I felt compelled to offer a version using glob to generate the combinations. It might not port so well to Python (but I don't really know what Python has).

#!perl
use strict;
use warnings;

my %digit_letters = (
    2 => '{a,b,c}',
    3 => '{d,e,f}',
    4 => '{g,h,i}',
    5 => '{j,k,l}',
    6 => '{m,n,o}',
    7 => '{p,r,s}',
    8 => '{t,u,v}',
    9 => '{w,x,y}',
);

my $digit_glob = '{' . join(',', 2..9) . '}';

my $limit = 20;
for my $num (glob($digit_glob x 4)) {
    (my $word_glob = $num) =~ s/(.)/$digit_letters{$1}/g;
    for my $word (glob $word_glob) {
        print "$num: $word\n";
    }
}
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