 
bisect, so all this is hypothetical. That's why I'm asking for real use-
cases.