you can mimic your semantic in Python by using a generator function acting as closure, defining a nested function roman_to_dec and returning it.

I don't have Python installed and am not a Py expert, so please accept my semantically equal Perl interpolation.

(untested)

sub generator {
    my %rtoa = ( M=>1000, D=>500, C=>100, L=>50, X=>10, V=>5, I=>1 );
    
    my $c_nested = sub {
             reduce { $a+$b-$a%$b*2 } map { $rtoa{$_} } split//, uc(sh
+ift);
         };

    return $c_nested;
}

*roman_to_dec = generator();
[download]

NB: This will look much easier in Python because it's automatically dereferencing.

UPDATE

I suppose most Pythonistas would rather prefer making rtoa a class variable and using roman_to_dec as a method. TIMTOWTDI. ;-)