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In the absence of an identity function, map accepting None is useful in 
the cases like this:
converters = {..}
y = map(converters.get(c), x)
That will now have to be rewritten as
conv = converters.get(c)
if conv is None:
 y = list(x)
else:
 y = map(conv, x)
and subtle bugs will be introduced if x is used instead of list(x) in 
the None case.
Another alternative,
y = map(converters.get(c, lambda x:x), x)
will be much slower.
Take my opinion with a grain of salt because I also believe None should 
be callable with None(x) -> x and None(x,y,..) -> (x,y,..).