You probably shouldn't do this in an inner loop: :-)

  m = re.match(r'(\d{4})', buf)

'map' is more general than just the single, convoluted statement of list comprehensions, which really seems as a kludge from unwillingness to accept multi-line lambdas. So something new was invented that had to be learned; then it was argued it is a boon. (Like references in Perl. :-) )

(There is some defaultdict functionality for autovivification in Python iirc.)

Yeah, they are all similar. I don't know enough Ruby to have an opinion.